K K TWT M Communication 2

Heng Chan;Mohawk College1 Communications 2 EE555 Heng Chan;Mohawk College2 CourseContent pIntroduction & Review pTransmission Line Characteristics pWaveguides & Microwave Devices pRadiowave Propagation pAntennas pMicrowave Radio & Radar Systems pFibre Optic Communications Heng Chan;Mohawk College3 Introduction&Review pMicrowaves are defined as radio waves in the frequency range > 1 GHz. pHowever, waves > 20 GHz are commonly known as millimeter wavespDistributed, rather than lumped, circuit elements must be used at microwave frequencies because of a phenomenon called skin effect. Heng Chan;Mohawk College4 SkinEffect pAt microwave frequencies current travels on the outer surface, or skin, of the conductor because of the increased inductance created. pThe skin depth , o (in m), for a conductor with permeability, (in H/m), conductivity, o (in S/m), and at a frequency, f (in Hz), is given by: o tof1=Heng Chan;Mohawk College5 SkinEffect(contd) p The current density, J, decreases with the distance beneath the surface exponentially. p At a depth o, the current density decreases to Jo/e. p As f increases, o + and resistance |. J Jo z J = Joe-z/o conductor surface direction of current Heng Chan;Mohawk College6 Transverse Electromagnetic Waves x y z Electric Field Magnetic Field In free space: Heng Chan;Mohawk College7 NotesonTEMWaves pThe E- and H-fields and the direction of motion of TEM waves are mutually perpendicular to each other. pVelocity of radio waves in free space is c = 3x108 m/s, but in a medium with dielectric constant cr: fv cvr= = c;Heng Chan;Mohawk College8 MicrowaveMaterials pGlass epoxy printed circuit boards are unsuitable for microwave use because of high dissipation factor and wide tolerance in thickness and dielectric constant. pInstead, materials such as Teflon fiberglass laminates, alumina substrates, sapphire and quartz substrates must be used (refer to text for details). Heng Chan;Mohawk College9 TypesofTransmissionLines pDifferential or balanced lines (where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair. pSingle-ended or unbalanced lines (where one conductor is grounded): e.g. concentric or coaxial cable. pTransmission lines for microwave use: e.g.striplines, microstrips, and waveguides. Heng Chan;Mohawk College10 Transmission Line Equivalent Circuit R L R L C G C G L L C C Lossy Line Lossless Line C j GL j RZoee++=CLZo =Zo Zo Heng Chan;Mohawk College11 NotesonTransmissionLine pCharacteristics of a line is determined by its primary electrical constants or distributed parameters: R (O/m), L (H/m), C (F/m), and G (S/m). pCharacteristic impedance, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo. Heng Chan;Mohawk College12 FormulasforSomeLines D d D d dDZdDCdDLro2ln120;2ln;2lnctct= = =For parallel two-wire line: For co-axial cable: dDZdDCdDLroln60;ln2; ln2ctct= = = = or; c = cocr; o = 4tx10-7 H/m; co = 8.854 pF/m Heng Chan;Mohawk College13 Transmission-Line Wave Propagation Electromagnetic waves travel at < c in a transmission line because of the dielectric separating the conductors. The velocity of propagation is given by: rcLCvc c= = =1 1m/s Velocity factor, VF, is defined as: rcvVFc1= =Heng Chan;Mohawk College14 PropagationConstant pPropagation constant, , determines the variation of V or I with distance along the line: V = Vse-x;I = Ise-x, where VS, and IS are the voltage and current at the source end, and x = distance from source. p = o + j|, where o = attenuation coefficient (= 0 for lossless line), and | = phase shift coefficient = 2t/(rad./m) Heng Chan;Mohawk College15 Incident&ReflectedWaves pFor an infinitely long line or a line terminated with a matched load, no incident power is reflected.The line is called a flat or nonresonant line. pFor a finite line with no matching termination, part or all of the incident voltage and current will be reflected. Heng Chan;Mohawk College16 ReflectionCoefficient The reflection coefficient is defined as: irirIIorEE= IIt can also be shown that: | Z I =+= Io Lo LZ ZZ ZNote that when ZL = Zo, I = 0; when ZL = 0, I = -1; and when ZL = open circuit, I = 1. Heng Chan;Mohawk College17 StandingWaves Vmin = Ei - Er With a mismatched line, the incident and reflected waves set up an interference pattern on the lineknown as a standing wave. The standing wave ratio is : I I += =11minmaxVVSWRVmax = Ei + Er 2 Voltage Heng Chan;Mohawk College18 OtherFormulas When the load is purely resistive: (whichever gives an SWR > 1)LooLZZorZZSWR =Return Loss, RL = Fraction of power reflected = |I|2, or -20 log |I| dB So, Pr = |I|2Pi Mismatched Loss, ML = Fraction of power transmitted/absorbed = 1 - |I|2 or -10 log(1-|I|2) dB So, Pt = Pi (1 - |I|2) = Pi - Pr Heng Chan;Mohawk College19 Time-DomainReflectometry ZL Pulse or Step Generator Oscilloscope Transmission Line TDR is a practical technique for determining the length of the line, the way it is terminated, and the type and location of any impedance discontinuities. The distance to the discontinuity is: d = vt/2, where t = elapsed time of returned reflection. d Heng Chan;Mohawk College20 Typical TDR Waveform Displays t RL > ZoRL < Zo ZL inductive ZL capacitive Vi Vr Vr Vi Heng Chan;Mohawk College21 Transmission-Line Input Impedance The input impedance at a distance l from the load is: ) tan() tan(l jZ Zl jZ ZZ ZL oo Lo i||++=When the load is a short circuit, Zi = jZo tan (|l). For 0 - l < /4, shorted line is inductive. For l = /4, shorted line = a parallel resonant circuit. For /4 < l - /2, shorted line is capacitive. Heng Chan;Mohawk College22 T-L Input Impedance (contd) When the load is an open circuit, Zi = -jZo cot (|l) For 0 < l < /4, open circuited line is capacitive. For l = /4, open-line = series resonant circuit. For /4 < l < /2, open-line is inductive. p A /4 line with characteristic impedance, Zo, can be used as a matching transformer between a resistive load, ZL, and a line with characteristic impedance, Zo, by choosing: L o oZ Z Z ='Heng Chan;Mohawk College23 