HP 83522A Service Notes page 184

A4 (ALC) CIRCUIT DESCRIPTION (CHANGE 8) (Cont'd)
Main ALCAmp L
Unleveled Signal
M
Both inputs to integrator U9 are at virtual ground under leveled power conditions, allowing for
immediate response to an input error voltage.
R15 optimizes the speed at which the loop responds to power level changes.
When Model 8350A/B RF BLANK is selected, L RFB goes low during retrace and UlD closes,
pulling current through C4, forging TP5 high and turning on the PIN modulator.
Under unleveled conditions, VR2 and VR3 will clamp the output of U9 at approximately
+
5
and -7 volts, preventing negative or positive saturation. When the output of U9 approaches
-2 volts, comparator U10 activates the front panel LED indicating unleveled power.
U8D is not used.
Collector current in common-base transistor Q1 is exponentially related to the base-emitter
voltage. The PIN modulator is driven exponentially to maintain constant loop gain.
Emitter-follower 42, CR5 and CR4 control the gain of the exponential current drive.
PIN Mod 0 Driver 0
RlOl compensates for the loss of modulator sensitivity with increasing bias current.
412 provides squarewave modulation, when selected.
CHANGE 8