Tektronix 494AP service manual A o a o, O o a a o o o, O o o o o o o a o o o a o o o

Theory of Operaton - 494A/4g4ApService,Vol. 1

Sincethe amplifiersare nearlyidentical,the follow- ing descriptionapplies to both amptifiers.The ac and dc signalpathsare treatedseparat€ly. Figures74 and 7-5 are simplifieddiagrams of the ac and dc signal paths.

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'PRINTED COMPONENT

Figure 7-4. Equivalent ac circuit of an B2g MH: amptifier.

fn th€ ac circuit(Figure7-41,C1,L1, andL2 form the input matchingnetwork. (tn the first stag€, L1 is actu- ally the series inductance of dc-blockingcapacitor cl016 at the inputof the amplifier.)Thecoilectorcircuit is matchedto 50o by L4 and c2. To a largeextent,L3 controlsthe gain of the stage. Highfrequencystability is enhancedby Rl and R2.

In the dc circuit (Figure 7-5), negativefeedback through the voltage divider,consistingof RO and R4, sets the collectorvoltage as a fixed proportionof the -12V reference supply. Collector current is deter- mined by R5. Current requirementsfor the first stage are less than the requirementsfor the secondbecause the first stage requires less intermodulationdistortion performance. Diode clamps are provided for each amplifier (CR1013and CR1022 at the bases of the amplifiersin the actualcircuits)to protectthe transistor againstreversebreakdownof the base-emitterjunction in casethe +12 V supptyfaits.

In the actual amplifiers(not shownin Figures7-4 and 7-5)Ll 014 and Cl 014 at the baseof e1017, Cl 0.tg at the collectorof Q1017,L1021andCl023 at the base of Q'1025,and Cl013 at the collectorof Q1017decou- ple the signalpath from the biasnetwork.

The 3 dB pad (R1026,R1027,R102B,and R1029) helpsmaintaina wide-band50O interfacebetweenthe second amplifierstage and the 829MHz Bandpass Filteron the 829 MHz 2nd Converterboard.

Test pointJ1029 at the outputport of the 3 dB pac, is usedfor checkingamplifierperformanceand to aid in adjustrnentof the 929 MHz band-passlilter on the

829MHz2nd Converterboard(A2347).

7-14

+ 1 2V

2?27-1A6A

Figure7-5. Equivalentdc circultot an829MHzamplifiee

829MHz 2nd Converter. Down-conversionlrom 829MHz to 110MHz lF occurs on the 829MHz 2nd Converter board (A23A7). The board contains a 829MHz Band-PassFilter,a 1.3 GHz Low-PassFilter, a 3 dB pad, a 450MHz High-PassFilter, a single- balancedmixer,and a 300MHz Low-PassFilter.

The 829MHz Band-PassFilter blocks unwanted inputs,primarilythe 609MHz image signal. lt consists of four, quarter-wave,coaxialtype resonators,mounted on the 829 MHz 2nd Converterboard. The end resona- tors are tapped near their grounded end to facilitate input and output couplingof the filter. lnter-resonator coupling is provided by printed "through-the-board" capacitorsthat connectb€tweenthe resonatorsat their high impedanceend. A bendabletab is locatedat the high impedanceend of each resonatorfor fine adjust- ment of resonantfrequency.The bendabletab acts as

asmall, variable capacitor to ground, making fine adjustmentsof resonant frequency possible. When properlytuned, the filter presentsat least 12 dB input returnlossandabout2 dB insertionlossat 829MHz.

The 1.3GHz Low-PassFilterblockshightrequency signals that would otherwasebe admitted at the re' entrant frequenciesof the band pass in excess of

2GHz. The functionof the 1.3GHz Low-PassFilteris sharedby the 1.2 GHzLow-PassFilteron the 829MHz Diplexerboard.

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