Tektronix 494AP service manual Theory of Operaton 494l4g4Ap Servlce,Vol

Theory of Operaton - 494[l4g4Ap Servlce,Vol. 1

* *10 dBm

	THS16 dB
LIN	SEGMENT
	MAY BE MOVED
ouT 80 dB Total	TO ANY POSITION
	ON LOG CURVE

dB IN 4416-1t3

Figure 7-12. Selecdon ol display positlon on lhe log scale.

Log modeamplifierU4030tineartyamptifiesthe sig- nal. (fhe log conversiontook placein the Log Amp cir- cuit.) At 2.2V input, the output of amptifierU40S0is 0 V. This is the only voltageat whichthe feedbackcir- cuit switchingnetworkresistorsof amplifierU4030can be switchedwithontchangingthe outputvoltage. The switchingnetworkis describedlaterin this discussion.

When the log mode is selected,the output signal from U4030is appliedthroughFETe5035to the output amplifier. Output Reference Level potentiometer Rl030, in the input circuitto U5030,adjuststhe output level for a full screen display after Input Reference Level potentiometerR1012is set for no changein the output of U4030when switchingbetweenthe 10 dB/div and2 dBldivmodes.

The gain switchingnetworkswitchesresistorsin or out of the fe€dbackpath of amplifierU4090. The net- work consists of Q4035,e4090, e4155, and e4150, and associatedresistors. The FETswitches(controlled

by data bits 0, 1, 2, and 3 from the instrumentdatabus) switch in feedbackresistorslor U4030in combinations determinedby the four data bits.

Linear Mode Circuits

The linear mode circuits acceptthe logarithmically scaled output from U4050and rescalethe signallevel to linearvalues. Sincethe input signalsare logarithmi- cally scaled,the signallevelis exponentiatedto operate the systemin the linearmod€. Highgain is requiredat the top of the screenand low gain is requiredat the bottom of the screento offset the characteristicsof the Log Amplifiercircuits.

In th€ linear mode, FET switch Q5150 is on. ena- bling the linear mode signal path; and Q5035 is off, disabling the log mode path. The output from preamplifierU4050is also appliedto linear modeopera- tional amplifierU4070,with a successiveresistornet- work switched into the feedback path. From this

amplifier,the output signal is applied through FET Q5150 to the summing node at the input of output amplifierU5030.

Witha *6 dBm inputto th€ Vid€oAmplifier,the out- put of U4070is 0 V. This is the level that represents the top ot the screen. At that level,the foedbackpath is only through resistors R4118 and R5112. Diode CR4125and R4122 areonlyactivawhen neededto limit negative excursions. The other feedback resistors (switchtransistoremitt€r r€sistors)ar€ not in the path, becausethe switch transistorsare biased off by the dividernetworkat the transistorbases.

As the displaymovesawayfrom full screen,th€ out- put voltage of U4070 increasesand turns transistor O4120on. This places R4124in parallelwith the tixed feedbackresistors,thus reducingthe gain. As the vol- tage output increases,transistors04125, Q4065.and 04060 start to conduct in sequence, adding their emitter resistors across the feedback path. This effectivelyreduces the gain of U4070 exponentially. The transistor characteristicssmooth the step transi- tions, producing a smooth exponential gain curve.

Diode 'l providesternperaturecompensationfor CR41 5

the switchingtransistors.The Lin Mode Balancecon' trol, R1025,sets the U4070output level to matchthe log modeoutput.

Pulse Stretch Circuit

The pulse stretch circuit consists of FET switch Q5026 and the associated components. When the pulse stretch mode is not sglect€d{data bit 7 on the instrum€ntdata bus is low), pin 13 of U4020Cpulls down to -15 V, and Q5026 biases off. This removes C5024from the circuitand also suppliessufficientnega- tive biasto keep CR5025fonvardbiased. WithCR5025 on, the feedbackloop for U5030,through Q5025and R5033is closed so the signaloutputwill fall as fast as its rise.

when the pulse stretchmodeis selected(databit 7 high),the open collectoroutput of u4020c (pin 13) is allowedto float. This turns Q5026on which completes the path for C5024to ground. Duringsignalrise time, C5024chargesthroughthe low impedanceof CR5025. The feedbackpath for U5030is still closed whichpro- videsa fast risetime.

When the output of U5030 begins to fall, CR5025 turnsoff and the signalfall time is now a functionof the RC time constantof R5031and C5024.sincethe f€ed-

7-34