HP 83522A Service Notes page 182

A4 (ALC), CIRCUIT DESCRIPTION (CHANGE 8) (Cont'd)
Sample and Hold Drivers K
Q10 and Qll act as complementary pairs, controlling the Input Sample and Hold, and Error
Sample-and Hold circuits respectively. The complementary pairs improve action of the Sam-
pling FETS Q5 and 4 6 by reducing the error signal passed through gate to source capacitance.
The sample and hold function of the ALC loop is used in conjunction with pulse and square
wave modulation. When L PULSE ENABLE is high, and L PULSE input is low, QlOA and Q11B
turn on causing QlOB and QllA to turn off' , thereby initializing the HOLD mode.
The frequency of the sampling mode is dependent on the L PULSE input. When the system is
used with the HP 8756A Scalar Network Analyzer, the L PULSE input is a 27.8 kHz square
wave, controlling the gates of Q5 (Block
I)
and 4 6 (Block
E).
(Refer to Model 8350A/B Operating
and Service Manual, Section V, for 27.8/1 kHz Oscillator adjustment). The sample level is
maintained during the OFF pulse, thus preventing saturation of the Log and Main ALC
amplifiers.
Input Sample and Hold E
The Input Sample and Hold function prevents the Log Amplifier from saturating during pulse
and squarewave modulation.
U16 is a unity gain follower with internal feedback which buffers the detector input. R78
compensates for the offset voltage of the operational amplifier. Q6 and C21 perform the sample
and hold function.
Power Meter Log Amplifier F
The Power Meter Log Amplifier is used in conjunction with the Log Amplifier in
ALC MODE
[MTR]. The Power Meter Log Amplifier sets the power level and takes care of low frequency
variations, while the Log Amplifier takes care of the high frequency variations.
U5B is
a
unity gain follower which buffers the input of RSD. Logarithmic scaling is performed by
Q3A in the feedback loop of U5D. The baseemitter voltage of Q3A is exponentially related to its
collector current, hence the logarithmic action of the amplifier. Q3B compensates the Log Amp
over temperature. USA is a standard non-inverting amplifier, with its gain controlled by R33
and R32. CR3 prevents oscillation in the Log Amplifier.
Log Amplifier G
The logarithmic scaling function is performed by Q9A in the feedback loop of U17.
Q9A
collector current is proportional to the voltage at TPlO and exponentially related to its base-
emitter voltage. Therefore, Q9A emitter voltage is logarithmically related to the input voltage at
TPlO.
Q9B compensates the Log Amp against changes in reverse saturation current with temperature.
CR9 clamps the output of U18 to 0.6V above the input voltage to U17, preventing oscillations.
CHANGE 8