TransmissionLineSummary or l < /4 l > /4 is equivalent to: l > /4 or l < /4 is equivalent to: = = /4 Zo Zo ZL /4-section Matching Transformer l = /4 Heng Chan;Mohawk College24 TheSmithChart pThe Smith chart is a graphical aid to solving transmission-line impedance problems. pThe coordinates on the chart are based on the intersection of two sets of orthogonal circles. p One set represents the normalized resistive component, r (= R/Zo), and the other the normalized reactive component, jx (= jX/Zo). Heng Chan;Mohawk College25 SmithChartBasics r = 0 r = 1 r = 2 +j0.7 -j1.4 j0 z1 z2 z1 = 1+j0.7 z2 = 2-j1.4 _ Heng Chan;Mohawk College26 ApplicationsofTheSmithChart pApplications to be discussed in this course: Find SWR, |I|Z|, RL Find YL Find Zi of a shorted or open line of length l Find Zi of a line terminated with ZL Find distance to Vmax and Vmin from ZL Solution for quarter-wave transformer matching Solution for parallel single-stub matching Heng Chan;Mohawk College27 SubstrateLines pMiniaturized microwave circuits use striplines and microstrips rather than coaxial cables as transmission lines for greater flexibility and compactness in design. pThe basic stripline structure consists of a flat conductor embedded in a dielectric material and sandwiched between two ground planes. Heng Chan;Mohawk College28 BasicStriplineStructure Ground Planes Centre Conductor Solid Dielectric b W t cr Heng Chan;Mohawk College29 NotesOnStriplines p When properly designed, the E and H fields of the signal are completely confined within the dielectric material between the two ground planes. p The characteristic impedance of the stripline is a function of its line geometry, specifically, the t/b and w/b ratios, and the dielectric constant, cr. p Graphs, design formulas, or computer programs are available to determine w for a desired Zo, t, and b. Heng Chan;Mohawk College30 Microstrip w t b Ground Plane cr (dielectric) Circuit Line Microstrip line employs a single ground plane, the conductor pattern on the top surface being open. Graphs, formulas or computer programs would be used todesign the conductor line width.However, since theelectromagnetic field is partly in the solid dielectric, and partly in the air space, the effective relative permittivity, ceff,has to be used in the design instead of cr. Heng Chan;Mohawk College31 StriplinevsMicrostrip pAdvantages of stripline: signal is shielded from external interference shielding prevents radiation loss cr and mode of propagation are more predictable for design pAdvantages of microstrip: easier to fabricate, therefore less costly easier to lay, repair/replace components Heng Chan;Mohawk College32 MicrostripDirectionalCoupler /4 Top View Cross-sectional View Conductor Lines Dielectric Ground Plane 1 2 3 4 Most of the power into port #1 will flow to port #3. Some of the power will be coupled to port #2 but only a minute amount will go to port #4. Heng Chan;Mohawk College33 Formulas For Directional Coupler The operation of the coupler gives rise to an even mode characteristic impedance, Zoe, and an odd mode characteristic impedance, Zoo, where: oo oe oZ Z Z =For a given coupling factor, C (which is V2/V1): CCZ ZCCZ Zo oo o oe+=+=11;11Heng Chan;Mohawk College34 CouplerApplications pSome common applications for couplers: monitoring/measuring the power or frequency at a point in the circuit sampling the microwave energy for used in automatic leveling circuits (ALC) reflection measurements which indirectly yield information on VSWR, ZL, return loss, etc. Heng Chan;Mohawk College35 BranchCoupler 1 2 3 4 /4 /4 Z1 = 0.707 Zo Input power at port #1 will divide equally betweenPorts 2 and 3 and none toport 4. Can provide tighter coupling and can handle higher power than directional coupler. Branches may consist of chokes, filters, ormatched load for more design flexibility. ZoZo Z1 Z1 Heng Chan;Mohawk College36 HybridRingCoupler Input power at port #1 divides evenly between ports 2 & 4 and none for port 3. Similarly, input at port #2 will divide evenly between ports 1 and 3 and none for port 4. One application: circulator. 1 2 3 4 /4 /4 /4 3/4 Heng Chan;Mohawk College37 Microstrip&StriplineFilters /4IN OUT Side-coupled half-wave resonator band-pass filter IN OUT L CCC L L Conventional low-pass filter L Heng Chan;Mohawk College38 ScatteringParameters p Microwave devices are often characterized by their S-parameters because: measurement of V and I may be difficult at microwave frequencies. Active devices frequently become unstable when open or short-circuit type measurements are made for h, Y or Z parameters. p An [S] matrix is used to contain all the S-parameters. Heng Chan;Mohawk College39 S-Variables&S-Parameters a1 b1b2 a2 V1V2 For port x: Vx = Vix + Vrx ; S-variables: orxxoixxZVbZVa = = ;Px = Pix - Prx =|ax|2-|bx|2 ((

((

=((

212212211121aaSSSSbbb1 = S11a1 + S12a2 b2 = S21a1 + S22a2 or 2-Port Network Heng Chan;Mohawk College40 S-Parametersof2-PortNetwork 02222 0211201221 011111 12 2;;= == == == =a aa aabSabSabSabSNote: when port 2 is terminated with a matchedload, a2 = 0. Similarly, a1 = 0 when port 1 is matched. S11, and S22 are reflection coefficients, i.e., I11, & I22.S21 represents the forward transmission coefficient. Thus, Insertion Loss/attenuation = -10 log (Po2/Pi1) = -20 log |S21| dB S12 is the reversed transmission coefficient. Heng Chan;Mohawk College41 PropertiesofS-Parameters pIn general, S-parameters have both magnitude and angle. pFor matched 2-port reflectionless networks, S11 = S22 = 0 pFor a reciprocal 2-port network, S12 = S21. pFor a lossless 2-port network, S12 = S21 = 1. pFor n-port, [b] = [S] [a].The n x n[S] matrix characterizes the network. Heng Chan;Mohawk College42 MicrowaveRadiationHazards p The fact that microwaves can be used for cooking purposes and in heating applications suggests that they have the potential for causing biological damage. p Health & Welfare, Canada specifies no limit exposure duration for radiation level of 1 mW/cm2 or less for