Microwave Generators

7/30/2019 Microwave Generators

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Microwave Generators


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MICROWAVE TUBE PRINCIPLES

The efficiency of conventional tubes is largely

independent of frequency up to a certain limit.

When frequency increases beyond that limit,

several factors combine to rapidly decrease tube

efficiency.

Tubes that are efficient in the microwave range

usually operate on the theory of VELOCITYMODULATION, a concept that avoids the

problems encountered in conventional tubes.


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Limitations of Conventional Tubes

Three characteristics of

ordinary vacuum tubes increasingly important

as frequency rises.

Inter electrode capacitance,

lead inductance, and

electron transit time.


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Higher the frequency, or the larger the inter-

electrode capacitance, the higher will be the current

through this capacitance (i=c dv/dt). The inter-electrode capacitance between the grid

and the cathode (Cgk) in parallel with the signal

source.

As the frequency of the input signal increases, the

effective grid-to-cathode impedance of the

tube decreases because of a decrease in the

reactance of the inter-electrode capacitance. If the signal frequency is 100megahertz or greater,

the reactance of the grid-to-cathode capacitance is

so small that much of the signal is short-circuited

within the tube.


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The lead inductances within a tube areeffectively in parallel with the inter-electrodecapacitance, the net effect is to raise the

frequency limit. However, the inductance of the cathode lead

is common to both the grid and plate circuits.

This provides a path for degenerativefeedback which reduces overall circuitefficiency.


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INTRODUCTION

TRANSIT TIME is basically time taken for

movement or transition of electron from one

electrode to another.

And the effect which is caused due to transit

time is known as TRANSIT TIME EFFECT.


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TRANSIT TIME EFFECT ON TUBES

AT VARIOUS FREQUENCIES

Some small amount of transit time is required

for electrons to travel from the cathode to the

plate, the time is insignificant at

low frequencies.

However, at high frequencies, transit time

becomes an appreciable portion of a signal

cycle andbegins to hinder efficiency.


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Transit times in excess of 0.1 cycle cause a significantdecrease in tube efficiency.

This decrease in efficiency is caused, in part, by a phaseshift between plate current and grid voltage.

If the tube is to operate efficiently, the plate current mustbe in phase with the grid-signal voltage and180 degrees outof phase with the plate voltage.

When transit time approaches 1/4 cycle, this phaserelationship between the elements does not hold true.

A positive swing of a high-frequency grid signal causeselectrons to leave the cathode and flow to the plate.

Initially this current is in phase with the grid voltage.

However, since transit time is an appreciable part of a cycle,the current arriving at the plate now lags the grid-signalvoltage.

As a result, the power output of the tube decreases and theplate power dissipation increases.


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Reducing Transit Time Effect

Transit time may be decreased by reducing the

spacing between electrodes.

Or by increasing the electrode voltages which

in turn increases electron velocity through the

tube.


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Two cavity and multi-cavity klystron


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Klystron Amplifier Klystron amplifiers are high power microwave vacuum

tubes. They are used in some coherent radar transmitters as

power amplifiers.

Klystrons make use of the transit-time effect by varying

the velocity of an electron beam.

A klystron uses special resonant cavities which

modulate the electric field around the axis of the tube

modulating the electric field around the axis the tube. In the middle of these cavities, there is a grid allowing

the electrons to pass the cavity.


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All electrons injected from cathods arrive at first

cavity with uniform velocity. Those electrons passing through first cavity gap at 0s

of the gap voltage for a signal voltage passed through

with unchanged velocity.

Those passing through the +ve half cycle of the gapvoltage undergo increasing velocity and those

passing through theve voltage undergo decreasing

velocity.

As a result, the electrons gradually bunch together as

they travel down the drift space.


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Due to the number of the resonant cavities

klystrons are divided up into

Two- or Multicavity klystrons, and

Reflex or Repeller Klystrons.


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Two-Cavity Klystron


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Two-Cavity Klystron

This klystron uses two resonating cavities. The first cavity together with the first coupling device

is called a buncher, while the second cavity with its

coupling device is called a catcher.


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The function of the catcher cavity is to absorb energy

from the electron beam.

The catcher grids are placed along the beam at a

point where the bunches are fully formed.

The location is determined by the transit time of the

bunches at the natural resonant frequency of the

cavities (the resonant frequency of the catcher cavity is the same as thebuncher cavity).

The air-cooled collector collect the energy of the

electron beam and change it into heat and X radiation.

Klystron amplification, power output, and efficiency

can be greatly improved by the addition of

intermediate cavities between the input and output

cavities of the basic klystron.


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The two-cavity amplifier klystron is readily turned into

an oscillator klystron by providing a feedback loop

between the input and output cavities.

It has the advantage of being among the lowest-

noise microwave sources available. It normally generates more power than the reflex

klystrontypically watts of output rather than

milliwatts.

Since there is no reflector, only one high-voltage supply

is necessary to cause the tube to oscillate, the voltage

must be adjusted to a particular value.
http://en.wikipedia.org/wiki/Feedbackhttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Microwavehttp://en.wikipedia.org/wiki/Feedback


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This is because the electron beam must produce the

bunched electrons in the second cavity in order togenerate output power.

Voltage must be adjusted to vary the velocity of the

electron beam (and thus the frequency) to a suitablelevel due to the fixed physical separation between

the two cavities.

