Assignment 1

Microwave Devices & Network

Microwave Devices & Network2015

[2015]

Adama Science &Technology UniversitySchool of Engineering Electrical & Computer Engineering Department

[Submitted by: Girma Abebe ID. :GSR/02935/07]

Microwave Devices & Network (EEng-6307) [Assignment 1] [ Submitted to: Dr M V Raghavendra ]

Microwave solid-state devices

Microwave solid-state devices

Semiconductor devices used for the detection, generation, amplification, and control of electromagnetic radiation with wavelengths from 30 cm to 1 mm (frequencies from 1 to 300 GHz). The number and variety of microwave semiconductor devices, used for wireless and satellite communication and optoelectronics, have increased as new techniques, materials, and concepts have been developed and applied. Passive microwave devices, such as pn and PIN junctions, Schottky barrier diodes, and varactors, are primarily used for detecting, mixing, modulating, or controlling microwave signals. Step-recovery diodes, transistors, tunnel diodes, and transferred electron devices (TEDs) are active microwave devices that generate power or amplify microwave signals.

Typical high-frequency semiconductor materials include silicon (Si), germanium (Ge), and compound semiconductors, such as gallium arsenide (GaAs), indium phosphide (InP), silicon germanium (SiGe), silicon carbide (SiC), and gallium nitride (GaN). In general, the compound semiconductors work best for high-frequency applications due to their higher electron mobilities.

Active devices

Transistors are the most widely used active microwave solid-state devices. At very high microwave frequencies, high-frequency effects limit the usefulness of transistors, and two-terminal negative resistance devices, such as transferred-electron devices, avalanche diodes, and tunnel diodes, are sometimes used. Two main categories of transistors are used for microwave applications: bipolar junction transistors (BJTs) and field-effect transistors (FETs). In order to get useful output power at high frequencies, transistors are designed to have a higher periphery-to-area ratio using a simple stripe geometry. The area must be reduced without reducing the periphery, as large area means large interelectrode capacitance. For high-frequency applications the goal is to scale down the size of the device. Narrower widths of the elements within the transistor are the key to superior high-frequency performance. A BJT consists of three doped regions forming two pn junctions. These regions are the emitter, base, and collector in either an npn or pnp arrangement. Silicon npn BJTs have an upper cutoff frequency of about 25 GHz (varies with manufacturing improvements). The cutoff frequency is defined as the frequency at which the current amplification drops to unity as the frequency is raised. The primary limitations to higher frequency are base and emitter resistance, capacitance, and transit time. To operate at microwave frequencies, individual transistor dimensions must be reduced to micrometer or submicrometer size. To maintain current and power capability, various forms of internal paralleling on the chip are used. Three of these geometries are interdigitated fingers that form the emitter and base, the overlaying of emitter and base stripes, and the matrix approach. Silicon BJTs are mainly used in the lower microwave ranges. Their power capability is quite good, but in terms of noise they are inferior to GaAs metal semiconductor field-effect transistors (MESFETs) at frequencies above 1 GHz and are mainly used in power amplifiers and oscillators. They may also be used in small-signal microwave amplifiers when noise performance is not critical.

Heterojunction bipolar transistors (HBTs) have been designed with much higher maximum frequencies than silicon BJTs. HBTs are essentially BJTs that have two or more materials making up the emitter, base, and collector regions (Fig. 1). In HBTs, the major goal is to limit the injection of holes into the emitter by using an emitter material with a larger bandgap than the base. The difference in bandgaps manifests itself as a discontinuity in the conduction band or the valence band, or both. For npn HBTs, a discontinuity in the valence band is required. In general, to make high-quality heterojunctions, the two materials should have matching lattice constants. For very thin layers, lattice matching is not absolutely necessary as the thin layer can be strained to accommodate the crystal lattice of the other material. Fortunately, the base of a bipolar transistor is designed to be very thin and thus can be made of a strained layer material. Combinations such as AlGaAs/InGaAs and Si/SiGe are possible. Field-effect transistors (FETs) operate by varying the conductivity of a semiconductor channel through changes in the electric field across the channel. The three basic forms of FETs are the junction FET (JFET), the metal semiconductor FET (MESFET), and the metal oxide semiconductor FET (MOSFET). All FETs have a channel with a source and drain region at each end and a gate located along the channel, which modulates the channel conduction (Fig. 2). Microwave JFETs and MESFETs work by channel depletion. The channel is n-type and the gate is p-type for JFETs and metal for MESFETs. FET structures are well suited for microwave applications because all contacts are on the surface to keep parasitic capacitances small. The cutoff frequency is mainly determined by the transit time of the electrons under the gate; thus short gate lengths (less than 1 m) are used.

Power devices consist of a number of MESFETs in parallel with air bridges connecting the sources. GaAs MESFET devices are used in low-noise amplifiers (LNAs), Class C amplifiers, oscillators, and monolithic microwave integrated circuits. The performance of a GaAs FET is determined primarily by the gate width and length. The planar structure of a MESFET makes it straightforward to add a second gate which can be used to control the amplification of the transistor. Dual-gate MESFETs can be used as mixers (with conversion gain) and for control purposes. Applications include heterodyne mixers and amplitude modulation of oscillators. The MOSFET has a highly insulating silicon dioxide (SiO2) layer between the semiconductor and the gate; however, silicon MOSFETs are not really considered microwave transistors. Compared with the GaAs MESFET, MOSFETs have lower electron mobility, larger parasitic resistances, and higher noise levels. Also, since the silicon substrate cannot be made semi-insulating, larger parasitic capacitances result. MOSFETs therefore do not perform very well above 1 GHz. Below this frequency, MOSFETs find application mainly as radio-frequency (RF) power amplifiers.

A disadvantage of the MESFET is that the electron mobility is degraded since electrons are scattered by the ionized impurities in the channel. By using a heterojunction consisting of n-type AlGaAs with undoped GaAs, electrons move from the AlGaAs to the GaAs and form a conducting channel at the interface. The electrons are separated from the donors and have the mobility associated with undoped material. A heterojunction transistor made in this fashion has many different names: high electron mobility transistor (HEMT), two-dimensional electron gas FET (TEGFET), modulation-doped FET (MODFET), selectively doped heterojunction transistor (SDHT), and heterojunction FET (HFET). The HEMT has high power gain at frequencies of 100 GHz or higher with low noise levels.

A monolithic microwave integrated circuit (MMIC) can be made using silicon or GaAs technology with either BJTs or FETs. For high-frequency applications, GaAs FETs are the best choice. A MMIC has both the active and passive devices fabricated directly on the substrate. MMICs are typically used as low-noise amplifiers, as mixers, as modulators, in frequency conversion, in phase detection, and as gain block amplifiers. Silicon MMIC devices operate in the 100-MHz to 3-GHz frequency range. GaAs FET MMICs are typically used in applications above 1 GHz.

Active microwave diodes

Active microwave diodes differ from passive diodes in that they are used as signal sources to generate or amplify microwave frequencies. These include step-recovery, tunnel, Gunn, avalanche, and transit time diodes, such as impact avalanche and transit-time (IMPATT), trapped plasma avalanche triggered transit-time (TRAPATT), barrier injection transit-time (BARITT), and quantum well injection transit time (QWITT) diodes.

A step recovery diode is a special PIN type in which charge storage is used to produce oscillations. When a diode is switched from forward to reverse bias, it remains conducting until the stored charge has been removed by recombination or by the electric field. A step recovery diode is designed to sweep out the carriers by an electric field before any appreciable recombination has taken place. Thus, the transition from the conducting to the nonconducting state is very fast, on the order of picoseconds. Because of the abrupt step, this current is rich in harmonics, so these diodes can be used in frequency multipliers.

For microwave power generation or amplification, a negative differential resistance (NDR) characteristic at microwave frequencies is necessary. NDR is a phenomenon that occurs when the voltage (V) and current (I) are 180 out of phase. NDR is a dynamic property occurring only under actual circuit conditions; it is not static and cannot be measured with an ohmmeter. Transferred electron devices (TEDs), such as Gunn diodes, and avalanche transit-time devices use NDR for microwave oscillation and amplification. TEDs and avalanche transit-time devices today are among the most important classes of microwave solid-state devices. The tunnel diode uses a heavily doped abrupt pn junction resulting in an extremely narrow junction that allows electrons to tunnel through the potential barrier at near-zero applied voltage. This results in a dip in the current-voltage (I-V) characteristic, which produces NDR. Because this is a majority-carrier effect, the tunnel diode is very fast, permitting response in the millimeter-wave region. Tunnel diodes produce relatively low power. The tunnel diode was the first semiconductor device type found to have NDR.