frequencies from 10 MHz to 300 GHz. p Avoid being in the direct path of a microwave beam coming out of an antenna or waveguide. Heng Chan;Mohawk College43 Waveguides pReasons for using waveguide rather than coaxial cable at microwave frequency: easier to fabricate no solid dielectric and I2R losses pWaveguides do not support TEM waves inside because of boundary conditions. pWaves travel zig-zag down the waveguide by bouncing from one side wall to the other. Heng Chan;Mohawk College44 E-FieldPatternofTE1 0 Mode a b g/2 End ViewSide View TEmn means there are m number of half-wave variations of the transverse E-field along the a side and n number of half-wave variations along the b side. The magnetic field (not shown) forms closed loopshorizontally around the E-field Heng Chan;Mohawk College45 TEandTMModes p TEmn mode has the E-field entirely transverse, i.e. perpendicular, to the direction of propagation. p TMmn mode has the H-field entirely transverse to the direction of propagation. p All TEmn and TMmn modes are theoretically permissible except, in a rectangular waveguide, TMmo or TMon modes are not possible since the magnetic field must form a closed loop. p In practice, only the dominant mode, TE10 isused. Heng Chan;Mohawk College46 WavelengthforTE&TMModes + Any signal with c will not propagate down the waveguide. + For air-filled waveguide, cutoff freq., fc = c/c Guide wavelength: ( ) ( )2 2/ 1 / 1 f forc cg = + TE10 is called the dominant mode since c = 2a is the longest wavelength of any mode. ( ) ( )2 2/ /2b n a mc+= Cutoff wavelength: Heng Chan;Mohawk College47 Other Formulas for TE&TMModes Group velocity:( )2/ 1cggc or c v =Phase velocity: ( )2/ 1cgpcor c v =Wave impedance: ( )( )22/ 1/ 1c o TMcoTEZ ZZZ ==Zo = 377 O for air-filled waveguide Heng Chan;Mohawk College48 Circular/CylindricalWaveguides p Differences versus rectangular waveguides : - c = 2tr/Bmn where r = waveguide radius, and Bmn is obtained from table of Bessel functions. All TEmn and TMmn modes are supported since m and n subscripts are defined differently. Dominant mode is TE11. p Advantages: higher power-handling capacity, lower attenuation for a given cutoff wavelength. p Disadvantages: larger and heavier. Heng Chan;Mohawk College49 WaveguideTerminations Dissipative Vane Side ViewEnd View Short-circuit Sliding Short-Circuit g/2 Dissipative vane is coated with a thin film of metal which in turn has a thin dielectric coating for protection.Its impedance is made equal to the wave impedance.The taper minimizes reflection. Sliding short-circuit functions like a shorted stub for impedance matching purpose. Heng Chan;Mohawk College50 Attenuators Resistive Flap Sliding-vane Type Rotary-vane Type Max. attenuation when flap is fully inside.Slot for flap is chosen to be at a non- radiating position. Max. attenuation when vaneis at centre of guide and min. at the side-wall. Atten.(dB) = 10 log (Pi/Po) = -20 log |S21| PiPo PiPo Heng Chan;Mohawk College51 IrisReactors = = = Inductive iris; vanes are vertical Capacitive iris; vanes are horizontal Irises can be used as reactance elements, filters or impedance matching devices. Heng Chan;Mohawk College52 TuningScrew s A post or screw can also serve as a reactive element. When the screw is advanced partway into the wave- guide, it acts capacitive.When the screw is advanced all the way into the waveguide, it acts inductive.In between the two positions, one can get a resonant LC circuit. Post Tuning Screws Heng Chan;Mohawk College53 WaveguideT-Junctions 1 2 3 1 2 3 E-Plane JunctionH-Plane Junction Input power at port 2 will split equally between ports 1 and 3 but the outputs will be antiphase for E-plane T and inphase for H-plane T.Input power at ports 1 & 3 will combine and exit from port 1 provided the correct phasing is used. Heng Chan;Mohawk College54 S-MatrixforT-Junctions For ideal T-junction: ((((((((

=21212121021212121] [SNote: + sign is used for H-plane T, and (-) sign for E-plane T. Also note that even though S22 = 0 (i.e. lossless), S11 and S33 are each equal to 1/2, i.e., input power applied to ports 1 and 3 will always suffer from reflection. Heng Chan;Mohawk College55 Hybrid-TJunction 1 2 3 4 (((((

=0 0 1 10 0 1 11 1 0 01 1 0 021] [SIt combines E-plane and H-plane junctions. Note : S11, S22, S33, and S44 are zero. Pin at port 1 or 2 will divide between ports 3 and 4. Pin at port 3 or 4 will divide between ports 1 and 2. Under matched & ideal conditions: Heng Chan;Mohawk College56 Hybrid-TJunction(contd) pIf input power of the same phase is applied simultaneously at ports 1 and 2, the combined power exits from port 4.If the input is out-of-phase, the output is at port 3. pApplications: Combining power from two transmitters. TX and a RX sharing a common antenna. Low noise mixer circuit. Heng Chan;Mohawk College57 DirectionalCoupler P1 P2 P4 Termination g/4 P3 2-hole Coupler Holes spaced g/4 allow waves travelling toward port 4 to combine.Waves travelling toward port 3, however, will cancel.Therefore, ideally P3 = 0. To broaden frequency response bandwidth, practical couplers would usually have multi holes. P1 P2 Heng Chan;Mohawk College58 DirectionalCoupler(contd) For ideal directional coupler: (((((

=0 00 00 00 0] [o |o || o| oSwhere o2 + |2 = 1 Definitions: Coupling Factor, | | log 20 log 10 ) (1441SPPdB C = =Directivity, 31413141log 20 log 10 ) (SSPPdB D = =Insertion Loss (dB) = 10 log (P1/P2) = -20 log |S12| Heng Chan;Mohawk College59 CavityResonators a b L Resonant wavelength for a rectangular cavity:2 2 2) / ( ) / ( ) / (2L p b n a mr+ += L r For a cylindrical resonator: 2 22|.|

\|+|.|

