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S
ERV
IC
E
S
HEE
T 2
PR
I
N
CI
PLE
S O
F
O
PER
ATIO
N
General
The RF input p o wer co upled to t he power senso r i s dissipate d by
t
h e
l
oad impe da nce of
t
he power sensin g d evi ce . Th e do output of
t
h e p ower sens ing
d
evice
i
s converte d to α 220 Hz ac si g na l by α
sa mplin g gate (cho pper) circuit. The ac sign a l, whi c h i s propor-
t i on a l to t h e RF input,
i
s a mplifi e d by
t
uned ac amplifi e r s in
t
h e
p ower sensor and Po wer Meter. Th e Synchronou s
D
etector con-
verts the ampli fie d 220 Hz ac si gna l back to α do
l
eve l whi c h also
i
s pr oportiona l to the RF input.
The RAN GE switc h attenuatoκ netwo rk s attenuate t h e ac si gna l
f or higher pow er inpu ts. Thi s allows equa l m easure m ent
r
esolu -
t i on f or hig h and l ow p ow er l eve l s. The Synchronous
D
etector and
α sa mpling gate c i rcui t (in
t
h e power sensor) are driv e n in ph ase
by
t
he 220 Hz Mul tivίbrator.
A
4 U4 B is connecte d as α
v
ol tage
f
ollower between
t
he input
sig nal
g
round and sig nal
g
round . Thi s circuit ensures α min-
imum
v
oltage d if ference exists b etween
t
hegrounds thereby elim-
inating t he
p
ossibility
o
f unreliabl e readin gs. High current flow,
through
t
h e ground r eturn
o
fca bl es which are greater
t
han 5
f
eet
l
on g, cau ses the
v
oltage diff e rence.
F irst Amplifier
The First Amplifier of the
P
ower M eter and
t
he power sensor's
amplifier stage
f
orm α low -noise high-gain hybrid operational
ampli fier (refer to the figure
b
elow). The ac gain is appr oxim ate ly
750 ; d o bias
i
s set by A4R 1,
R
2,
R
6,
R
10 and R11.
Γ
~
P
O
WER SEN
SO
R
SAMPLEDAC
INPUT 220 Hz
1+12
Vd
c
31 .6
_
1070 . IF
1000
PO WER
S
ENSO R CABLE
H
y
b
ri
d
Operational Amplifier
P
O
WER ME
T
ER
- -
3 -ο-- - OUTPU T
SH
A
P
I
N
G
NE
T
W
O
RK
-12 ναε
22.1 Κ
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