Avalanche diodes are junction devices that produce a negative resistance by appropriately combining impact avalanche breakdown and charge-carrier transit time effects. Avalanche breakdown in semiconductors occurs if the electric field is high enough for the charge carriers to acquire sufficient energy from the field to create electron-hole pairs by impact ionization. The avalanche diode is a pn-junction diode reverse-biased into the avalanche region. By setting the DC bias near the avalanche threshold, and superimposing on this an alternating voltage, the diode will swing into avalanche conditions during alternate half-cycles. The hole-electron pairs generated as a result of avalanche action make up the current, with the holes moving into the p region, and the electrons into the n region. The carriers have a relatively large distance to travel through the depletion region. At high frequencies, where the total time lag for the current is comparable with the period of the voltage, the current pulse will lag the voltage. By making the drift time of the electrons in the depletion region equal to one-half the period of the voltage, the current will be 180 out of phase. This shift in phase of the current with respect to the voltage produces NDR, so that the diode will undergo oscillations when placed in a resonant circuit.

A Gunn diode is typically an n-type compound semiconductor, such as GaAs or InP, which has a conduction band structure that supports negative differential mobility. Although this device is referred to as a Gunn diode, after its inventor, the device does not contain a pn junction and can be viewed as a resistor below the threshold electric field (Ethres). For applied voltages that produce electric fields below Ethres, the electron velocity increases as the electric field increases according to Ohm's law. For applied voltages that produce electric fields above Ethres, conduction band electrons transfer from a region of high mobility to low mobility, hence the general name transferred electron device. Beyond Ethres, the velocity suddenly slows down due to the significant electron transfer to a lower mobility band producing NDR. For GaAs, Ethres is about 3 kV/cm. The Gunn effect can be used up to about 80 GHz for GaAs and 160 GHz for InP. Two modes of operation are common: nonresonant bulk (transit-time) and resonant limited space-charge accumulation (LSA).

Impact avalanche and transit-time diodes (IMPATTs) are NDR devices that operate by a combination of carrier injection and transit time effects. There are several versions of IMPATT diodes, including simple reverse-biased pn diodes, complicated reverse-biased multidoped pn layered diodes, and reverse-biased PIN diodes. The IMPATT must be connected to a resonant circuit. At bias turn-on, noise excites the tuned circuit into a natural oscillation frequency. This voltage adds algebraically across the diode's reverse-bias voltage. Near the peak positive half-cycle, the diode experiences impact avalanche breakdown. When the voltage falls below this peak value, avalanche breakdown ceases. A 90 shift occurs between the current pulse and the applied voltage in the avalanche process. A further 90 shift occurs during the transit time, for a total 180 shift which produces NDR. An IMPATT oscillator has higher output power than a Gunn equivalent. However, the Gunn oscillator is relatively noise-free, while the IMPATT is noisy due to avalanche breakdown.

A trapped plasma avalanche triggered transit-time (TRAPATT) diode is basically a modified IMPATT diode in which the holes and electrons created by impact avalanche ionization multiplication do not completely exit from the transit domain of the diode during the negative half-cycle of the microwave signal. These holes and electrons form a plasma which is trapped in the diode and participates in producing a large microwave current during the positive half-cycle.

A barrier injection transit-time diode (BARRITT) is basically an IMPATT structure that employs a Schottky barrier formed by a metal semiconductor contact instead of a pn junction to create similar avalanche electron injection.

A variety of approaches have been investigated to find alternative methods for injecting carriers into the drift region without relying on the avalanche mechanism, which is inherently noisy. Quantum well injection transit-time diodes (QWITT) employ resonant tunneling through a quantum well to inject electrons into the drift region. The device structure consists of a single GaAs quantum well located between two AlGaAs barriers in series with a drift region of made of undoped GaAs. This structure is then placed between two n+-GaAs regions to form contacts.

How radar works

Whether it's mounted on a plane, a ship, or anything else, a radar set needs the same basic set of components: something to generate radio waves, something to send them out into space, something to receive them, and some means of displaying information so the radar operator can quickly understand it.

The radio waves used by radar are produced by a piece of equipment called a magnetron. Radio waves are similar to light waves: they travel at the same speedbut their waves have much longer wavelengths and higher frequencies. Both light and radio waves are part of the electromagnetic spectrum, which means they're made up of fluctuating patterns of electrical and magnetic energy zapping through the air. The waves a magnetron produces are actually microwaves, similar to the ones generated by a microwaveoven. The difference is that the magnetron in a radar has to send the waves many miles, instead of just a few inches, so it is much larger and more powerful.

Photo: A typical military radar screen, located in the flight tower at Eielson Air Force Base, Alaska. Photo by Christopher Griffin courtesy of US Air Force.

Once the radio waves have been generated, an antenna, working as a transmitter, hurls them into the air in front of it. The antenna is usually curved so it focuses the waves into a precise, narrow beam, but radar antennas also typically rotate so they can detect movements over a large area. The radio waves travel outward from the antenna at the speed of light (186,000 miles or 300,000 km per second) and keep going until they hit something. Then some of them bounce back toward the antenna in a beam of reflected radio waves also traveling at the speed of light. The speed of the waves is crucially important. If an enemy jet plane is approaching at over 3,000 km/h (2,000 mph), the radar beam needs to travel much faster than this to reach the plane, return to the transmitter, and trigger the alarm in time. That's no problem, because radio waves (and light) travel fast enough to go seven times around the world in a second! If an enemy plane is 160 km (100 miles) away, a radar beam can travel that distance and back in less than a thousandth of a second.

The antenna doubles up as a radar receiver as well as a transmitter. In fact, it alternates between the two jobs. Typically it transmits radio waves for a few thousandths of a second, then it listens for the reflections for anything up to several seconds before transmitting again. Any reflected radio waves picked up by the antenna are directed into a piece of electronic equipment that processes and displays them in a meaningful form on a television-like screen, watched all the time by a human operator. The receiving equipment filters out useless reflections from the ground, buildings, and so on, displaying only significant reflections on the screen itself. Using radar, an operator can see any nearby ships or planes, where they are, how quickly they're traveling, and where they're heading. Watching a radar screen is a bit like playing a video gameexcept that the spots on the screen represent real airplanes and ships and the slightest mistake could cost many people's lives.

There's one more important piece of equipment in the radar apparatus. It's called a duplexer and it makes the antenna swap back and forth between being a transmitter and a receiver. While the antenna is transmitting, it cannot receiveand vice-versa. Take a look at the diagram in the box below to see how all these parts of the radar system fit together.

How does radar work?

Here's how radar works:

1. Magnetron generates high-frequency radio waves.

2. Duplexer switches magnetron through to antenna.

3. Antenna acts as transmitter, sending narrow beam of radio waves through the air.

4. Radio waves hit enemy airplane and reflect back.

5. Antenna picks up reflected waves during a break between transmissions. Note that the same antenna acts as both transmitter and receiver, alternately sending out radio waves and receiving them.

6. Duplexer switches antenna through to receiver unit.

7. Computer in receiver unit processes reflected waves and draws them on a TV screen.

8. Enemy plane shows up on TV radar display with any other nearby targets.Radaris an object-detection system that usesradio wavesto determine the range, altitude, direction, or speed of objects. It can be used to detectaircraft, ships,spacecraft,guided missiles,motor vehicles,weather formations, and terrain. The radar dish (or antenna) transmits pulses of radio waves ormicrowavesthat bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna that is usually located at the same site as thetransmitter.

Radar was secretly developed by several nations before and duringWorld War II. The termRADARwas coined in 1940 by theUnited States Navyas anacronymforRAdioDetectionAndRanging. The termradarhas since enteredEnglishand other languages as a common noun,losing all capitalization.

The modern uses of radar are highly diverse, including air and terrestrial traffic control,radar astronomy,air-defense systems,antimissile systems;marine radarsto locate landmarks and other ships; aircraft anticollision systems;ocean surveillancesystems, outer space surveillance andrendezvoussystems;meteorologicalprecipitation monitoring; altimetry andflight control systems;guided missiletarget locating systems; andground-penetrating radarfor geological observations. High tech radar systems are associated withdigital signal processingand are capable of extracting useful information from very highnoiselevels.

Other systems similar to radar make use of other parts of theelectromagnetic spectrum. One example is "lidar", which uses ultraviolet, visible, or near infrared light fromlasersrather than radio waves.

BJT Bipolar Junction Transistor

This name is a representation for a device having transfer resistors. As we have seen a semiconductor offers less resistance to flow of current in one direction and high resistance in another direction, we call the device made of semiconductors as a transistor.There are basically two types of transistors:

1. Point contact2. Junction transistor

Junction transistors are more in use compared to point type transistors. They are preferred due to their ruggedness and small size. Junction transistors are further classified into two types

a. PNP

b. NPN

Each has 3 electrodes called emitter, base, and collector. These are made of P and N types semiconductors depending on the type.

TRANSISTOR

The transistor was invented by William Shockley in 1947. A transistor consist of two PN junctions. The junctions are formed by sandwiching either P-type or N-type semiconductor layers between a pair of opposite types. There are two types of transistors one is called PNP transistor and other is called NPN transistor.

A PNP transistor is composed of two P-type semiconductors separated by a thin section of N-type as shown in Figure (a). Similarly, NPN transistor is composed of two N-type semiconductors separated by a thin section of P-type as shown in Figure (a). the symbol used for PNP and NPN transistors are also shown with the diagrams.