\|=LprBmnrttHeng Chan;Mohawk College60 CavityResonators(contd) pEnergy is coupled into the cavity either through a small opening, by a coupling loop or a coupling probe.These methods of coupling also apply for waveguides pApplications of resonators: filters absorption wavemeters microwave tubes Heng Chan;Mohawk College61 FerriteComponents p Ferrites are compounds of metallic oxides such as those ofFe, Zn, Mn, Mg, Co, Al, and Ni. p They have magnetic properties similar to ferromagnetic metals and at the same time have high resistivity associated with dielectrics. p Their magnetic properties can be controlled by means of an external magnetic field. p They can be transparent, reflective, absorptive, or cause wave rotation depending on the H-field.. Heng Chan;Mohawk College62 ExamplesofFerriteDevices Attenuator Isolator u Differential Phase Shifter 1 2 3 4 4-port Circulator Heng Chan;Mohawk College63 NotesOnFerriteDevices p Differential phase shifter - u is the phase shift between the two directions of propagation. p Isolator - permits power flow in one direction only. p Circulator - power entering port 1 will go to port 2 only; power entering port 2 will go to port 3 only; etc. p Most of the above are based on Faraday rotation. p Other usage: filters, resonators, and substrates. Heng Chan;Mohawk College64 SchottkyBarrierDiode Semi- conductor Layer Substrate Contact SiO2 Dielectric Metal Electrode Metal Electrode Its based on a simple metal- semiconductor interface. There is no p-n junction but a depletion region exists. Current is by majority carriers; therefore, very low in capacitance. Applications: detectors, mixers, and switches. Heng Chan;Mohawk College65 VaractorDiode Circuit Symbol V Cj Co Junction Capacitance Characteristic Varactors operate under reverse-bias conditions. The junction capacitance is: mbojV VK CC) ( =where Vb = barrier potential (0.55 to 0.7 for silicon) and K = constant (often = 1) Heng Chan;Mohawk College66 EquivalentCircuitforVaractor Cj Rj Rs The series resistance, Rs, and diode capacitance, Cj, determine the cutoff frequency: j scC Rft 21=The diode quality factor for a givenfrequency, f, is: ffQc=Heng Chan;Mohawk College67 VaractorApplications pVoltage-controlled oscillator (VCO) in AFC and PLL circuits pVariable phase shifter pHarmonic generator in frequency multiplier circuits pUp or down converter circuits pParametric amplifier circuits - low noise Heng Chan;Mohawk College68 ParametricAmplifierCircuit Pump signal (fp) Input signal (fs) L1 C1 C2 L2 D1 L3 C3 Signal tank (fs) Idler tank (fi) Nondegenerative mode: Upconversion - fi = fs + fp Downconversion - fi = fs - fp Power gain, G = fi /fs Regenerative mode: 4 negative resistance 4 very low noise 4 very high gain fp = fs + fi Degenerative Mode:fp = 2fs Heng Chan;Mohawk College69 PINDiode P+ I N+ +V R RFC C1 C2 S1 D1 In Out PIN as shunt switch PIN diode has an intrinsic region between the P+ and N+ materials.It has a very high resistance in the OFF mode and a very low resistance when forward biased. Heng Chan;Mohawk College70 PINDiodeApplications pTo switch devices such as attenuators, filters, and amplifiers in and out of the circuit. pVoltage-variable attenuator pAmplitude modulator pTransmit-receive (TR) switch pPhase shifter (with section of transmission line) Heng Chan;Mohawk College71 TunnelDiode Symbol Ls Cj Rs -R Equivalent Circuit i V Vv Ip Vp Characteristic Curve Heavy doping of the semiconductor material creates a very thin potential barrier in the depletion zone which leads to electron tunneling through the barrier. Note the negative resistance zone between Vp and Vv. B C A Heng Chan;Mohawk College72 MoreNotesOnTunnelDiode The resistive, and self-resonant frequencies are: 2) (1 121; 121j j sss jrRC C LfRRC Rf = =t tTunnel diodes can be used in monostable (A or C), bistable (between A and C), or astable (B) modes. These modes lead to switching, oscillation, and amplification applications.However, the power output levels of the tunnel diode are restricted to a few mW only. Heng Chan;Mohawk College73 TransferredElectronDevices p TEDs are made of compound semiconductors such as GaAs. p They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded. p The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band. Heng Chan;Mohawk College74 GunnDiode The Gunn diode is a transferred electron device that can be used in microwave oscillators or one-portreflection amplifiers.Its basic structure is shown below.N-, the active region, is sandwiched between two heavily doped N+ regions.Electrons from theN- Metallic Electrode N+ Metallic Electrode cathode (K) drifts to the anode (A) in bunched formation called domains. Note that there is no p-n junction. AK l Heng Chan;Mohawk College75 GunnOperatingModes p Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect. p Transit Time (TT) Mode: operating frequency, fo = vd / l where vd is the domain velocity, andl is the effective length.Output power < 2 W, and frequency is between 1 GHz to 18 GHz. p Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W. Heng Chan;Mohawk College76 Gunn Diode Circuit and Applications Tuning Screw Diode Resonant Cavity Iris V Gunn diode applications: microwave source for receiver local oscillator, police radars, and microwave communication links. The resonant cavity is shocked excited by current pulses from the Gunn diode and the RF energy is coupled via the iris to the waveguide. Heng Chan;Mohawk College77 Avalanche Transit-Time Devices p If the reverse-bias potential exceeds a certain threshold, the diode breaks down. p Energetic carriers collide with bound electrons to create more hole-electron pairs. p This multiplies to cause a rapid increase in reverse current. p The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon. Heng Chan;Mohawk College78 IMPATTDiode A single-drift structure ofan IMPATT (impact avalanche transit time) diode is shown below: P+NN+ - + l Drift Region Avalanche Region Operating frequency: lvfd2=where vd = drift velocity Heng Chan;Mohawk College79 NotesOnIMPATTDiode p The current build-up and the transit time for the current pulse to cross the drift region cause a 180o phase delay between V and I; thus, negative R. p IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are possible. p They must operate in conjunction with an external high-Q resonant circuit. p They have relatively high output power (>100 