Tektronix 070-8462-03, Instruction Manual

Page: 84 / 271

Theory of Operation
1735 HD High Definition Waveform Monitor
4–5
The effect of the DC feedback path is to supply bias current for U52, and to short
circuit DC block capacitor C239. The high frequency feedback path (R476,
R477, and R478) provides a video gain of +1.12 from U52 pin 3 to U52 pin 6,
and a clamp gain of –0.12 from clamp output (U49B pin 7) to U52 pin 6. The
DC feedback path (R450, R448, and R474) provides unity gain from U52 pin 3
to U43B pin 5, and subtracts the clamp output. In the DC-coupled mode, U43B’s
gain is unity, and integrator U43A delivers bias current through R509 to U52 pin
3, so that the voltage at U43B pin 5 tracks the voltage at the video input. At
higher frequencies, the DC feedback loop, working through R531, cannot affect
the voltage across DC block capacitor C239, and relinquishes control of U52 to
the high frequency feedback path. The DC feedback match to unity gain, and
C139, C209, C195, R391, and C182 help to smooth this transition.
In the AC-coupled mode, the integrator at U43B is enabled, so the DC feedback
path forces a DC average of zero at U43B, pin 5. The integration time constant
(0.75 sec) is determined by R333 and C154. At higher frequencies, the gain
through U43B is unity, as it was in the DC-coupled mode, so the gain is
unaffected for frequencies above frame rate.
The back porch clamp is used to restore the DC level of AC-coupled video
signals. The integrate/hold amplifier U49B integrates during during the back
porch sample time, charging its hold capacitor (C125), and moving the back
porch voltage level of the video output (U52 pin 6) toward zero. An additional
hold capacitor (C141) is switched in for slow clamp mode. The high-frequency
feedback and DC feedback paths are scaled to subtract the clamp output (U39B
pin 7) and the video output (U52 pin 6), so that the clamp does not affect the
operation of the DC feedback loop.
At high frequencies where the DC feedback loop is inactive, R532, R509 and the
input impedance of U52 pin 3 appear as resistance to ground. At low frequen-
cies, the DC feedback loop makes the input resistance infinite, and is therefore
unstable without a resistor to ground. A single resistor would be sufficient for
DC feedback loop stability, but resistors R531, R506, C238, R533, R508, and
C125 are scaled to provide a broadband constant input resistance (approximately
18K). C238 effectively un-grounds the 18K formed by this network at frequen-
cies where the DC feedback loop is inactive. C226 cancels the effect of the
clamp.
Input Relays.
The high-input impedance of the input amplifiers (U52 for CH 1A)
is lost when the power is turned off. Comparator U8A turns off the channel input
relays (K1 for CH 1A) whenever it detects low power supply voltage. When
power is switched off, the relays disconnect the video input amplifiers from the
rear-panel video loop-through input, so video routed through the 1735 HD is not
affected. These relays are also used to provide a ground level check controlled
from the front panel. To reduce displayed crosstalk, serial register U59 switches
the relays to disconnect undisplayed input channels.