Basically, transistor has three portions known as emitter, base and collector. The portion on one side is the emitter and the portion on the opposite side is the collector. The middle portion is called the base and forms two junctions between the emitter and collector.

EMITTER

The portion on one side of transistor that supplies charge carriers (i.e. electrons or holes) to the other two portions. The emitter is a heavily doped region. The emitter is always forward biased with respect to base so that it can supply a large number of majority carriers. In both PNP and NPN transistors emitter base junction always should be forward biased. Emitter of PNP transistor supplies hole charges to its junctions with the base. Similarly, the emitter of PNP transistor supplies free electrons to its junction with the base.

COLLECTOR

The portion on the other side of the transistor (i.e. the side opposite to the emitter) that collects the charge carriers (i.e. electrons or holes). The collector is always larger than the emitter and base of a transistor. The doping level of the collector is in between the heavily doping of emitter and the light doping of the base. In both PNP and NPN transistors the collector base junction always should be reverse biased. It function is to remove charge carriers from junction with the base. Collector of PNP transistor receives hole charges that flow in the output circuit. Similarly, the collector of NPN transistor receives electrons.

BASE

The middle potion which forms two PN junctions between the emitter and the collector is called the base. The base of transistor is thin, as compared to the emitter and is a lightly doped portion. The function of base is to control the flow of charge carrier. The emitter junctions forward biased, allowing low resistance emitter circuit. The base collector junction is reverse biased and showing high resistance in the collector circuit.

TRANSISTOR CONSTRUCTION

The techniques used for manufacturing transistor are given as below:

1. Grown Junction

2. Alloy or Fused Junction

3. Diffused Junction

4. Epitaxial Junction

5. Point Contact Junction

Grown Junction

This junction is prepared by using either the Czochralski or floating zone technique. The apparatus used for Czochralski technique is shown in the below Figure. It consists of a graphite crucible, a quartz container, a rotating pulling rod and the induction heating coils placed around the graphite crucible. The graphite crucible contains the molten semiconductor material.

First of all, a single semiconductor seed is immersed in the molten semiconductor. Then it is gradually withdrawn, while the rod holding the seed is slowly rotating. The PN junctions are grown by first adding P-type impurities to the melt and then changing it to N-type.

Alloy or Fused Junction

The alloy junction technique produces PN junctions, which have high (PIV) peak-in-voltage and current ratings. Such junctions have large capacitance, due to their large junction area. In the alloy junction technique, a small dot of aluminum is placed on the N-type silicon water as shown in Figure (d). It is ten heated to temperature of about 150C. At this temperature, the aluminum melts and dissolves some of the silicon. Then its temperature is lowered and silicon refreezes to form a single crystal having a PN junction as shown in Figure.

Diffused Junction

This technique gives us a precise control of impurity concentration for manufacturing PN junction. N-type silicon wafer called substrate (or base) is exposed to a gaseous impurity of P-type as shown in Figure (e). Then the wafer is heated to a sufficiently high temperature at which the impurities diffuse slowly into the surface of the water. After the diffusion, portions of the surface are protected and the rest are etched out as shown in Figure.

Epitaxial Junction

This junction differs from diffused junction only in one aspect that the junction is fabricated not on the substrate but on the epitaxial layer grown, above the substrate. The epitaxial junctions have an advantage of low resistance.

Point Contact Junction

It consists of an N-type semiconductor (silicon, or germanium) wafer, whose one face is soldered to a metallic base and the other face has a phosphor bronze (or tungsten) spring (called Cats whisker) pressed against it as shown in Figure (f). The whole assembly is encapsulated in a ceramic or glass envelops to give it mechanical strength.

The PN junction is formed by passing a large current (about 200 mA) for 1 to 100 millisecond duration. The junction is formed at the contact point because of the melting of silicon surface and diffusion of the whisker material into the surface at that point as shown in Figure (f).

The point contact junction has very low value of the capacitance. Because of this, such junctions are very useful for operation at the frequencies as high as 10 GHz.

BJT Modes of Operation

There are two junctions in bipolar junction transistor. Each junction can be forward or reverse biased independently. Thus there are four modes of operations:

1. Forward Active

2. Cut off

3. Saturation

4. Reverse active

FORWARD ACTIVE

In this mode of operation, emitter-base junction is forward biased and collector base junction is reverse biased. Transistor behaves as a source. With controlled source characteristics the BJT can be used as an amplifier and in analog circuits.

CUTT OFF

When both junctions are reverse biased it is called cut off mode. In this situation there is nearly zero current and transistor behaves as an open switch.

SATURATION

In saturation mode both junctions are forward biased large collector current flows with a small voltage across collector base junction. Transistor behaves as an closed switch.

REVERSE ACTIVE

It is opposite to forward active mode because in this emitter base junction is reverse biased and collector base junction is forward biased. It is called inverted mode. It is no suitable for amplification.However the reverse active mode has application in digital circuits and certain analog switching circuits.

FET & JFET

FET stands for "Field Effect Transistor" it is a three terminal uni polar solid state device in which current is control by an electric field.

FET can be fabricated with either N- Channel or P- Channel, for the fabrication of N-Channel JFET first a narrow bar of N-type of semiconductor material is taken and then two P-Type junction are defused on opposite sides of it's middle part, called channel. The two regions are internally connected to each other with a signal lead, which is called Gate terminal. One lead is called Source terminal and the other is called Drain terminal.Construction of FET

P-Channel JFET is similarly is constructed except that it use P- type of bar and two N- types of junctions.

Source:-It is the terminal through which majority carriers are entered in the bar, so it is called Source.

Drain:-It is the terminal through which the majority carriers leads the bar, so it is called the drain terminal.

Gate:-These are two terminals which are internally connected with each other and heavily doped regions which form two PN-Junctions.

Working / Operation FET or JFET

Gate are always in reverse biased, hence the gate current IG is practically zero. The source terminal is always connected to end of the drain supply, which provides the necessary chares carrier, in N- Channel JFET Source terminal is connected to the negative end of the drain voltage source. The electrons flow from source to drain through the channel from D to S is started,

the current ID increases as VDS is increased from zero on ward. This relation ship between VDS and ID continuous till VDS reaches certain value called "Pinch OFF" VPO.

When VDS is equal to zero and VGS is decreased from zero, the gate reverse bias increases the thinks of the region, as the negative value of the VGS is increase a stage cones when the two dip lections regions touch each other, in this conduction the channel is said to be Cut OFF.

JFET as Amplifier

One of the application of the JFET is an Amplifier, it amplified the weak signal connected in the Gate terminal , the input is always reversed biased, a small change in the reverse bias on the gate produce large change in the drain current, this fact make JFET capable of amplifing the weak signals

Working / Operation

When negative signal is applied at in put of the amplifier, the gate bias is increase, duplication layer is decrease, Channel resistance is increase, ID is decreased, Drop across Load Resistor is decreases, and the positive signal is present at output through C2.When the positive signal is applied at the input the action will be the wise versaThis seen that there is phase inveration between the input signal at the gate and the output signal at the drain.

Application of JFET

JFET is used at large scale in amplifiers circuits, analog switches; it is also used in AGC system, voltage regulators, buffer amplifiers.FET & BJT ON THE BASIS OF APPLICATIONSBipolar Junction TransistorField Effect Transistor

1) A bipolar transistor requires a small amount of current flowing to keep the transistor on. While the current for one transistor may be negligible, it adds up when millions are switching simultaneously. The heat dissipated on bipolar limits the total number of transistors that can be built on the chip2) A BJT will consume more power in the on- state.it cannot switch with less than a 0.3V voltage drop.

3) BJTs function as regulators of currents as small current is regulating a large current.

4) The bipolar transistor is liable for thermal runway(over heating) due to a negative temperature co-efficient.

5) BJTs are preferred for low current applications.

6) BJTs have low-medium input impedance(~1k -3k ohms).

7) BJTs are used where we need high gain & fast response.

8) BJT's have a higher cutoff frequency and a higher maximum current then FET's.

9) To operate BJTs at high switching frequencies & high current, we have to prevent the devices from going into haerd saturation as this will increase storage times( making it difficult to switch off quickly) but then cause the device to dissipate more power due to higher Vce-sat.

10) BJTs are relatively greater in size than FET of same rating.11) BJT is temperature sensitive at higher level.12) BJTs have high switching speed but they are noisy also.

13) BJT Have small duty cycles.

1) Once the gate terminal on an FET has been charged, no more current is needed to keep that transistor on (closed) for the duration of time required.2) FETs are preferred in Wide line or load variations & have low power consumption.

3) FET function as voltage regulators as applied voltage on gate control the output characteristics.

4) FET have a positive temperature co-efficient, stopping thermal runway.