W pulsed) but are very noisy and not very efficient. Heng Chan;Mohawk College80 MicrowaveTransistors p Silicon BJTs and GaAsFETs are most widely used. p BJT useful for amplification up to about 6 MHz. p MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz. p FETs have higher input impedance, better efficiency and more frequency stable than BJTs. Heng Chan;Mohawk College81 MicrowaveTransistorPowerGain Max. power gain of a unilateral transistor amplifier with conjugate matched input and output: Transistor Go Matching Network Gs Matching Network GL ZL Zs Vs 222221211max| | 11| || | 11SSSG G G GL o s = =Note that Go = |S21|2 is the gain of the transistor.Forunconditional stability, |S11| < 1 and |S22| < 1. Heng Chan;Mohawk College82 NoiseFactor&NoiseFigure Noise Factor, Fn = SNRin/SNRout Noise Figure, NF (dB) = 10 log Fn = SNRin (dB) - SNRout (dB) Equivalent noise temperature, Te = (Fn -1) To where To = 290 oK For amplifiers in cascade, the overall noise factor: 1 2 1 2 13121...1. . .1 1+ +++ =nnTG G GFG GFGFF Fwhere Gn = amplifier gain of the nth stage. Heng Chan;Mohawk College83 MicrowaveTubes pClassical vacuum tubes have several factors which limit their upper operating frequency: interelectrode capacitance & lead inductance dielectric losses & skin effect transit time pMicrowave tubes utilize resonant cavities and the interaction between the electric field, magnetic field and the electrons. Heng Chan ;Mohawk College84 Magnetrons It consists of a cylindrical cathode surrounded by the anode with a number of resonant cavities. Waveguide Output Coupling Window Cathode Anode Interaction Space Cavity Its a crossed-field device since the E-field is perpendicular to the dc magnetic field. At a critical voltage the electrons from thecathode will just graze the anode. Heng Chan;Mohawk College85 MagnetronOperation p When an electron cloud sweeps past a cavity, it excites the latter to self oscillation which in turn causes the electrons to bunch up into a spoked wheel formation in the interaction space. p The continuous exchange of energy between the electrons and the cavities sustains oscillations at microwave frequency. p Electrons will eventually lose their energy and fall back into the cathode while new ones are emitted. Heng Chan;Mohawk College86 MoreNotesOnMagnetrons p Alternate cavities are strapped (i.e., shorted) so that adjacent resonators are 180o out of phase.This enables only the dominant t-mode to operate. p Frequency tuning is possible either mechanically (screw tuner) or electrically with voltage. p Magnetrons are used as oscillators for radars, beacons, microwave ovens, etc. p Peak output power is from a few MW at UHF and X-band to 10 kW at 100 GHz. Heng Chan;Mohawk College87 Klystrons pKlystrons are linear-beam devices since the E-field is parallel to the static magnetic field. pTheir operation is based on velocity and density modulation with resonating cavities to create the bunching effect.pThey can be employed as oscillators or power amplifiers. Heng Chan;Mohawk College88 Two-CavityKlystron Filament RF InRF Out Control Grid Cathode Anode Buncher Cavity Catcher Cavity Collector Gap Drift Region Effect of velocity modulation v Electron Beam Heng Chan;Mohawk College89 KlystronOperation p RF signal applied to the buncher cavity sets up an alternating field across the buncher gap. p This field alternately accelerates and decelerates the electron beam causing electrons to bunch up in the drift region. p When the electron bundles pass the catcher gap, they excite the catcher cavity into resonance. p RF power is extracted from the catcher cavity by the coupling loop. Heng Chan;Mohawk College90 MulticavityKlystrons p Gain can be increased by inserting intermediate cavities between the buncher and catcher cavity.p Each additional cavity increases power gain by 15- to 20-dB. p Synchronous tuned klystrons have high gain but very narrowbandwidth, e.g. 0.25 % of fo. p Stagger tuned klystrons have wider bandwidth at the expense of gain. p Can operate as oscillator by positive feedback. Heng Chan;Mohawk College91 ReflexKlystron Output Anode Filament Cathode Repeller Cavity Vr Electron Beam Condition for oscillation requires electron transit time to be: T n t|.|

\|+ =43where n = an integer and T = period of oscillation Heng Chan;Mohawk College92 ReflexKlystronOperation p Electron beam is velocity modulated when passing though gridded gap of the cavity. p Repeller decelerates and turns back electrons thus causing bunching. p Electrons are collected on the cavity walls and output power can be extracted. p Repeller voltage, Vr, can be used to vary output frequency and power. Heng Chan;Mohawk College93 NotesOnReflexKlystrons pOnly one cavity used. pNo external dc magnetic field required. pCompact size. pCan be used as an oscillator only. pLow output power and low efficiency. pOutput frequency can be tuned by Vr , or by changing the dimensions of the cavity. Heng Chan;Mohawk College94 Travelling-WaveTube RF InRF Out Collector Helix AttenuatorElectron Beam The TWT is a linear beam device with the magnetic field running parallel to tube lengthwise. The helix is also known as a slow wave structure to slow down the RF field so that its velocity down the the tube is close to the velocity of the electron beam. Heng Chan;Mohawk College95 TWTOperation p As the RF wave travels along the helix, its positive and negative oscillations velocity modulate the electron beam causing the electrons to bunch up. p The prolonged interaction between the RF wave and electron beam along the TWT results in exponential growth of the RF voltage. p The amplified wave is then extracted at the output. p The attenuator prevents reflected waves that can cause oscillations. Heng Chan;Mohawk College96 NotesOnTWTs p Since interaction between the RF field and the electron beam is over the entire length of the tube, the power gain achievable is very high (> 50 dB). p As TWTs are nonresonant devices, they have wider bandwidths and lower NF