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_ The 2GHz Limit€r(A10)operatesfrom 100kHz to 2 GHz. lt hasa lineartwo-porttransfercharacteristicof unity (-1 dB) until the input exceeds+5 dBm. Above this point, the internai detEctor OioOesconduct, reflecting part of the RF input energy back to the source.. A: lhe input level rises, the-Limiterreflectsmore signal,.limiting amountthat can passthrough

the

the mixer, thus protectingthe mixer from beingover_ driven.

The 1.gGHz Low-passFitter (FL10) strips the incoming signal of any frequen"y cbmpon"ntsabove

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1.8 GHzto FitterSetectorswiicnSi2"olnion"nt..

The presetector

(FL12)is a 1.7-1gGHzyttrium_ lron-Garnet(ylG)filterthat providesnighsetectivityanO y1S"-jl"qyency rejection. Tuning c-urrent,which is near 500mA at 21 GHz,is provideJby the preselector Driver (A42) circuits. The presele"ior op"rates on bands2, g, 4, and S. Becausethe preselector

is sensi- tive to outputtoadimpedance,a g dB Attenuator(AT11) is insertedbetweenthe pr€s€lectoroutputandone port of the FitterSelectswitchto hetpisotateoutputloading.

1st Mixer

The 1st Mixer {A12)circuitconsistsof a singlebal_ ancedmixer,a coupler,and a g0ophaseshifter. A bal- anced mixer inherentlyhas less conversionloss com_ Par€d to an unbalancedmixer, and local oscillator feedthroughto ths RF port is minimized. The locat oscillator input is split through a broad-bandmulti-sectioncouplerwhoseoutputeare equal in powerbut 9p deoqegsout. of phase. an aOOiitonattiO degree phase shift is cascadedwith the appropriatesignalto create a 180 degree phase differencettrat is ipptied across a pair of series-connected

Schottkydiodes.The result is that the diodesare alternatetyswitcnedon and off as the localoscillatorcycles.

The node betweenthe two diodesis isotatedfrom 11"-,,1^"j!O ^ilpu. by about O0dB so the RF input is apptteoto this node. The blocking capacitorat the input connectorpermits broadbandsignal application from the RF port, whileblockingthe dc'diodebiasfrom g.ettingto. the RF port and the ipectrum analyzerinput. Mixer biasis suppliedfrom the ist LO Driverboardvia the 829MHz lF circuits,4.5 GHzFilter,Diplexet,Direc_ tionalFilter,andTransferSwitch. Biasreturnis through assemblyA11 to ground.

Excludinglossesin the lF filteringcircuitry,the fun_ -Conuener

dam€ntalconversionloss of ths l st is about 14 dB, and the thirdharmonicconversionlossis about 24.d8. The.Schottkydiodes are mountedin a mixer suo-assembty(A12A1)so that they can be easily replaced.

Theoryof Operation- 4g4Al4g4ApServtce,Vot l

1st Local Oscillator

The lst LO (A16) is a ytG (yttrium-tron-Gamet) oscillatorthat has a tuningrangeof 2.072to 6.4GHz. The oscillator assemblyincludesthe interfacecircuit board that couplesoperatingand tuning voltagesfrom the l.st LO Driver, Span Attenuator,and Error Amptifier circuitsto the oscillator.

The +15 Vl voltageprovidesoperatingbias for the p,l"jl1!or.- The_suppty is protected and iecoupted by VRl010,Cl 0t 6, and L1011. Th: s9c9ndsuppty,+t S t2: is not used in this instrument.VRlolg anctVR1019 clamptransientvoltagesfrom the tune voltagecoil. lt atso protects the driving circuits from the transients inducedwhendegaussing.

Whenthe FM coil is used to sweep the osciilator, relay K101Scloses and couptesC1d12 and C1014 across the tune coil, The capacitorstower the noise bandwidthof the main coil drivingcircuit while the FM coil is in operation. The heater provides temperature stability.

Power Divider

The Power Divider(A13)splitsthe output of the 1st LO (YlGosciilator)to isolatethe 1st Mixei from thg tst !O OUjP,qT .front-panetconnector. Basicaily, the Power Divideris two multi-sectiondirectionalcouplersthat are cascadedto producetwo ports havingequal power. The isolationbetweenoutputports is 15 dg or mors at the op€rating frequency. The power Divider alsoprovidesan improvedloadto the localoscillator.

Transfer Switch

The Transfer Switch (S13)is a three-portcoaxial relay that selects 1st lF signals from either the 1st Mixeror from the EXTERNALMIXERinput. This allows the useof an externalmixerby by-passingthe lst Con- verter circuitry. The functionis controlledby circuitry on the RF Interfaceboard. lt is autornaticallyactuated when waveguidebands ars selectedor the front-panel EXTMIXERpush buttonis pressed.

ln Option07 and 08 instruments,the externalmixer capability is detet€d. In those instruments,W125 directlyconnectsthe 1st Mixer outputto the Directional Filter. In Option 07 instruments,the transfer switch selectsbetweenthe 50O and 7SO inputs to feed the StepAttenuator.

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