5) FETs are preferred in Low-voltage applications ( less than 250V).6) no current flows through the gate, the input impedance of the FET is extremely large (in the range of 1010,1016).the large input impedance of the FET makes them an excellent choice for amplifier inputs.7) FETs have low-medium gain.8) FETs are preferred in High frequency applications ( greater than 200kHz).9) FETs are low switching devices.FET is therefore used for power switch design and high power functions ( less than 500W output power).

10) FET are smaller in size.so area consumption of FET is less so Ics made by FETs provide higher packing density as compared to BJTs.

11) FET is more stable to temperature & therefore it is used in high temperature applications.

12) As FET introduce lower noise level to the system so where sensitivity to the noise is very critcal and cannot be neglected,JFET amplifiers are preferred over BJT.

13) JFET used for micro wave communication such as VHF, UHF receivers.14) FET's are easy to fabricate in large scale and have higher element density the BJT's.15) JFET is mostly used in digital circuits.16) Switch mode power supplies (SMPS): Hard switching above 200kHz 17) Switch mode power supplies (SMPS): ZVS below 1000 watts

Gunn diodesGunn diodes are also known as transferred electron devices, TED, are widely used in microwave RF applications for frequencies between 1 and 100 GHz.

The Gunn diode is most commonly used for generating microwave RF signals - these circuits may also be called a transferred electron oscillator or TEO. The Gunn diode may also be used for an amplifier in what may be known as a transferred electron amplifier or TEA.

As Gunn diodes are easy to use, they form a relatively low cost method for generating microwave RF signals.

Gunn diode basics

The Gunn diode is a unique component - even though it is called a diode, it does not contain a PN diode junction. The Gunn diode or transferred electron device can be termed a diode because it does have two electrodes. It depends upon the bulk material properties rather than that of a PN junction. The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.

The mechanism behind the transferred electron effect was first published by Ridley and Watkins in a paper in 1961. Further work was published by Hilsum in 1962, and then in 1963 John Battiscombe (J. B.) Gunn independently observed the first transferred electron oscillation using Gallium Arsenide, GaAs semiconductor.

Gunn diode symbol for circuit diagrams

The Gunn diode symbol used in circuit diagrams varies. Often a standard diode is seen in the diagram, however this form of Gunn diode symbol does not indicate the fact that the Gunn diode is not a PN junction. Instead another symbol showing two filled in triangles with points touching is used as shown below.

Gunn diode symbolGunn diode construction

Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are gallium Arsenide, GaAs and Indium Phosphide, InP. However other materials including Ge, CdTe, InAs, InSb, ZnSe and others have been used. The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not holes found in a p-type material.

Within the device there are three main areas, which can be roughly termed the top, middle and bottom areas.

A discrete Gunn diode with the active layer mountedonto a heatsink for efficient heat transferThe most common method of manufacturing a Gunn diode is to grow and epitaxial layer on a degenerate n+ substrate. The active region is between a few microns and a few hundred micron thick. This active layer has a doping level between 1014cm-3and 1016cm-3- this is considerably less than that used for the top and bottom areas of the device. The thickness will vary according to the frequency required.

The top n+ layer can be deposited epitaxially or doped using ion implantation. Both top and bottom areas of the device are heavily doped to give n+ material. This provides the required high conductivity areas that are needed for the connections to the device.

Devices are normally mounted on a conducting base to which a wire connection is made. The base also acts as a heat sink which is critical for the removal of heat. The connection to the other terminal of the diode is made via a gold connection deposited onto the top surface. Gold is required because of its relative stability and high conductivity.

During manufacture there are a number of mandatory requirements for the devices to be successful - the material must be defect free and it must also have a very uniform level of doping.Gunn diode operation basics

The operation of the Gunn diode can be explained in basic terms. When a voltage is placed across the device, most of the voltage appears across the inner active region. As this is particularly thin this means that the voltage gradient that exists in this region is exceedingly high.

The device exhibits a negative resistance region on its V/I curve as seen below. This negative resistance area enables the Gunn diode to amplify signals. This can be used both in amplifiers and oscillators. However Gunn diode oscillators are the most commonly found.

Gunn diode characteristicThis negative resistance region means that the current flow in diode increases in the negative resistance region when the voltage falls - the inverse of the normal effect in any other positive resistance element. This phase reversal enables the Gunn diode to act as an amplifier and oscillator.

Gunn diode operation at microwave frequencies

At microwave frequencies, it is found that the dynamic action of the diode incorporates elements resulting from the thickness of the active region. When the voltage across the active region reaches a certain point a current is initiated and travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created.

It can be seen that the time taken for the current pulse to traverse the active region largely determines the rate at which current pulses are generated, and hence it determines the frequency of operation.

To see how this occurs, it is necessary to look at the electron concentration across the active region. Under normal conditions the concentration of free electrons would be the same regardless of the distance across the active diode region. However a small perturbation may occur resulting from noise from the current flow, or even external noise - this form of noise will always be present and acts as the seed for the oscillation. This grows as it passes across the active region of the Gunn diode.

Gunn diode operationThe increase in free electrons in one area cause the free electrons in another area to decrease forming a form of wave. It also results in a higher field for the electrons in this region. This higher field slows down these electrons relative to the remainder. As a result the region of excess electrons will grow because the electrons in the trailing path arrive with a higher velocity. Similarly the area depleted of electrons will also grow because the electrons slightly ahead of the area with excess electrons can move faster. In this way, more electrons enter the region of excess making it larger, and more electrons leave the depleted region because they too can move faster. In this way the perturbation increases.

Gunn diode operation - electrons in the peak move more slowlyThe peak will traverse across the diode under the action of the potential across the diode, and growing as it traverses the diode as a result of the negative resistance.

A clue to the reason for this unusual action can be seen if the voltage and current curves are plotted for a normal diode and a Gunn diode. For a normal diode the current increases with voltage, although the relationship is not linear. On the other hand the current for a Gunn diode starts to increase, and once a certain voltage has been reached, it starts to fall before rising again. The region where it falls is known as a negative resistance region, and this is the reason why it oscillates.

IMPATT Microwave Diode

IMPATT diode , IMPact ionisation Avalanche Transit Time microwave diode is used for many microwave RF applications where it is one of the highest power microwave diodes.IMPATT diode theory basics

Like any other diode, an IMPATT has a relatively standard IV characteristic. In the forward direction it will conduct after it has reached the forward conduction point. In the reverse direction it will block current.

However at a certain voltage the diode will break down and current will flow in the reverse direction.

Graphical representation of the IMPATT diode IV characteristic The IMPATT diode is operated under reverse bias conditions. These are set so that avalanche breakdown occurs. This occurs in the region very close to the P+ (i.e. heavily doped P region). The electric field at the p-n junction is very high because the voltage appears across a very narrow gap creating a high potential gradient. Under these circumstances any carriers are accelerated very quickly.

As a result they collide with the crystal lattice and free other carriers. These newly freed carriers are similarly accelerated and collide with the crystal lattice freeing more carriers. This process gives rise to what is termed avalanche breakdown as the number of carriers multiplies very quickly. For this type of breakdown only occurs when a certain voltage is applied to the junction. Below this the potential does not accelerate the carriers sufficiently.

In terms of its operation the IMPATT diode can be considered to consist of two areas, namely the avalanche region or injection region, and secondly the drift region.

These two areas provide different functions. The avalanche or injection region creates the carriers which may be either holes of electrons, and the drift region is where the carriers move across the diode taking a certain amount of time dependent upon its thickness.

The two types of carrier drift in opposite directions.

Charge carrier movement within an IMPATT diode IMPATT diode operation

Once the carriers have been generated the device relies on negative resistance to generate and sustain an oscillation. The effect does not occur in the device at DC, but instead, here it is an AC effect that is brought about by phase differences that are seen at the frequency of operation. When an AC signal is applied the current peaks are found to be 180 out of phase with the voltage. This results from two delays which occur in the device: injection delay, and a transit time delay as the current carriers migrate or drift across the device.

IMPATT diode voltage & current waveforms The voltage applied to the IMPATT diode has a mean value where it is on the verge of avalanche breakdown. The voltage varies as a sine wave, but the generation of carriers does not occur in unison with the voltage variations. It might be expected that it would occur at the peak voltage. This arises because the generation of carriers is not only a function of the electric field but also the number of carriers already in existence.

As the electric field increases so does the number of carriers. Then even after the field has reached its peak the number of carriers still continues to grow as a result of the number of carriers already in existence. This continues until the field falls to below a critical value when the number of carriers starts to fall. As a result of this effect there is a phase lag so that the current is about 90 behind the voltage. This is known as the injection phase delay.

When the electrons move across the N+ region an external current is seen, and this occurs in peaks, resulting in a repetitive waveform.

IMPATT circuits

IMPATT diodes are generally used at frequencies above around 3 GHz. It is found that when a tuned circuit is applied along with a voltage around the breakdown voltage to the IMPATT, and oscillation will occur.

Compared to other devices that use negative resistance and are available for operation at these frequencies, the IMPATT is able to produce much higher levels of power. Typically figures of ten or more watts may be obtained, dependent upon the device.