Often several "modes" of oscillation can be observed

in a given klystron.
http://en.wikipedia.org/wiki/Velocityhttp://en.wikipedia.org/wiki/Velocity


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Reflex Klystron


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Fig: Reflex Klystron


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The reflex klystron contains a REFLECTOR PLATE, referred to as

the REPELLER, instead of the output cavity used in other types

of klystrons. The electron beam is modulated as it was in the other types

of klystrons by passing it through an oscillating resonant

cavity, but here the similarity ends.

The feedback required to maintain oscillations within thecavity is obtained by reversing the beam and sending it back

through the cavity.

The electrons in the beam are velocity-modulated before the

beam passes through the cavity the second time and will giveup the energy required to maintain oscillations.

The electron beam is turned around by a negatively

charged electrode (repeller) that repels the beam.


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Three power sources are required for reflex

klystron operation:

filament power,

positive resonator voltage (often referred to as

beam voltage) used to accelerate the electrons

through the grid gap of the resonant cavity, and

negative repeller voltage used to turn the electronbeam around.


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Travelling Wave Tube


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Cutaway view of a HELIX TWT


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1. Electron Gun

2. RF input

3. Magnets

4. Attenuator

5. Helix Coil

6. RF Output

7. Vacuum tube8. Collector


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Electron Gun: produces and then accelerates an

electron beam along the axis of the tube.

The surrounding static magnet provides a

magnetic field along the axis of the tube to focusthe electrons into a tight beam.

A longitudinal helix slow wave non-resonantguide is placed at the centre of the tube that

provides a low impedance transmission line for

the RF energy within the tube.


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The TWT is designed with helix delay structure to

slow the travelling wave down to or below thespeed to the electrons in the beam.

The RF signal wave injected at the input end of thehelix travels down the helix wire at the speed of the

light but the coiled shape causes the wave to travel

a much greater distance than the electron beam.


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Changing the number of turns or diameter of the turns

in the helix wire, the speed at which RF signal wavetravels in the form ofaxial E field, can be varied.

The helical delay structure has the added advantage ofcausing a large proportion of electric fields that are

parallel to the electron beam, provides maximum

interaction between the fields and the moving

electrons to form bunching.


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Velocity modulation

The electrons entering the helix at zero field are not

affected by the signal wave; those electronsentering the helix at the accelerating field are

accelerated, and those at the retarding field are

deccelerated.

This velocity modulation causes bunching of

electrons at regular intervals of one wavelength.


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As the bunches release energy to the signal on the

helix, amplification begins.

This amplified signal causes a denser electron

bunch which in turn amplifies the signal even more.

This process continues as the RF wave and the

electron beam travel down the length of the tube.When the loss in the system is compensated by this

energy transfer, a steady amplification of the

microwave signal appears at the output end.


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Why attenuator?

An attenuator is placed over a part of the helix on

midway to attenuate any reflected waves generateddue to the impedance mismatch.

It is placed after sufficient length of the interactionregion so that the attenuation of the amplified signal

is insignificant compared to the amplification.


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Comparison of TWTA and Klystron Amplifier

Klystron Amplifier TWTA

1. Linear beam or 1. Linear beam or O

O type Device type device

2. Uses Resonant cavities 2. Uses non resonantfor input and output wave circuits

circuits

3. Narrowband device 3.Wideband device

li i


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Applications

Mediumpower satellite

Higher

power satellite transponder output.


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Magnetron/ Cross Field Amplifier


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Magnetrons

Inherently efficient

Delivers large powers (up to GW pulsedpower and MW cw)

Limited electronic tuning, i.e., BW limited

Low cost

Industrial uses

microwave ovens

industrial heating drying wood

processing and bonding materials


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Circular Magnetron

(conventional geometry)

Electrons tend to

move parallel to

the cathode. After

a few periods in

the cylindrical

geometry theelectron cloud so

formed is known

as the Brillouin

cloud. A ring

forms around the

cathode.


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Circular Magnetron Oscillator


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Construction

Each cavity in the anode acts as an inductor having only one turnand the slot connecting the cavity and the interaction spaceacts as a capacitor.

These two form a parallel resonant circuit and its resonantfrequency depends on the value ofL of the cavity and the Cof the slot.

The frequency of the microwaves generated by the magnetronoscillator depends on the frequency of the RF oscillationsexisting in the resonant cavities.


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If the magnetic field strength is increased slightly, thelateral force bending the path of the electron as given

by the path b in Fig iii.

If the strength of the magnetic field is made sufficientlyhigh then the electrons can be prevented fromreaching the anode as indicated path c in Fig iii,

The magnetic field required to return electrons back tothe cathode just grazing the surface of the anode iscalled the critical magnetic field (B

c) or the cut off

magnetic field.

If the magnetic field is larger than the critical field (B >B

c), the electron experiences a greater rotational force

and may return back to the cathode quite faster.


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Electron trajectories in the

presence of crossed electric

and magnetic fields

(a) no magnetic field

(b) small magnetic field

(c) Magnetic field = Bc

(d) Excessive magnetic field

Fig (iii)


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Applications

1. Pulsed radar is the single most importantapplication with large pulse powers.

2. Voltage tunable magnetrons are used in

sweep oscillators in telemetry and inmissile applications.

3. Fixed frequency, CW magnetrons are

used for industrial heating andmicrowave ovens.

Microwave Generators

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