HP 435B - Service Sheet 3

S ERV IC E S HEE T 3
General
Γ
PR I N CI PLE S O F O PER ATIO N
The input
from t he Synchronous D
etector passes through α L ow
P
ass Filter before i t i s amplified by t
he
D
C Amplifier . The output
drives
the M eter, the Servo Amplifier,
and may a lso b e coupled
t hrough
t he RECORDER O UTPUT j
ack to drive 'an external
device
such as an χ -y recorder . The gain of t
he
D
C Amplifier is set
by t he CAL FACTOR switch .
The
Servo Amplifier generates
an
error
voltage if t he D C Ampli-
fier o
utput i s n ot ground potential . Without
an RF input coupled
to the
p ower sensor, the D C Amplifier o utput i
s v ery c lose to 0 Vdc .
W
hen
t he ZERO switch is pressed, the
Servo Amplifier e rror
o ffset v
oltage
i
s coupled to the Auto Z ero circuits
. The error vol-
tage i s processed,
attenuated a nd coupled across the
p ower sen-
sor's p ower sensing
d evice output as αz eroing correction
voltage .
This correction
voltage i s of e qual do l evel but opposite
p plarity to
the output of t he p
ower sensing d evice
(no
RF
input) . With the
corrected input v
oltage, the DC Amplifier o utput i s
exactly zero .
When t he
ZERO switch is r eleased, the Servo Amplifier
o utput
v oltage i s
stored within t he Auto Z ero circuits
and t he correction
voltage r
emains coupled across the output of t he p
ower sensing
d evice . (Refer
to the Auto Z ero f eedback diagram below .)
PO WER
S EN SI N G - I NPU T

Ι

Ι

_ AC

SYNC

DC
D
E V IC E

AMPL I F I ER

A MPL I F I ER S
DET E CTO R A MPL I F I ER
A U TO ZERO
®
~ .

_
Z ER O (~

1

SERV O
A u to Zero F ee db ac k P at h
A
MPL
I F I ER
-'