than klystrons. p TWTs operate from 0.3 to 50 GHz. p The Twystron tube is a combination of the TWT and klystron.It gives better gain and BW over either the conventional TWT or klystron. Heng Chan;Mohawk College97 Radio- WaveInFreeSpace Radio waves propagate as TEM waves in free space. For an isotropic (i.e. omnidirectional) source:dPEdPPrrD30;42= =twhere PD = power density (W/m2); E = electric field intensity (V/m); Pr = total radiated power (W); and d = distance from source (m). d Point Source Heng Chan;Mohawk College98 OpticalPropertiesOfRadioWaves p Since light waves and radio waves are part of the electromagnetic spectrum, they behave similarly. p Thus, radio waves can: refract at the boundary between two different media reflect at the surface of a conductor diffract around the edge of an obstacle interfere with one and another to degrade performance p Propagation of radio wave in the atmosphere is greatly influenced by the frequency of the wave. Heng Chan;Mohawk College99 RadioWavePropagationModes pIn every terrestrial radio system, there are three possible modes of propagation: Ground-wave or surface-wave propagation Space-wave or direct-wave propagation Sky-wave propagation p At frequencies < 2 MHz, ground wave is best. p Sky waves are used for HF signals. p Space waves are used for VHF and above. Heng Chan;Mohawk College100 Ground-WavePropagation Ground waves start out with the electric field being perpendicular to the ground. Due to the gradient density of the earths atmosphere the wavefront tilts progressively. Direction of wave travel Increasing Tilt Earth Wavefront Heng Chan;Mohawk College101 NotesOnGroundWaves p Advantages: Given enough power, can circumnavigate the earth. Relatively unaffected by atmospheric conditions. p Disadvantages: Require relatively high transmission power. Require large antennas since frequency is low. Ground losses vary considerably with terrain. p Applications: MF broadcasting; ship-to-ship and ship-to-shore comms; radio navigation; maritime comms. Heng Chan;Mohawk College102 Space-WavePropagation Most terrestrial communications in the VHF or higher frequency range use direct, line-of-sight, or tropospheric radio waves.The approximate maximum distance of communication is given by: ( )R Th h d + = 17where d = max. distance in km hT = height of the TX antenna in m hR = height of the RX antenna in m Heng Chan;Mohawk College103 NotesOnSpace-Waves p The radio horizon is greater than the optical horizon by about one third due to refraction of the atmosphere. p Reflections from a relatively smooth surface, such as a body of water, could result in partial cancellation of the direct signal - a phenomenon known as fading. Also, large objects, such as buildings and hills, could cause multipath distortion from many reflections. Heng Chan;Mohawk College104 Sky-Wave Propagation p HF radio waves are returned from the F-layer of the ionosphere by a form of refraction. p The highest frequency that is returned to earth in the vertical direction is called the critical frequency, fc. p The highest frequency that returns to earth over a given path is called the maximum usable frequency (MUF).Because of the general instability of the ionosphere, the optimum working frequency (OWF) = 0.85 MUF, is used instead. Heng Chan;Mohawk College105 FormulasForSkyWaves pFrom geometry (assuming flat earth): d = 2hv tan ui where hv = virtual height of F-layer pFrom theory (secant law): MUF = fc sec ui ui hv d F-Layer Earth Heng Chan;Mohawk College106 Free-SpacePathLoss pDefined as the loss incurred by a radio wave as it travels in a straight line through a vacuum with no absorption or reflection of energy from nearby objects. pFormula: Lp (dB) = 92.4 + 20log f + 20log d where f = frequency of radio wave in GHz and d = distance in km. pIf f is in MHz, replace 92.4 above by 32.4. Heng Chan;Mohawk College107 FadeMargin p To account for changes in atmospheric conditions, multipath loss, and terrain sensitivity,a fade margin, Fm,must be added to total system loss: Fm (dB) = 30log d + 10log(6ABf) - 10log(1-R) -70 where d = distance (km), f = frequency (GHz), R = reliability (decimal value), A = terrain roughness factor (0.25 to 4), and B = factor to convert worst-month probability to annual probability (0.125 to 1 depending on humidity or dryness). Heng Chan;Mohawk College108 AntennaBasics p An antenna is a passive reciprocal device. p It acts as a transducer to convert electrical oscillations in a transmission line or waveguide to a propagating wave in free space and vice versa. p It functions as an impedance matcher between a transmission line or waveguide and free space. p All antennas have a radiation pattern which is a plot of the field strength or power density at various angular positions relative to the antenna. Heng Chan;Mohawk College109 AntennaEfficiency An antenna has an equivalent radiation resistance, Rr given by: 2iPRrr =where Pr = power radiated and i = antenna current at feedpoint Antenna efficiency: 100 100 xR RRxP PPe rrd rr+=+= qwhere Pd = power dissipated; and Re = effective antenna resistance. All the power supplied to the antenna is not radiated. Heng Chan;Mohawk College110 DirectiveGain&PowerGain Directive gain of an antenna is given by: DrDPPD =where PD = power density at some point with a given antenna; PDr = power density at the same point with a reference antenna. Reference antenna is generally the isotropic source. When antenna efficiency is taken into account directive gain becomes power gain: Ap = q D. In decibels, power gain is 10 log Ap Maximum directive gain is called directivity. Heng Chan;Mohawk College111 Effective Isotropic Radiated Power EIRP is the equivalent power that an isotropic antennawould have to radiate to achieve the same power density at a given point as another antenna: EIRP = PrAt = PinAp where Pr = total radiated power; Pin = antenna input power;At = TX antenna directive gain; and Ap = antenna power gain. Therefore, the power density at a