Tektronix 494AP service manual A o o o O o o o O a o, A o o o o o o, 110MHzFTLTERS Diagram17

Theoryof Operaton - 494A/4gdApService,Vot.1

CRl029 addedfor maximumattenuation.

lf Rl015 is set higher(nearer+i 5 V),diodeCR3030 is forward biased and starts to conduct. Since the

2mA supplycurrent is relativelyconstant,this subtracts fromthe currentthroughCRl02g. Thus,the impedance of the diodes is relativelyconstant,which resultsin a good impedance match over a broad range. The RF signal path is determinedby the exact amount of curent throughCR3030;part of the RF signalpath is through CR3030 to th€ output amptifierind part is through R2032 and diode CRlO2g to ground. This resultsin reducedsignalattenuation.

lf R1015is set to the positivetimit,the entire2 mA flows through CR3030. This routes the RF signal through CR3030 (which exhibits titil€ resistance with high current) to the output amplifierwith almost no att€nuation.Cl-heinsertionloss is approximatelyI dB.)

the adiustableattenuator,the signalis applied _From

to the linal amplifier09018. This stagets a broad'-6andfeedback amplifierthat supplies relativelysubstantial outputcurrentand exhibitsgood intermodulationdistor_ tion performance.Thisis providedprimarilythroughthe

large curr^ent by negativefeedbackthiough

.capacity,

resistorR3014,and emitterdegenerationthroughresls- tor R4029. Theseresistorsare sizedto providea r€a- sonablygood impedancematch at 110MHz. Nominal gainof the stageis 13 dB.

With Gain potentiomet€rRi0i5 set for maximum gain (least attenuation),the gain of the 110MHz tF Amplifieris approximatety26dB to 27d8. R.t01Sis normallyadjustedfor totalgainof 21 dB.

Th€ outputsignalfrom the 110MHz tF Amplifieris appliedthroughthe 110MHz band-passfitterFL36and low-passfilterFL37to the 3rd Converter.

110MHzFTLTERS (Diagram17)

The 110MHz band-passfitter(FLg6)determin€sthewidest resolutionbandwidthof the analyzer,provides i-I1g-g-rejection to prevent the mixer fiom producing 10 MHz outputsfrom inputsignalsof 90 MHz,and also limitsthe noisespectrumthat appearsat th€ 1OMHz lF circuits. The low-passfilter (FL3Z)that followsturther reducesharmonicsand spursfrom the 2nd Converters. Bothfiltersare sealedunitswith no internalservicing.

The band-passfilter consistsof four helicalresona- tors that are tuned with multi-turntrimmercapacitors. For purposesof impedancematching,the fittei is sym_ metrical.

7-20

Adjustmentof the filter for minimurnattenuationis performedby settingthe trimmercapacitors. Insertion loss is approximately4 dB to 4.5 dB. From this filter. the 110 MHz signalis appliedto the separatelow-pass filter. There is no adjustment for the low-pass filter. The signalthenfeedsthe 3rd Converterboard.

3rd CONVERTER(Diagram17)

The 3rd Gonverterconsists of a 100MHz crystat oscillatorand a mixer. lt outputsth€ 3rd lF of 10 MHz, for the Variable Resolution(VR) circuits, and a stable 100MHz referencefor other circuits within th€ instru- ment.

100 MHz Oscillator

AColpitts oscillator is formed by Q2038, yg03g, L1041, C1038, and related compon€nts. Y3038 is a 100MHz crystal that op€rates in a series resonant modein the fEedbackloop of the oscillator.The oscilla- tor output couples through C2042 to differential amplitier4204402441. The two separateoutputs of approximately2V peak-to-peakamplitudego to three hybrids(mixerU3051,distributionamplifierU30il1.and cafibratorU2022)on the 3rd Converterboard.

lnductor L3041,varactor diode CR3039,and crystal Y3038 form a series resonator that tunes th€ oscillator approximately*1 kHz. The RPL VOLTS TUNE tine varies from 0V to +12V, changingCR3039'scapacL tanceto phaselock the oscillatorto the ReferenceLock source. RPL GND is tied to ground in the Reference Lockmodule.

Mixer

At mixEr U3051, 100 MHz enters on pin 2 and is amplifiedto drive a ring diode mixer. 110MHz enters on pin 10 and is mixedwith the 100MHzto yieldmixing productsat 10 MHz and 90 MHz. The 1OMHz signal pass€sthrougha low-passfilter and is sentto the Vari- able Resolution Input circuat, while the unwanted 90 MHz signalis terminatedwithinthe mixer,

Distribution Amplilier

U3031 distributes a 100MHz signal to other modulesin the instrument. The input levelon pin 2 is typically 2 V peak-to-peak,while the output level is 0 dBm intoa 50 ohm load.

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