One of the main drawbacks of the IMPATT diode in its operation is the generation of high levels of phase noise as a result of the avalanche breakdown mechanism. It is found the devices based around Gallium Arsenide technology are much better than those using Silicon. This results from the much closer ionisation coefficients for holes and electrons.

In many respects, the IMPATT diode structure is very similar to that of many other forms of diode, and in particular the standard PN junction diode or the Schottky diode.

However the differences in its construction and fabrication mean that it is able to operate in its avalanche mode whether the transit time provides the negative resistance.

IMPATT diode construction

There is a variety of structures that are used for the IMPATT diode. All are variations of a basic PN junction and usually there is an intrinsic layer, i.e. a layer without any doping that is placed between the P type and N type regions.

The structures use a PN junction which is reverse biased so that avalanche multiplication occurs within the high field region. In most structures a Schottky barrier can be used as the injecting junction.

The most common method of fabricating an IMPATT diode is to use a vertical structure where there is vertical current flow.

.

IMPATT diode vertical structure For this format of diode, the layers are generally grown epitaxially. Where very high frequency devices are to be made layers can become very thin. For these layers, techniques including MBE, molecular beam epitaxy, or MOCVD, metallo-organic chemical vapour deposition can be used.

For a typical Read diode the n-layer may be only 1 to 2 m thick, and the intrinsic layer may be between 2 and 20m thick. For very high frequency operation, these dimensions are reduced.

The dopants needed for the different layers may be introduced using one of a number of techniques including diffusion, ion implantation or even in-situ doping during the epitaxial growth process for a given layer.

Apart from the vertical or mesa fabrication, a horizontal structure may also be used using more traditional planar technology.

IMPATT diode horizontal structure Packaging

The devices are normally used as microwave power sources and as a result, heat dissipation is a key issue. As a result the devices are mounted into packages where the heat can be transferred away from the active areas of the devices as fast as possible. To this end, the devices are often mounted in what may be termed an upside down fashion where the active layers are closest to the heat sinking provided by the package.

Often the package is coaxial in format so that the correct transmission line properties are presented to the RF signal which may be at many tens of GHz. As a result the package is often quite intricate and accordingly very expensive, especially when very high frequencies are used.

The most commonly used materials for IMPATT devices are Silicon and Gallium Arsenide, but other materials including Germanium, and Indium Phosphide or Gallium Aluminium Arsenide may also be used.

Electric field profiles

One major element of the structure of an IMPATT diode is the way in which the electric field profile occurs. The diagram below shows the electric field profile an also the areas of highest electric field represented by the grey areas show where the avalanche breakdown occurs.

Electric field profiles for common IMPATT diode structures The diagram shows the main types of avalanche diode. The p+ n, i n+ diode (leftmost on the diagram) is the Read diode and the rightmost diode structure, p+ i n+ is also known as the Misawa diode.

Applications

IMPATT diodes are ideal where small cost effective microwave radio sources are needed. The main drawback of generators using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process that is key to their operation. Nevertheless these diodes make excellent signal sources for many RF microwave applications.

Typically the device is used in a number of applications including:

Alarms

Radar

Detectors using RF technology

Discovery & development

The original idea for the diode was put forward by Shockley in 1954. He thought of the idea of creating negative resistance using a transit time delay mechanism. The method of injection for the carriers was a forward biased PN junction. He published this in the Bell Systems Technical Journal in 1954 in an item entitled: 'Negative resistance arising from transit time in semiconductor diodes.'

However it was not until 1958 that W.T. Read of Bell Laboratories proposed the p+ n i n+ diode structure which was later called the Read diode. This diode used the avalanche multiplication as the injection mechanism. Again this was published in the Bell Systems Technical Journal in 1958 under the title: A proposed high-frequency, negative resistance diode.'

Although the injection mechanism and diode had been postulated, it was not until 1965 that the first practical operating diodes were made that enabled oscillations to be observed. The diode used for this demonstration was fabricated using silicon and had a p+ n structure.

After this, operation of the Read diode was demonstrated and then in 1966 a p i n diode was also demonstrated to work.

IMPATT basics

In many respects the IMPATT diode is an unusual diode in that it is able to provide high power RF signals at microwave frequencies using a structure that is not that far different from the basic PN junction. However it has been developed to enable its different mode of operation to be utilised properly.

Theory &operation: The IMPATT diode relies upon a negative resistance effect caused by the transit time of the carriers. This negative resistance enables the diode to act as an oscillator, creating signals at microwave frequencies. Fabrication & structure: There are a number of structures and fabrication methods used for IMPATT diodes. Each one has its own advantages and disadvantages

Practical operation

The main application for IMPATT diodes is in microwave generators. An alternating signal is generated simply by applying a DC supply when a suitable tuned circuit is applied.

The output is reliable and relatively high when compared to other forms of microwave diode. In view of its high levels of phase noise it is used in transmitters more frequently than as a local oscillator in receivers where the phase noise performance is generally more important. It is also used in applications where phase noise performance is unlikely to be of importance.

To run an IMPATT diode, a relatively high voltage, often as high as 70 volts or higher may be required. This often limits their application as voltages of this order are not always easy to use in some pieces of equipment. Nevertheless IMPATT diodes are particularly attractive option for microwave diodes for many areas.

M-type microwave device with slanted field emitterMicrowave tubes can be broadly classifies into two categories

1.O-TYPE Linear Tubes(Travelling tube amplifiers,Klystrons)In O-Type tube , a magnetic field whose axis coincides with the electron beam is used to hold the beam togetheras it travels the length of the tube 2.M-TYPE Tubes(Magnetrons and cross field devices)

This is just a rough classification of the microwave tubes. Basically there are only main two types of microwave tubes

1. Tubes with electromagnetic cavity(klystrons and magnetrons)2. Tubes with slow wave circuits(traveling wave tubes)The present invention relates to M-type microwave devices and is aimed to improve effectiveness of using a working surface of field-electron emitters, to improve their reliability while increasing stability of field emission and service life of the device. These objects are solved in the design of a M-type microwave device, comprising an anode encircling a cylindrical evacuated cavity and a cathode assembly disposed co-axially inside the anode, said cathode assembly comprising a cylindrical rod with its surfaces having elements in the form of planar (film) field-electron emitters and secondary-electron emitters that provide a primary and a secondary electron emission, respectively. In doing so, the normal to planar field-electron emitters is not parallel and makes therewith an angle of more than 0 degrees. An end-face of the field-electron emitter is protected by a tunnel-thin dielectric layer containing impurities of various materials and materials having a low work function.

Description

RELATED APPLICATION

The present application claims priority of Russian Application Ser. No. 98/10/0560, filed on Jan. 5, 1999, entitled M-TYPE MICROWAVE DEVICE, which turn claims priority from Russian Application Ser. No. 98/10/0569 filed Jan. 5, 1999, entitled MAGNETRON, the disclosures of which are incorporated by reference herein in their entirety.

1. Field of the Invention

The present invention relates generally to the field of electronics and, more particularly, to vacuum electronic devices intended to generate microwave electromagnetic radiation using an electron-transit time, namely to devices known as M-type microwave devices.

More specifically, the present invention relates to structural elements of such devices, namely to cathodes requiring no preliminary incandescence to perform electronic emission.

2. Background of the Invention

In the M-type microwave devices, there are widely used cathodes (which, due to complexity of their structure, would be more accurately identified as cathode assemblies), which make use of a combination of secondary electron emission caused by return to a cathode of a part of electrons traveling in the inter-electrode space along epicycloids, as well as ion bombardment with respect to the cathode, and field emission, that is the phenomenon of electron ejection from a conductor surface under the action of a fairly strong electric field, with the latter emission initiating and maintaining said secondary electron emission.

Methods of improving secondary-emission properties of the cathode are generally known and include fabrication thereof (or its surface coating) from materials such as oxides, in particular oxides of thorium, etc.

A required quantity of field emission is primarily afforded by the shape of corresponding elements and selection of their material, which governs operation of the electron release from a given material into vacuum. Among other things, planar elements (films) having microscopic points (roughness, unevenness) on their lateral surfaces are used as a field-electron emitter. So, the use of such field-emitter located on a focusing flange of the device is described in USSR Inventor's Certificate No. 320,852 granted Nov. 4, 1971 to L. G. Nekrasov et al., for Cathode For M-Type Microwave Devices, Int. Cl. H01J 1/32.

Location of field-electron emitters made in the form of washers along a cathode assembly rod is described in RU Patent No. 2,040,821 granted Jul. 27, 1995 to V. I. Makhov et al., for M-Type Microwave Device, Int. Cl. H01J 1/30. The RU Patent No. 821 is the closest prior art with respect to the present invention.

A need for improving effectiveness of using a working surface of field-electron emitters is still popular in the state of the art, since a field-emission current value is proportional to an emitting area of the field-electron emitter. In view of the fact that a magnetron anode constitutes a cylindrical surface cut by cavity slots, a primary current of the magnetron is dependent upon the location of field-electron emitters relative to an anode cylindrical part having a minimum distance to a working surface of the field-electron emitter.

