Tektronix 1710J Series, Instruction Manual

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Theory of Operation
5–4
1710J-Series Waveform Monitors
An accurate 100 kHz waveform from the microcontroller is amplified and its
amplitude set and controlled by the calibrator. Calibrator output is enabled and
output through the vertical amplifier low pass filter. The calibrator enable is also
generated by the microprocessor.
The rear-panel Channel A and B inputs are high-impedance bridging loop-
through inputs compensated for use in 75
W
systems. Each amplifier has its own
DC restorer that is turned on or off by repositioning a plug jumper (See Diagram
5). Restorers are either both on or off.
Because the Channel A and Channel B input amplifiers are identical, circuit
numbers for the Channel A amplifier are used to simplify the remainder of this
discussion.
The input amplifiers are inverting feedback operational amplifiers with a gain
of –1. The input resistor (R
i
) is R196 and the feedback resistor (R
f
) is R198. A
plug and jumper is provided to select input coupling. J197 is factory set to the
1-2 position for ac coupling; it can however, be moved to the 2-3 position to
provide dc coupling by bypassing C197, the ac-coupling capacitor.
The DC restorer is a feedback sample-and-hold circuit. Sampling occurs when
U395A closes at back porch time to charge the hold cap (C398) When the
restorer is selected U395D closes and the loop-compensated buffer amplifier
(U495A) drives the input amplifier input summing junction through R197. The
time constant of the restorer does not attenuate 50/60 Hz hum by more that 10%.
The input amplifier output signals drive the channel switch, U492. The signal
selection is determined by the level of the CH-B signal at U492 (pin 10). When
CH-B is low, channel B is selected; when it is high, channel A is selected. The
channel switch output (pin 6) drives a current mirror with three current sources.
Q792 is the current source for the internal sync signal and Q791 is the current
source for the remainder of the vertical. With a 1 V input signal there will be
1.11 mA of signal current flowing through R392 (or R393) into the channel
switch. This signal current is available to drive the vertical through Q791.
Only 0.5 mA of signal current is available to drive the sync stripper through
Q792. Its emitter resistor, R694, is twice the resistance of R792 and R693, the
emitter resistance for Q790.
The external sync signal from the rear-panel EXT REF loop-through is buffered
by an operational amplifier consisting of U795A and B. It has a gain of –1,
which is determined by the combination of input resistor (R
i
) R997 and feedback
resistor (R
f
) R898. The operational amplifier output drives Q798, which is one
current source for the source switch (U795D).
Input Amplifiers
Channel Switch
External Sync Input and
Source Switch