Tektronix 494AP service manual O o O o o

Theory of Operation- 494A/494ApServlce,Vol. 1

After mixing,the l0MHz output signalis fittered with a second monotithic crystat fitter (y2020). This filter is importantto the systemoperation.Withoutit, a large signalat 9.75MHz woutdbe presentin the wide_ bandVR amplifiersthatfoltow.

Local Osclllator. The Local Oscillator assembly (469A4)providesthe 9.75MHz squarewaveLO signali needed for mixing. This is derivedfrom a 19.SMHz 9ry_sFloscillator by using a duat D-type flip-flop in divide-by-twocircuits. Since both sections are avail_ able, one is used for each mixer. This providesexcel- lent isolataon.One outputdrivesthe bufferamptifierto !he.11 Mixer (A69At),and the otherdir€cflydrivesthe 2nd Mixer (A69A3). A +5 V regutatoron this board powers the flip-flops.

10 dB Gain Steps (Diagram 20)

The 10 dB GainStepscircuitprovidessystemgain,

a10 Hz gain adiustment,a 10 dB switchabtigain itep, and the front-paneloverall gain (AMpL CAL) control. The circuit consists of three stages of amplification. The nominalinput signallevelfrom the 1st FilterSelect carcuit is -25dBm for a resolution bandwidth of 100kHz. (All levelslisted in this descriptionrelateto the 100 kHz resolution.)

The input signat is apptied through impedance transformerT4019to the first amplifierstageconsisting of a differentialpair, 03016 and e2027, drivingemitter follower 01036. The signalfeeds back to the base of Q2027throughdividerR2034and R20S1.Signatoutpui resistor R2035presentsapproximatetyS0ohms output impedanceto the next stage.

Gainof th€ inputstageis the samefor all resolution bandwidths€xcept 10H2. When 10Hz is selected, Q2015connects10 Hz LevelcontrolR2025and R3029 acrossR2031.

Tle lst stage output drivescommonemitterstage 02043. Gain of this stage changesby +10 dB when Q4039is switchedon. Data bit 0 froni the gain steps decoder circuit on the VR Mother board #2 {A6gA2) controls this galn step. When the bit is high, emitter resistor R2048sets the stage gain. When low. e40gg saturatesand shuntsR2049with R3039and 10 dB Gain adjustmentR3035. This increasesthe stage gain by 10dB.

The output of Q2043drives the input of the third amplifierstage. This stage operatesthe same as the first stage except the gain is adjustableby the front panel AMPL CAL screwdriver control. plN diode CR1053and resistorR1056shunt resistorFt1060to

7-26

control the gain of this stage. The AMPL CAL control biases CR1053. The amount of current through the diode determinesits high-frequencyresistance.As the current through the diode increases.the resistance decreasesand the gain of the stage increases.Gain rangeis approximately14 dB.

Outputimpedanceof the stageis 50 ohmsas set by resistor R1064.Nominaloutput level is -1dBm for a full screen display. This level may be as high as

+9 dBm when MIN NOISEis active. In the MtN NOTSE mode, '10

dB of att€nuationis removedfrom the instru- mentinputstep attenuator.vR Inputsignalsare higher. H€nce,10 dB of gainis removedfrom th€ VR.

20 dB Gain Steps Circuit (Diagram 20)

This circuit provides gains of -8dB, +2 dB, +12 dB, and+22 dB in precise10 dB steps. The nomi- nal -1dBm input is suppliedthroughpin P from the 10 dB Gain Steps circuit. This signal is appliedto a chain of three amplifiers,each usingemitterdegenera- tion. A changeof the emitter resistancechangesthe amplifiergain. The gain step decoderon the VR Mother board #2 suppliesthe switchingsignalsthat s€lectthe amplifiergain. Theseampliftersare similarto the 10 dB Gain Step amplifier previously described. On this board, the first two amplifiersare cascadedfor the 20 dB step and the third amplifierprovidesthe addi- tional10 dB step.

The nominalgain of the compl€tecircuit is -8dB, with th€ gain steps switched off. This providesa nomi- nal -9dBm output. In this condition.controlpinsV and Y are high,biasingQ2018,Q204{ andQ1062off.

For the 20 dB gain step. 02018 and Q2042turn on (pinV is low), increasing the gain of the first two ampfifiersby 10 dB each,for a 2OdB gainstep. Poten- tiometer A2029 (20 dB Gain) adjusts the first stage (O1025)gain shift while the secondstage(Q1035)gain shift is fixed at about +10 dB. The adjustmentallows settingthe gain step to exactly+20 dB.

For the 10 dB step, pin Y is low, saturatingQ1062. This raises the gain of the third amplifier(Q1043)by 10 dB, as set by R2060.

Gain of the 20 dB and 10 dB gain step circuitsis controffed by data bits 0, 1, and 2. Datais latchedon the output of decoder U3017on the VR Motherboard #2. When the bits are high. transistor04035, 03035, and Q4037switch on. The resultantlow out turns on the respectivegain step circuit. Table 74 shows the stateof bits 2, 1, and 0 and the gainshiftsobtained.

The output signalfrom the 20 dB GainStepscircuit is appliedthrougha coaxialcableto the VR BandLevel- ing circuit.

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