The increase in primary current to a required value is possible by two ways: either by decreasing a film thickness of the field-electron emitter, resulting in the stepping-up of an electric-field intensity near the surface of an emitter end-face, or by the second wayat the expense of increasing an area participating in the emission, by enlarging a number of field-electron emitters. In doing so, the first way is characterized by augmentation of an effect exerted by electromechanical forces on a field-emission cathode, resulting in the decrease in its mechanical reliability and degradation of its volt-ampere characteristics, whereas the second way is characterized by the fact that a cathode structure of the magnetron becomes more complex, less adaptable to efficient manufacture and less reliable.

SUMMARY OF THE INVENTION

The principal objects of the present invention are: to improve effectiveness of using a working surface of the field-electron emitters; to improve their reliability while increasing stability of field emission and service life of a M-type microwave device, comprising an anode and a cathode having a cylindrical rod with field-electron emitters located on its surface and fabricated as planar discs, and secondary-electron emitters located in the plane perpendicular to a cathode axis, the said emitters providing a primary and secondary emission, respectively.

In accordance with the present invention, these objects are achieved in the arrangement of a M-type microwave device, comprising an anode encircling a cylindrical evacuated cavity and a cathode assembly disposed inside the anode, said cathode assembly comprising a cylindrical rod which is co-axial with the anode, a field-electron emitter made in the form of one or several planar elements mechanically and electrically connected to the cylindrical rod and extending therefrom with a working end-face towards the anode, and a secondary-electron emitter made in the form of one or several sections having an increased secondary electron-emission coefficient, said sections being located on the cylindrical rod surface, the above objects are solved when locating said planar elements such that the normal thereto makes an angle of more than 0 degrees with an axis of the cylindrical rod.

In a preferred embodiment of the present invention, a field-electron emitter in the form of a planar element is located at an angle of more than 5 degrees with respect to a radial plane which is perpendicular to the cylindrical rod axis.

In another preferred embodiment of the present invention, the field-electron emitter in the form of a planar element is located on a spiral path having an axis extending in register with the cylindrical rod axis.

In still another preferred embodiment of the present invention, the field-electron emitter in the form of a planar element is located such that the normal to the surface of said field-electron emitter is perpendicular to the cathode axis. In other words, the planar element surface is located in the plane parallel with an axis passing through the cylindrical rod axis.

According to the present invention, planar elements constituting the field-electron emitter may be isolated with a vacuum gap from those regions (cylindrical rod coatings) which constitute a secondary-electron emitter.

In the preferred embodiments of the present invention, material of field-electron emitters may include impurities of electropositive materials, or impurities of material of the same kind, or both simultaneously, where impurities of material of the same kind are advantageously located at a depth greater than that of the electropositive material.

It is also preferred that a working end-face of said field-electron emitter be fabricated from an amorphous material.

For a number of practical applications, a planar element constituting the field-electron emitter may have cavities in which a film of electropositive material is received. It may be also fabricated with its end-face in the form of a multilayer metal-insulator-metal structure, with each layer having a depth of 2-10 nm.

The field-electron emitter may be fabricated from either tungsten, molybdenum, tantalum, niobium, titanium, or hafnium silicides. It may be also fabricated from amorphous conducting metals and carbide-based alloy, including impurities of electropositive materials.

It is preferred that the working end-faces of planar elements of field-electron emitters be coated with a tunnel-thin dielectric layer also containing impurities of electropositive materials.

Essential distinctions of the proposed M-type microwave device consist in the presence of elements affording primary emission, the elements being disposed on the surfaces the normal to which is not parallel with the cathode axis and makes therewith an angle of more than 0 degrees.

This distinctive feature gives rise to the solution of objectives in accordance with the present invention. In doing so, a primary current increase is attained at the expense of more efficient usage of the working surface of field-electron emitters, since, in accordance with the present design, emission occurs from the larger surface of the emitter.

An additional advantage of the present invention consists in a device simplification at the expense of possibility to reduce a number of field-electron emitters used.

The third advantage of the present invention consists in the stepping down of operating voltage of the device, which makes it possible to expand types of devices used and structural capabilities of field-electron emitters and to employ a wider range of materials and alloys providing stability of volt-ampere characteristics and an extended service life of the devices.

Additional objects and advantages of the present invention will be set forth in the detailed description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Single conductor multi-coil multi-beam microwave device O

A traveling wave tube is provided with a low-wave, electron beam interaction means in the form of a single wire conductor coiled into at least two parallel series of aligned turns; in each series, the turns are of identical size and configuration. In one form, all the turns have rectilinear stretches of equal length that are in a common plane; an electron beam source, a collector, a sole electrode and electric and magnetic fields direct a sheet beam alongside the rectilinear stretches outside the turns. In another form, the turns of the single wire conductor are almost entirely circular and in figure-eight configuration viewed endwise; electron beams are directed through either one or both series of turns. In still other forms of the invention, the slow-wave means viewed endwise has more than two turns and the axes are coplanar or in a circular distribution.

OUTPUT SINGLE CONDUCTOR MULTI-COIL MULTI-BEAM MICROWAVE DEVICE BACKGROUND OF THE INVENTION The most common form of slow wave means for a traveling wave tube is the wire conductor coil of identical aligned turns with a constant pitch, though coils may be designed with a pitch that changes along its length to compensate for slowing of the electrons in the beam as they give up energy. The geometry of the coil as seen endwise is related to whether the traveling wave tube is designed for O-type linear operation or for M- type crossed field operation. For O-type operation, the geometry of the slow-wave means, viewed endwise is a circle; for M-type operation, the geometry of the slowwave means viewed endwise is a loop that includes a rectilinear stretch. Regardless of the type of operation, the coil has been formed as a single series of aligned loops. In O-type operation, the electron beam is projected through the wire helix and the focusing field resists spreading of the beam eletrons laterally beyond the interaction space. In crossed field operation, a high current beam is projected and then redirected by a perpendicular magnetic field to a course that runs between a sole electrode and the coplanar rectilinear stretches of the coil for wave-beam interaction proximate to the rectilinear stretches of the coil. If the traveling wave tube is designed for as much power amplification as is practical, the beam voltage and beam current are made as high as is feasible, the focusing current is high, the wire'size of the coil must be large enough to carry the amplified RF and the collector must be designed for very high heat dissipation and the electron beam source must be designed to emit the high density beam.

Phase velocity of a propagated RF wave is related to the helix design i.e. the helix diameter and pitch. In the interaction space, the electron beam must have a velocity somewhat greater than the phase velocity to transfer energy to the RF wave. The electron beam accelerating voltage level is set to impart the correct velocity to the beam electrons for interaction and energy transfer to the RF wave. Beam current may be adjusted but the DC focusing field needs to be adjusted too for the resultant change in beam current density. If the tube needs to be operated at much higher power level during one time interval compared to that during another time interval, the tube needs to be designed to operate with the higher beam current though it may be operated at the lower beam current most of the time. Also, a traveling wave tube amplifier may have to operate in continuous-wave and pulse modes sequentially. Each time mode is switched, the power level at which the tube operates needs to be switched; in order that there be no loss in data which would occur during switching, the shift in power level needs to be essentially instantaneous. However, the DC focusing field and the beam voltage cannot be switched between widely different levels of operation at a rate fast enough for widely different levels of dual power operation. One problem is that the beam current does not continue in close proximity to the helical structure during and immediately following switching; there is degradation in gain and efficiency. It is well known that a 2:1- change in power level is about the maximum that can be tolerated with out adversely affecting efficiency to an unacceptable degree.

If two traveling wave tubes are connected in parallel to meet the above requirements, not only is the cost high but sophisticated phasing techniques are needed particularly for operation over a wide frequency band. If a traveling wave tube is made with two slow-wave helices connected in parallel and with two electron beam sources that can be energized singly or in combination, sophisticated phasing techniques are still required; additionally the input RF energy divides between the two paralleLmounted helices, a 3db drop prior to amplification.

SUMMARY OF THE INVENTION Instead of providing a traveling wave tube with a slow-wave coil of one continuous wire conductor having identical turns that are coaxial, this invention provides the traveling wave tube with a slow-wave multicoil of one continuous wire conductor that forms one complete element of the multi-coil by looping alternately clockwise and counterclockwise crossing over itself at least once to form two or more turns side-byside, e.g. a figure-eight or a chain of three or more turns side-by-side; the looping repeats to form a series of identical such elements in line and with a predetermined pitch. The geometry of the novel multi-coil viewed endwise may be a plurality of almost circular loops; the circles are incomplete to a minor extent at the crossovers. A multi-coil with circular loops may be operated with one beam projected through one of the sub-coils or with a plurality of beams up to the number of sub-coils projected through the respective sub-coils. For crossed field operation, the loops of the novel coil include rectilinear stretches that are of equal length and coplanar. The multi-coil with the rectilinear stretches is used with a single beam source.