distance, d, from an antenna is:2 24 4 dA PdEIRPPt rDt t= =Heng Chan;Mohawk College112 AntennaMiscellany p Power captured by the receiving antenna with an effective area, Aeff, is C = PDAeff.Note that Aeff includes the gain and efficiency of the antenna. p Antennas can be linearly, elliptically or circularly polarized depending on their E-field radiated. p Antenna beamwidth is the angular separation between the two half-power points on the major lobe of the antennas plane radiation pattern. p Antenna input impedance, Zin = Ei/Ii Heng Chan;Mohawk College113 Half-WaveDipole Balanced Feedline Symbol /2 Simple and most widely used at f > 2 MHz. Its a resonant antenna since its length is 2 x /4. Zin = 73 O approx.; Zmax = 2500 O approx. at ends Radiation pattern of dipole in free space has two main lobes perpendicular to the antenna axis. Has a gain of about 2.15 dBi Heng Chan;Mohawk College114 Free-SpaceRadiationPatternofDipole Heng Chan;Mohawk College115 Ground& LengthEffectsOnDipole p Since the ground reflects radio waves, it has a significant effect on the radiation pattern and impedance of the half-wave dipole. p Generally speaking, the closer the dipole is to the ground, the more lobes will form and the lower the radiation impedance. p Length also has an effect on the dipole antenna: dipoles shorter than /2 is capacitive while dipoles longer than /2 is inductive. Heng Chan;Mohawk College116 Marconi/MonopoleAntenna Main characteristics: vertical and /4 good ground plane is required omnidirectional in the horizontal plane 3 dBd power gain impedance: about 36O Heng Chan;Mohawk College117 AntennaImpedanceMatching p Antennas should be matched to their feedline for maximum power transfer efficiency by using an LC matching network. p A simple but effective technique for matching a short vertical antenna to a feedline is to increase its electrical length by adding an inductance at its base.This inductance, called a loading coil, cancels the capacitive effect of the antenna. p Another method is to use capacitive loading. Heng Chan;Mohawk College118 AntennaLoading Inductive Loading Capacitive Loading Heng Chan;Mohawk College119 AntennaArrays p Antenna elements can be combined in an array to increase gain and get desired radiation pattern. p Arrays can be classified as broadside or end-fire, according to their direction of maximum radiation. p In a phased array, all elements are fed or driven; i.e. they are connected to the feedline. p Some arrays have only one driven element with several parasitic elements which act to absorb and reradiate power radiated from the driven element. Heng Chan;Mohawk College120 Yagi-UdaArray p More commonly known as the Yagi array, it has one driven element, one reflector, and one or more directors. Radiation pattern Heng Chan;Mohawk College121 CharacteristicsofYagiArray unidirectional radiation pattern (one main lobe, some sidelobes and backlobes) relatively narrow bandwidth since it is resonant 3-element array has a gain of about 7 dBi more directors will increase gain and reduce the beamwidth and feedpoint impedance a folded dipole is generally used for the driven element to widen the bandwidth and increase the feedpoint impedance. Heng Chan;Mohawk College122 FoldedDipole p Often used - alone or with other elements - for TV and FM broadcast receiving antennas because it has a wider bandwidth and four times the feedpoint resistance of a single dipole. 2 Feed line Zin = 288 O Heng Chan;Mohawk College123 Log-Periodic DipoleArray(LPDA) Feed line o L6 L5 L4 L3L2 D6 D5 Direction of main lobe Apex Heng Chan;Mohawk College124 Characteristics ofLPDA feedpoint impedance is a periodic function of log f unidirectional radiation and wide bandwidth shortest element is less than or equal to /2 of highest frequency, while longest element is at least /2 of lowest frequency reasonable gain, but lower than that of Yagi for the same number of elements design parameter, t = L1/L2 = D1/D2 = L2/L3 = . used mainly as HF, VHF, and TV antennas Heng Chan;Mohawk College125 TurnstileArray omnidirectional radiation in the horizontal plane, with horizontal polarization gain of about 3 dB less than that of a single dipole often used for FM broadcast RX and TX Half-wave dipoles fed 90o out-of phase Heng Chan;Mohawk College126 CollinearArray all elements lie along a straight line, fed in phase, and often mounted with main axis vertical result in narrow radiation beam omnidirectional in the horizontal plane when antenna is vertical Half-wave Elements Feed Line Quarter-wave Shorted Stub Heng Chan;Mohawk College127 BroadsideArray all /2 elements are fed in phase and spaced /2 with axis placed vertically, radiation would have a narrow bidirectional horizontal pattern Feed Line Half-wave Dipoles 2 Heng Chan;Mohawk College128 End-FireArray dipole elements are fed 90o out of phase resulting in a narrow unidirectional radiation pattern off theend of the antenna Feed Line 4 RadiationPattern Half-wave Dipoles Heng Chan;Mohawk College129 Non-resonantAntennas p Monopole and dipole antennas are classified as resonant type since they operate efficiently only at frequencies that make their elements close to /2. p Non-resonant antennas do not use dipoles and are usually terminated with a matching load resistor. p They have a broader bandwidth and a radiation pattern that has only one or two main lobes. p Examples of non-resonant antennas are long-wire antennas, vee antennas, and rhombic antennas. Heng Chan;Mohawk College130 LoopAntenna Main characteristics: very small dimensions bidirectional greatest sensitivity in the plane of the loop very wide bandwidth efficient as RX antennawith single or multi-turn loop Feedline Heng Chan;Mohawk College131 HelicalAntenna S D Ground Plane Coaxial Feedline End-fire Helical Antenna broadband (+ 20% of fo) circularly polarized Ap= 15 dB; u-3dB = 20o are typical when S, D, & # of turns increase: Ap increases and u decreases to get higher gain and narrower beamwidth, use an array applications: