HP 435B Service Manual

SERV IC E SHEE T 3 (Cont'd)
DC Amplifier
The input to the
D
C
Amplifier is filtered by α two-stage L ow
Pass
Filter A4R29
and C17 ; R 30 and C18 . On the three m ost sensitive
r
anges
additional filtering i s introduced by components which
a re
m
ounted on
the Α 1 Switch Assembly .
The D
C
Offset control A4R32 i s set to eliminate any do offset
v oltage
introduced by t he D
C
Amplifier
.
The
gain
of t he D
C
Amplifier is
controlled
by A4R38, A 4R33
and
AlR
1 -15 . The gain
is v ariable
from
a pproximately 5 .3 to 6 .2
in
15
one-percent steps
as
determined by t
he
CAL
FACTOR
switch . The CAL FACTOR
s witch setting
i
s d ependent on the
frequency response of t he
p ower
sensing
d evice (refer to the chart on the p
ower sensor case) .
The D C Amplifier drives the M eter, Servo Amplifier a nd an
external instrument through t he r ear panel RECORDER O UT-
PUT j ack . The M eter control, A 4R35, i s u sed to calibrate the m eter
with α known input . Thermistor A4RT1 compensates f or c hanges
in sensitivity o f t he m eter due to temperature . Diodes CRl1 and
C R12 at the output of t he D C Amplifier, U 5, prevent the m eter
needle from being d amaged if e xcess p ower is applied to the
m eter .
Servo Amplifier
The DC
Amplifier
output i s coupled to A 4R39, theServo Amplifier
input . B ecause of t he high do gain (=7000) α s mall do output from
t he D C Amplifier U 5 produces α l arge error voltage at the Servo
Amplifier U6 output . When t he ZERO switch is pressed, this error
voltage i s coupled to the Auto Z ero circuit .
Capacitor A4C21 with R43 gives the Servo Amplifier t he charac-
teristics of α l ow p ass filter . The Auto Z ero Offset Control A4R42
i s set to r emove any do offset v oltage introduced by t he Servo
Amplifier .
AutoZero Circuit
When t he front p anel ZERO switch S2 i s pressed, A 4Q17 i s turned
on, the collector voltage goes positive which places αd o v oltage
across r elays
Α
4 Κ 1 and Α 4 Α 1 Κ 1 . The RF BLANKING OUTPUT
is now coupled to ground by Α 4 Κ 1 and t he Servo Amplifier error
voltage
i
s coupled to
A
4A1Q1 and A4A1C1 by
Α
4
Α
1
Κ
1 .
The error voltage from t he Servo Amplifier biases Q1 which pro-
duces an equivalent error voltage at Q1 source . This v oltage i s
attenuated by A4A1R2, A 4A1R4and A4R74 . The v oltage i s
further attenuated in t he p ower sensor a nd is coupled across the
ambient temperature d o output of t he p ower
sensing d
evice as α
correction voltage . The algebraic sum o
f
t he d o v oltages
i
s
ampli-
fied
a
nd coupled
back to
the Auto Z
ero
input
.
B
ecause the
f
eed-
ack
loop
i s α n egative p ath, the correction voltage across the
p ower sensingd evice output b egins to change and continues to d o

HP 435B Specifications

General

Brand	HP
Model	435B
Category	Measuring Instruments
Language	English