Terrestrial communication system using satellite transmission techniques

A terrestrial communication system, facilitating audio, video, data, and any other type of communication within a local geographical area, and with an extremely large number of communication channels being made available simultaneously at a very low cost. It comprises at least two local terrestrial satellite (LTS), preferably located in a mast or any other supporting structure, each LTS having high frequency communication equipment substantially corresponding to a conventional geostationary satellite for digital transmission of video, audio, or data, arranged to transmit in the L Band (1-2 GHz), the S Band (2-4 GHz), or a high frequency band with a relatively low power output and having a preferably omnidirectional antenna installation for transmission in a substantially horizontal plane. The signals can be digitalized according to MPEG-2 or according to any known signal algorithm.

Description

TECHNICAL FIELD

The present invention relates to a terrestrial communication system, facilitating audio, video, data, and any other type of communication within a local geographical area, and with an extremely large number of communication channels being made available simultaneously at a very low cost.

BACKGROUND ART

Audio, video, and data communication are rapidly increasing sectors of interest, and existing terrestrial communication circuits, radio communication channels and satellite communication systems are used extensively. Dedicated cable systems are also being used to cover local geographical areas with a large number of TV-channels, and these cable systems may also be used for data transmission purposes.

Digital transmission techniques have been developed, and as a result, digital consumer TV transmissions are now available from satellite transponders, and due to the digital techniques utilized, the number of programs transmitted by each transponder is no longer one program only, since several programs can be transmitted simultaneously by each transponder.

However, with regard to terrestrial transmissions techniques, a suitable system has not yet been developed, but it is hoped that such a dedicated system will be made available within the next few years. This will require development of suitable encoders and decoders, specifically developed for terrestrial digital transmission techniques and frequency bands today used for terrestrial TV transmission purposes. Considerable efforts and large sums of money have been invested in the development of suitable techniques, but so far, these efforts have not been successful.

DISCLOSURE OF THE INVENTION

The present invention is based on the discovery that present satellite transmission techniques also can be used for terrestrial transmission.

As a result, a user having equipment intended for reception of digital satellite signals can also use the very same equipment for reception of terrestrial transmissions. This is achieved by arranging communication equipment of the type used in satellites as Local Terrestrial Satellites (LTS) in a mast or other suitable foundation. Transmission from a LTS is based on one of the conventional standards for digital transmission as today used by conventional orbiting geostationary satellites, e.g., MPEG-2 or MPEG-1 or any other standard suitable for audio, video or data transmission purposes. A preferred standard today is frequency modulated MPEG-2 (also compatible with MPEG-1), but other known or future standards may also be used, and amplitude modulated (AM) transmission techniques can also be used as an alternative to frequency modulated (FM) transmission techniques. Each LTS is transmitting using relatively low output power and with an antenna arrangement preferably having none or little directional effect. As a result, a suitable number of LTS can be arranged to cover a local geographical area, giving the inhabitants access to individual communication channels for data, video (TV), and audio communication. The advantages of such a system will be more fully discussed later.

Microwave Tube Devices Klystron

klystron

Atypeofvacuumtubeusedasanamplifierand/oroscillatorforUHFandmicrowavesignals.Itistypicallyusedasahigh-powerfrequencysourceinsuchapplicationsasparticleaccelerators, UHFTVtransmissionandsatelliteearthstations.TheklystronwasinventedatStanfordUniversityin1937andoriginallyusedastheoscillatorinradarreceiversduringWorldWarII.

Aklystrontubemakesuseofspeed-controlledstreamsofelectronsthatpass througha resonatingcavity.Electronsinaklystronareacceleratedtoacontrolledspeedbytheapplicationofseveralhundredvolts.Astheelectronsleavetheheatedcathodeofthetube,theyaredirectedthroughanarrowgapintoaresonatingchamber,wheretheyareacteduponbyanRFsignal.Theelectronsbunchtogetherandaredirectedintooneormoreadditionalchambersthataretunedatornearthetube'soperatingfrequency.StrongRFfieldsareinducedinthechambersastheelectronbunchesgiveupenergy.Thesefieldsareultimatelycollectedattheoutputresonatingchamber.Seemagnetronanddiode.

Anevacuatedelectron- beamtubeinwhichaninitialvelocitymodulationimpartedtoelectrons inthebeamresultssubsequentlyindensitymodulationofthebeam;usedasanamplifierinthemicrowaveregionorasanoscillator.

Klystron

Anevacuatedelectron-beamtubeinwhichaninitialvelocitymodulationimpartedtoelectrons inthebeamresultssubsequentlyindensitymodulationofthebeam.Aklystronisusedeitherasanamplifierinthemicrowaveregionorasanoscillator.

Foruseasanamplifier,aklystronreceivesmicrowaveenergyataninputcavitythroughwhichtheelectronbeampasses.Themicrowaveenergymodulatesthevelocitiesofelectronsinthebeam,whichthenentersadriftspace.Herethefasterelectronsovertaketheslowertoformbunches.Inthismanner,theuniformcurrentdensityoftheinitialbeamisconvertedtoanalternatingcurrent.Thebunchedbeamwithitssignificantcomponentofalternatingcurrentthenpassesthroughanoutputcavitytowhichthebeamtransfersitsacenergy.

Klystronsmaybeoperatedasoscillatorsbyfeedingsomeoftheoutputbackintotheinputcircuit.Morewidelyusedisthereflexoscillatorinwhichtheelectronbeamitselfprovidesthefeedback.Thebeamisfocusedthroughacavityandisvelocity-modulatedthere,asintheamplifier.Thecavityusuallyhasgridstoconcentratetheelectricfieldinashortspacesothatthefieldcaninteractwithaslow,low-voltageelectronbeam.Leavingthecavity,thebeamentersaregionofdcelectricfieldopposingitsmotion,producedbyareflectorelectrodeoperatingatapotentialnegativewithrespecttothecathode.Theelectronsdonothaveenoughenergytoreachtheelectrode,butarereflectedinspaceandreturntopassthroughthecavityagain.Thepointsofreflectionaredeterminedbyelectronvelocities,thefasterelectronsgoingfartheragainstthefieldandhencetakinglongertogetbackthantheslowerones.Reflexoscillatorsareusedassignalsourcesfrom3to200GHz.Theyarealsousedasthetransmittertubesinline-of-sightradiorelaysystemsandinlow-powerradars.

klystron

Electronsleavetheheatedcathode,andareacceleratedandfocusbythefocusingelements.Theyaredeceleratedandbunchedbythedecelerationgrid.TheyU-turnattherepeller.Theirfrequencyisdependentonsize.

Aformofelectrontubeusedforgenerationandamplificationofmicrowaveelectromagneticenergy.Itisalinear-beamtube; itincorporatesanelectrongun,oneormorecavities,andanapparatusformodulatingthebeamproducedbytheelectrongun.Themostcommonlyusedklystrontubesarethetwo-cavity,themulticavity,andthereflexklystron.

Klystronanultrahigh-frequency electronicvacuumdeviceinwhichasteadystreamofelectronsis convertedtoanalternatingstreambymodulatingtheelectronvelocitieswithanultrahigh-frequencyelectricfieldwhiletheelectronsmovethroughthegapofacavityresonator.Modulatingthevelocitieshastheeffectofgroupingtheelectronsintobunches,owingtodifferencesinvelocityinadriftspace,asectionthatisfreefromtheultrahigh-frequencyfield.

Twotypesofklystronsareinuse:thefloatingdriftandthereflex.Inthefloatingdriftklystron,electronspasssuccessivelythroughthegapsofcavityresonators(seeFigure1).Velocitymodulationoccursinthegapoftheinputresonator,theultrahigh-frequencyfieldinthegapperiodically accelerating(halfacycle)anddecelerating(halfacycle).Acceleratedelectronscatchupwithretardedelectronsinthedriftspace,resultingintheformationofelectronbunches.Intransitthroughthegapoftheoutputresonator,theelectronbunchesinteractwiththeresonatorsultrahigh-frequencyfield;mostaredecelerated,andsomeoftheirkineticenergyisconvertedtotheenergyofultrahigh-frequencyoscillations.