V/UHF antenna; satellite tracking antenna Heng Chan;Mohawk College132 UHF&MicrowaveAntennas p highly directive and beamwidth of about 1o or less p antenna dimensions >> wavelength of signal p front-to-back ratio of 20 dB or more p utilize parabolic reflector as secondary antenna for high gain p primary feed is either a dipole or horn antenna p use for point-to-point and satellite communications Heng Chan;Mohawk College133 ParabolicReflectorAntenna Power gain and-3 dB beamwidth are: DDAputq70;22 2= =where q = antenna efficiency (0.55 is typical); D = dish diameter (m); and = wavelength (m) Heng Chan;Mohawk College134 Hog-hornAntenna The hog-horn antenna, often used for terrestrial microwave links, integrates the feed horn and a parabolic reflecting surface to provide an obstruction-free path for incoming and outgoing signals. Parabolic Section Feed Horn Heng Chan;Mohawk College135 MicrowaveRadioCommunications p Can be classified as either terrestrial or satellite systems. p Early systems use FDM (frequency division multiplex) technique. p More recent systems use PCM/PSK (pulse code modulation/phase shift keying) technique. p Microwave system capacities range from less than 12 VB (voice-band) channels to > 22,000. p Operate from 24 km to 6,400 km. Heng Chan;Mohawk College136 SimplifiedBlockDiagram Preemphasized Baseband Input FM Modulator Upconverter MixerBPF Ch. Combiner IF Oscillator RF Oscillator RF Out Deemphasized Baseband Output FM Detector Downconverter MixerBPF Ch. Separator RF Oscillator RF In FM Microwave Receiver FM Microwave Transmitter Amp Amp Heng Chan;Mohawk College137 NotesOnFMMicrowaveRadioSystem p Baseband signals may comprise one or more of : Frequency-division-multiplexed voice-band channels Time-division-multiplexed VB channels Broadcast-quality composite video or picturephone Wideband data p IF carrier is typically 70 MHz p Low-index frequency modulation is used p Common microwave frequencies used: 2-, 4-, 6-, 12-, and 14-GHz bands. Heng Chan;Mohawk College138 MicrowaveRadioSystems(contd) p The distance between transmitter and receiver is typically between 24 to 64 km. p Repeaters have to be used for longer distances. p To increase the reliability of microwave links, the following techniques can be used: frequency diversity - two RF carrier frequencies space diversity - two or more antennas are used polarization diversity - vertical and horizontal polarization Heng Chan;Mohawk College139 SystemGain pSystem gain for microwave radio link is: Gs (dB) = Pt - Cmin= Fm + Lp + Lf + Lb - At - Ar where Pt = transmitter output power (dBm) Cmin = min. receiver input power (dBm) Fm=fade margin for a given reliability objective (dB) Lp = free-space path loss between antennas (dB) Lf, Lb = feeder, coupling, & branching losses (dB) At, Ar = Tx and Rx antenna gain respectively (dB) Heng Chan;Mohawk College140 IntroductionToPulsedRadar t PRT Pulse of energy Pulse Repetition Time Pulse repetition frequency, PRF = 1/PRT Range to target, R = ct/2, where c = speed of light, and t = time between TX pulse and echo return. Dead zone, Rdead, and resolution, AR, are both = ct/2. Duty cycle, D = t/PRT Resolution can be improved by pulse compression. Heng Chan;Mohawk College141 RadarPower&RangeEquation Average power, Pa = Ppt(PRF) = Ppt/PRT = PpD where Pp = peak power. Ideal radar range equation: 4 32 2) 4 ( RG PPaRto =where PR = signal power returned (W)G = antenna gain = wavelength of signal (m) o = radar cross section of target (m2) In the real world, losses and noise must be added to above equation. Heng Chan;Mohawk College142 PulsedRadarBlockDiagram Video Amp Video Detector IF Amp LO MixerRF Amp T/R Switch Control Section Modulator Timer Transmitter Signal Processor Display Receiver Section Antenna Heng Chan;Mohawk College143 RadarDisplayModes Range Target Elevation N Beam Sweep Targets E-Scan Plan Position Indicator Heng Chan;Mohawk College144 CWDopplerRadar MicrowaveOscillator Doppler Mixer TX RX fd The Doppler effect can be used fordetermining the speed of a moving target. v = fd/2(m/s) where fd = doppler shift (Hz) = radar wavelength (m) Circulator Basic block diagramof CW Doppler radar Heng Chan;Mohawk College145 FM Doppler Radar Both distance and velocity can be determined if an FM Doppler radar is used. fd+ fd- t fi fo TX RX af f cRd d4) ( + +=Range: Velocity: od dff f cv4) (+ =where a = slope of line or rate of change of fi Heng Chan;Mohawk College146 OpticalFibreCommunications p Advantages over metallic/coaxial cable: much wider bandwidth and practically interference-free lower loss and light weight more resistive to environmental effects safer and easier to install almost impossible to tap into a fibre cable potentially lower in cost over the long term p Disadvantages: higher initial cost in installation & more expensive to repair/maintain Heng Chan;Mohawk College147 OpticalFibreLink Input Signal Coder or Converter Light Source Source-to-fibre Interface Fibre-to-light Interface Light Detector Amplifier/Shaper Decoder Output Fibre-optic Cable Transmitter Receiver Heng Chan;Mohawk College148 TypesOfOpticalFibre Single-mode step-index fibre Multimode step-index fibre Multimode graded-index fibre n1 core n2 cladding no air n2 cladding n1 core Variable n no air Light ray Index porfile Heng Chan;Mohawk College149 ComparisonOfOpticalFibres p Single-mode step-index fibre: minimum signal dispersion; higher TX rate possible difficult to couple light into fibre; highly directive light source (e.g. laser) required; expensive to manufacture p Multimode step-index fibres: inexpensive; easy to couple light into fibre result in higher signal distortion; lower TX rate p Multimode graded-index fibre: intermediate between the other two types of fibres Heng Chan;Mohawk College150 Acceptance Cone & Numerical Aperture n2 cladding n2 cladding n1 core Acceptance Cone Acceptance angle, uc, is the maximum angle in which external light rays may strike the air/fibre interface and still propagate down the fibre with