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--.The.assemblyconsistsof four subassemblies:1st Mix-er(A69A't),Bandpass Fitter (A69A2),2nd Mixer (A69A9), and Local Osciilator (A69i4). These subass€mbliesare on individualciriuit boards con- tained in shieldedcompartmentswithin a metal cas€ mountedaboveth€ crt.

lst Mlrer. The lst Mixer (A6gA1)is the input
mixer. lt converts the l0 MHz lF signal down to
250kHz. An inputfilter reducesthe signil skirtsbefore
goingto the mixer.
The buffered
lator (LO)mixes with	9.ZSi4Hz LocalOscil-
	19 MHz signat,producing		the
250kHzmixeroutput lhe	-bandpass
that drivesttre		filter.

-The input signal consists of the 10MHz lF signal with a dc controlsignalalso ridingon the line. TtreOc control signal selects the 10 Hz or 100Hz lilter. The 10 MHz signal couples through a capacitor and transformerto feed the input filter. The dc control sig- nal feeds througha resistor and capacitorto isolatettie 10Ir4Hzsignaland providea cleandc signalto the filter switcheson the BandpassFitterboard(469A2).

Input filter Y2025is a monolithicsecond-ordercry-stal fflt€r with a i kHz bandwidth. The tilter limits tarje signals outside the bandpass before they enter the mixer. This reduces th€ intermodulationOistortion (lMD)in the circuit. The l0 MHz signaldriv€sthe fitter at about-20dBm signatlevet.

Thefilter driveshigh levelmixerUSO20,builtfrom a monolithicring of MOSFETswitches. Differentiatpair 05020 and Q5025 buffers the g.7SMHz LO signal and drivesthe mixer at the LO input. The bufferprovides oppositepolarityhigh amplitudesquarewavesto drive the mixerdifferentialLO inputs. The squarewavehelps providelow mixer lMD. A potentiomet€r

adjustsrnixer bias to help providelow IMD and low mixer insertion loss. Theadjustmentis madefor bestconversiongain.

The mainpowersuppliesalso enterthis board. The 117V power suppty is re-regutatedto +1SV to avoid loadingthe existing +.1SV suppty. The _15 V suppty acts as the referencesupply for th€ +1SV regutaior whichconsistsof US010B,e5010,and eS015.hesis- tors R5125and R5'l27act as a voltagedividerto set the outputvoltage.

Bandpass Fitter. The 250kHz signatproducedby -ampiified

the previous mixer is filtered andin the BandpassFilter board (A69A2). The filter consistsof thr€e nearly identical stages. Bandwidthis changed from 10 Hz to 100 Hz with transistorswitchesin each ijaS.". The controlsignalfor this switchingcomesfrom the lst Mixerboard(A69Al).

Theory of Operalion - 4g4A/lg4Ap Service,Vol. 1

The filtErsectionsconsistof crystalsin serieswith the signal path. Each crystal is driven from a low impedancesource. Resistive loading then sets the bandwidthof each section.Ttre switihing transistors connectsmallerresistorsin parallelwith the filter load resistorsto reducethe bandwidth.

Each crystal is embeddedin a balancednetwork. The balancEadjustmentcompensatesfor the effects of crystal parallelcapacitance.This improvesfitter stop- bandattenuationand shapesymmetry. each stagealio containsa variablecapacitorin serieswith theLrystal to provide a fine frequency adjustment with a tuning rangeof about 15 Hz tor each stage.

_ lach amplifierconsistsof a feedbackcircuitusinga JFET and a PNP, providingvoltage gain of about trio. The stageis completedby drivingan emitterfollowerto providethe low output impedanceneEdedto drive the nextcrystalor the outputmixer(2nctMixer). Thevatues of the f€edbackresistorand the input resistorsdeter_ minethe stagegain.

The 250 kHz signal from the mixer drives a smatl attenuatorand the transformerTgoi 6. The attenuator terminatesthe mixer and transformers. The balanced output from T3016 containscrystal y2o2o and the Fre_ guencyadjust capacitorson one side and the Balance adjustmentcapacitoron the other side. Load resistor R4163 terminatesthe crystal network When 10Hz bandwidthis selected,Q4159turns on, ptacingR9162 in parallelwith load resistorR4l69. This raisesthe e and reducesthe bandwidth. lt also increasethe inser- tion loss, so Q2159 also switcheson, placingR3162in parallelwith R4027to increasethe gain and overcome the addedinsertionloss.

Seriesnetwork R2145and C2150 provide positive feedbackfor the amplifier.This looks inductiveand so compensatesfor the capacitancean the impedance seen at the amplifierinput. CapacitorCg14g,across feedback resistor R3160, rolls ofr the amplifiergain above250 kHzto prevent10 Mhz feedthrough.

Emitterfollower Q4145provides a low impedance drivefor the next filterstage.

The secondstag€filterusesQ4140to invertthe sig- nal for BalanceadjustmentC4045. Otherthanthat, the operationof this stageis identicalto the first. The third stageis identicalto the second.

2nd Mker. The 2nd Mixer (A694A)convertsthe filteredsignal trom 250kHz back up to 10 MHz. The mixer in this circuit is a MOSFETring (U5022)like that usedin the 1st Mixer stage. Less LO voltageis needed becauseIMD requirementsare less stringentin this stage. A potentiometeradjusts mixer gate bias. The adjustmentis madefor bestconversiongain.

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