HP 435B - Service Sheet 2

S ERV IC E S HEE T 2
PR I N CI PLE S O F O PER ATIO N
General
Th e RF inpu t po w e r couple d to t h e po w e r se n so
r i
s di ss ip ate
d
by
t
h
e
l
oa dimp e da n ce of t h e p o w er se n s in
gd
e vi
ce
.
Th
e d oou t pu t o f
t
h
e p owe r se n s in g d e vi ce i s co nv er te
d
to
α
220
Hz ac
s i
g
n a
l
by α
sa mplin g gate (c h o pp e r) ci r c ui t . Th e ac
s i gn
a
l, w hi
c
h i
s propo r-
t
i
o n a
l
to t
h
e RF inpu t, i s a
mplifi
e
d
by
t un e d
ac
a mplifi
e
r
s in t
h
e
powe r se n so r andP o w e r M ete r . Th
e S yn c hr o n o u
s
D
etecto r con-
verts t h e a mpli
fie
d 220 Hz ac s i gn a
l
back to αd o
l
e v e
l whi
c
h
al so
i s pro p ort i o na
l
to t h e RF inpu t .
Th e RAN GE sw itc h atte nu ato κ n etwo
rk
s atte nu ate t
h
e ac s i g n a
l
f
or higher p o w er inpu ts .
Thi
s allows e qu a
l m
eas ur e
m
e n t resolu
-
t
i
o n
f
o r hi g
h
and
l
ow po w e r
l
e v e
l
s . Th e Syn c hro n ou s
D
etecto r and
α sa
mplin g
gate c
i
rcui t
(in t h
e
p owe r se n so r ) are driv
e
n
in
ph
ase
by t h e 220 Hz Mul t iv ί br ator .
A 4 U 4 B i s co nn ecte d as α vo l tage f o ll owe r b etwee n
t
h e inpu t
s i g n a l g r o und and si g n a l g r o und . Thi s c ir c ui t e n s ur es α
min-
imum v o l tage d
if
f e re n ce ex i sts
b
etwee n t h e g ro und s t h e r e by elim-
in at ing
t
h e p oss ibili t y o
f
unr e li a bl
e
r ea din gs . High curre n t flo w ,
t hr o ugh
t
h
e
g r o und r et urn of ca
bl
es wh ic h ar e g r eate r t h a n
5
f eet
l o n g, ca u ses t h e vo l tage diff
e
re n ce .
F
irst
Amplifier
Th e Fir st Ampli fie r
of
t h e Po w e r M ete r and t h e p owe r se n so r 's
amplifie r stage f o rm α lo w - n o i se hi gh -ga in hybrid operational
a mpli fie r (r e fe r to t h e fig ur e b e l o w) . Th eac ga in i s a ppr o xim ate ly
750 ; d o bi as i s set by A 4 R 1, R 2, R 6, R 10 a nd R 11 .
Γ

~
PO WER S EN SO R
SA MPLE D AC
I NPU T 220 Hz
1+12 Vd c
31 .6 _
1070 . IF
1000
PO WER S EN SO R CABLE
H y
b
ri
d Operational Amplifier
PO WER ME T ER
- -

3 - ο -- - OU T PU T
SH A P I N G
NE T W O RK
-12 ναε
22 .1 Κ