Figure1.Diagramsoffloating-driftklystrons:(a)klystronamplifier,(b)klystronoscillator;(1)cathode,(2)focusingcylinder,(3)electronstream,(4)inputcavityresonator,(5)inputapertureforultrahighfrequencyenergy,(6)resonatorgap,(7)driftspace,(8)outputcavityresonator,(9)outputapertureforultrahighfrequencyenergy,(10)electronstreamcollectors,(11)intermediatecavityresonators,(12)anodeDCpowersupply,(13)heaterpowersupply,(14)firstcavityresonator,(15)couplingslotthroughwhichsomeultrahighfrequencyenergypassesfromsecondresonatortofirstresonator,(16)secondcavityresonator

Mostfloatingdriftklystronsaremanufacturedasmulticavityklystronamplifiers(seeFigure1,a).Intermediatecavityresonatorslocatedbetweentheinputresonatorandtheoutputresonatormakeitpossibletobroadenthefrequencypassband,increaseefficiency,andincreasegain.Klystronamplifiersarebuiltforoperationinnarrowfrequencyrangesofthedecimeterorcentimeterwavelengths.Pulse-modeklystronshaveanoutputfromseveralhundredwatts(W) to40megawatts (MW); continuous-modeklystrons,fromafewwattsto1MW. Thegainusuallyrunsfrom35to60decibels(dB).Efficiencyvariesfrom40to60percent.Thepassbandislessthan1percentinthecontinuousmodeandupto10percentinpulsemode.TheprincipalareasofapplicationofklystronamplifiersareinDopplerradar,communicationswithearthsatellites,radioastronomy,andtelevision(continuous-modeklystrons),aswellasinlinear accelerationofelementaryparticlesandpoweroutputamplificationinlongdistancehigh-resolutionradar(pulse-modeklystrons).

Asmallnumberofindustriallymanufacturedklystronsarecontinuous-modeklystronoscillators,usuallywithtwocavityresonators(seeFigurel,b).Asmallfractionoftheultrahigh-frequencyoscillatorypowergeneratedinthesecondresonatoristransmittedthroughacouplingslottothefirstresonatorinordertomodulateelectronvelocities.Thetypicaloutputofsuchklystronsisfrom1to10W,andtheirefficiencyislessthan10percent.Klystronoscillatorsareusedmainlyinparametricamplifiersandinradiobeaconswithwavelengthsinthecentimeterormillimeterrange.

Figure2.Diagramofareflexklystron:(1)cathode,(2)focusingcylinder,(3)electronstream,(4)acceleratinggrid,(5)cavityresonator,(6)resonatorgap,(7)reflector,(8)secondresonatorgrid,(9)firstresonatorgrid,(10)vacuum-tightceramicwindowservingaslead-outforultrahighfrequencyenergyfromresonator,(11)resonatorvoltagesupply,(12)heaterpowersupply,(13)reflectorvoltagesupply

Reflexklystronsarethoseinwhichtheelectronstream,havingpassedthroughtheresonatorgap,arrivesatthedeceleratingfieldofthereflector,toberepelledbythefieldandpassthroughtheresonatorgapintheoppositedirection(seeFigure2).Duringthefirsttransitthroughthegap,theultrahighfrequencyelectricfieldofthegapmodulatestheelectronvelocities.Thesecondtime,movingintheoppositedirection,theelectronsarriveatthegapgroupedinbunches.Theultrahighfrequencyfieldinthegapretardsthesebunchesandconvertssomeoftheirkineticenergytotheenergyofultrahigh-frequencyoscillations.Electronbunchesareformedbecausetheacceleratedelectronsfollowalongerpathinthespacebetweencavityresonatorandreflectorandthusspendmoretimetherethandothedeceleratedelectrons.Ifthenegativereflectorvoltageischanged,thentheelectrontransittime,thearrivalphaseofthebunchesatthegap,andthefrequencyofoscillationsgeneratedwillalsobechanged(seeFigure3).

Figure3.Reflexklystronfrequencyandoutputpowerasafunctionofreflectorvoltage:(a)oscillationbandwidth,(b)oscillationbandwidthathalfpower,(f1)oscillationfrequencyatcenterofbandwidth,(f)frequencydeviationfromf1,(c)electronictuningrangeathalfpower

Thepossibilityofchangingthefrequencyofoscillationisusedinelectronictuning.Thismakesitpossibletocontroloscillationfrequency,practicallyinertia-freeandwithoutpowerloss,infrequencymodulationandautomaticfrequencycontrol.Mechanicalfrequencytuningcanbeaccomplishedbychangingthegap,eitherbydeflectingtheface(adiaphragm)ofametallicklystron(seeFigure4,a)orbymovingatuningpistonofadetachablepartofthecavityresonatorthatisjoinedtotheedgesofmetallicdisksprotrudingfromtheklystronsglassorceramicshell(seeFigure4,b).Inadditiontothisprimarycavityresonator,manyreflexklystronshaveasecondcavityresonatorlocatedoutsidethevacuumenvelope(seeFigure4,c).Mechanicalfrequencytuningisaccomplishedinthiscasebymovingastub,therebychangingthegapofthesecondcavityresonator.Suchdesignsmakepossibleanunlimitednumberoffrequencyretunings.Theincorporationofahigh-Qresonatorimprovesfrequencystabilitybutreducestheklystronsoutputpower.

Figure4.Mechanicalfrequencytuningmethodsinareflexklystron:(a)bydeflectingdiaphragm,(b)bymovingpistonindetachablepartofcavityresonator,(c)bymovingstubincavityresonatoroutsidevacuumenvelope;(1)diaphragmwhosedeflectionchangesresonatorgap(increasingthegapincreasesoscillationfrequency),(2)edgesofmetaldiskstowhichdetachablepartofcavityresonatorisjoined,(3)detachablepartofresonator,(4)pistonwithincavityresonator(loweringdecreaseslengthofresonatorandincreasesoscillationfrequency),(5)vacuum-tightceramiccouplingwindowbetweencavityresonators,(6)stub(raisingstubincreasesresonatorgapandoscillationfrequency),(7)outputapertureforultrahighfrequencyenergy

Reflexklystronsarethemostwidelyusedultrahigh-frequencydevice. Theyaremanufacturedforoperationinthedecimeter,centimeter,andmillimeterwavebands.Their outputpowerrangesfrom5mWto5W.Theirmechanicalfrequency-tuningrange isasmuchas10percent(forklystronswithdetachablecavityresonators,severaldozenpercent).Theirelectronictuningrangeisusuallylessthan1percent.Theirefficiencyisabout1percent.Reflexklystronsareusedasheterodynesinsuperheterodyneradioreceivers,asdrivingoscillatorsinradiotransmitters,aslow-poweroscillatorsinradar,inradionavigation, andinmeasurementengineering.As shown in the given diagram. The multi cavity klystron consist of a glass envelope in which there is an electron gun composed of heater and cathode. After the electron gun there are two focusing electrodes used to keep the electron beam in the center around the glass envelope. There are two cavities known as buncher and catcher cavity. Between the cavities around the glass envelope a magnet is used in order to keep the electron beam in the center and in concentrated form at the end inside the glass envelope. There is anode used to attract the electrons emitted from the cathode.

Working of Klystron

When switch on the circuit, the electrons starts emitting from the cathode. These electrons move at a uniform speed towards the anode until they are attracted by it. Now we apply the R.F input signal to the buncher cavity with the help of loop coupling. We suppose the negative half cycle of the input signal. When this negative half cycle is applied to the buncher cavity, the negative charges will develop and the speed of electron will be reduced between the cathode and cavity. As a result the bunch of electrons will be formed near the buncher cavity. Now this bunch will travel towards the anode. At the movement when the positive half cycle is applied, the speed of electrons will increase from the previous condition. These electrons will join the bunch produced by negative half cycle and the field strength of the field of the bunch will further increase.

In this manner bunches will continue to be form. when the R.F signal is present at the buncher cavity.

when the nunch of electrons reaches in front of catcher cavity, due to its strong field strength the excitation of this cavity will take place and we will get an amplified output from the catcher cavity.

Application of Klystron

This type of klystron is mostly used for the purpose of amplification of microwave length of frequencies. it means that the high requencies can be amplified by multi cavity klystron. Which is impossible and not feasible to use other components for this purpose.

Reflex Klystron

Construction

As shown in the given diagram it is composed of heater, cathode, focusing electrodes and repeller anode. All these components are enclosed in a glass envelope. After the focusing electrodes there is a cavity resonator which encircles the glass envelope. A loop coupling is used inside the cavity to get the output.

Working

When we switch on the klystron, the heater of reflex klystron is heated up; the cathode starts emission of electrons. The focusing electrodes keep the flow of electrons in the center of glass envelope. In font of cavity resonator a huge crowed of electrons assembles and excites cavity, the oscillation it the cavity takes place and hence we get the R.F output from the cavity with the help of loop coupling.

Two cavity Klystron

2-cavity klystron amplifier works on the following principles

Velocity modulation

Current modulation

As we know in any tube amplifier we need to have electron beam produced in cathode to anode region.So electrons in electron beam are produced in the cathode to anode region and accelerated by the means of an anode voltage V0 .These electrons are allowed to pass through a pair of buncher grid across which an RF(radio frequency) voltage V1sint and these electrons are accelerated or de-accelerated depending on the part of cycle during which they enter gap.The accelerated electrons emerge with a velocity higher than the entering velocity v0 and de-accelerated electrons emerge with a velocity lower than v0.While some electrons pass through zero RF field and hence there is no change in their velocity. This phenomenon of the variation of electrons in electron beam is known as Velocity Modulation.

The analysis for velocity modulation is carried out with following assumptions1.Electrons leave the cathode with velocity=0 and the beam has uniform density in cross section of beam2.Space charge effets are not considered3.Magnitude(V1) of input signal