HP 435B Operating And Service Manual page 87

S
ERV
IC
E
S
HEE
T 3
General
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The input from the Synchronous D etector passes through α Low
P ass Filter before
i
t is amplified by t he D C Amplifier. The output
drives the
M
eter, the Servo Amplifier, and may also b
e coupled
through the RECORDER OUTPUT j
ack to drive 'an external
device such as an χ-y recorder . The gain oft he D C Amplifier is set
by the CAL FACTOR switch .
The Servo Amplifier generates an error voltage ifthe DC Ampli-
fier
o
utput is not ground potential. Without an RF input coupled
to the power sensor, the DC Amplifier output i s very
c
lose to 0 Vdc.
W
hen the ZERO switch is pressed, the Servo Amplifier error
offset v oltage i s coupled to the Auto Zero circuits . The error vol-
tage is processed, attenuated and coupled across the power sen-
sor's power sensing device output as α zeroing correction voltage.
This correction voltage is of
e
qual do
l
evel but opposite pplarity to
the output of the p ower sensing device (no RF input) . With the
corrected input v oltage, the DC Amplifier output is exactly zero.
When the ZERO switch is released, the Servo Amplifier output
voltage is stored within t
he Auto Zero circuits and the correction
voltage r emains coupled across the output of the p ower sensing
device. (Refer to the Auto Zero feedback diagram below.)
PO
WER
S
EN SI N G - I NPUT Ι Ι
_ AC SYN C DC
D EV IC E
A MPL I F I ER A MPL I F I ER S
DETE CTO R AMPL I F I ER
A U TO ZERO
®
~. _
Z ERO (~
1
S
ERV O
A
u
to Zero
F
ee
db
ac
k P
at
h
A MPL I F I ER -'