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C.
DEMODUIAMR CIRCUIT
The demodulator
circuit
employs a four-diode
bridge
circuit
with
silicon
diodes.
A balanced
configuration
is used so that
careful
balance
of the
transformer
secondary
is not necessary.
The demodulator
is driven
synchronously
with
the neon lamps which witch
the input
modulator.
The modulator
output
is a pulsating
dc signal
which is fed through
R119 to the input
grid
of the dc amplifier.
D.
DC AMPLIFIER
-
The dc amplifier
is required
for two reasons.
At the carrier
frequency,
the dc amplifier
is a feed back integrator.
The integrating
capacitor,
C115, is in a local
feedback
loop from output
cathods
to input
grid.
The gain of the dc amplifier
is about 500 so that
the value of Cl15 is
effectively
multiplied
by 500,
eliminating
the need for a large
value
of capacity
for filtering.
At dc, the demodulated
signal
is amplified,
increasing
the loop gain by about 500.
This additional
feedback
results
in a ver.v high input
resistance
for the null-detector.
Exclusive
of the
divider,
the input
resistance
is in excess of 300 megohms on most ranges.
The dc amplifier
circuit
is conventional
and consists
of V3 connected
as
a differential
amplifier,
Vb used as a dc an.7lifier,
and the output
cathode-
follow@r.
Rl28,
DC AMP BAL, adjusts
the balance
of the dc amplifier.
Once set,
this
control
requires
very infrequent
adjustment.
A misadjust-
ment is evident
if,
on the 10~ log range the meter is not exactly
on zero.
E.
THE WHOLE LOOP DESCRIPTION
---
Sections
of the amplifier
are combined.
The input
fil.ter
re-
moves any high frequency
components from the input
signal.
The modulator
converts
the filtered
dc signal
to ac, which is amplified
by the ac amplifier.
The output
of the ac amplifier
is converted
into
a
pulsating
dc signal
filtered
by the dc amplifier
acting
as an integrator,
and further
amplified
by the dc amplifier.
The output
signal
is fed back to the input
by means of ~160 through
R16h
and Rlh9 through
Rl52.
The feedback
is applied
to the low end of PD102,
the modulator
diode.
Applying
feedback
at this
point
not only stabilizes
the gain,
but raises
the input
resistance
Vera substantially.
The sensitivity
is changed from .l millivolt
to 10 millivolts
by changing
the feedback
factor.
An input
divider
is used above 10 millivolts.
The ~*logarithmicfl
ranges are obtained
through
the use of a non-linear
"thyrite"
resistor
Pi166 in the feedback
loop.
?
IV - 2
0763
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