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Model
339A
Section
VIII
indicates
the
rms
value
of
the
input
signal
(voltmeter
function).
RELATIVE LEVEL
-
The
relative level
function
permits
the user to
measure
the
rms
value of
the
input
signal relative to a pre-set
reference
(dB and
VU
measurements).
8-25.
In
addition
to
the
Distortion
ANalyzer
input, the
339A
also includes
an
AM
DETECTOR
INPUT
which
detects the
AM
modulation
signal of
an
RF
carrier.
This
allows
the user
to
measure
the
total
harmonic
distortion
of
the
modulation
signal.
8-26.
Input Amplifier.
The
339A
input
amplifier
is
an
operational
^Plplifier
circuit
which
uses a
combination
of attenuation
and
gain
to limit
the
full-scale
output
of the amplifier
to
3
V
rms. Figure 8-2
shows
a simplified
schematic
of the
input amplifier
and
lists
the attenuation
and
gain
for
each
INPUT
RANGE
setting.
The
output
signal
of the buffer
amplifier
is
applied
to
the
fundamental
rejection
circuit
(notch
filter)
and
auto
set-level circuit
of the
analyzer
section.
The
voltmeter attenuator provides
the necessary-
attenuation
to
maintain
a
10
mV
rms
full-scale
output
signal
to
the
voltmeter buffer
amplifier.
8-28.
Input
Overload
Protection.
8-29.
The
input
amplifier
is
protected
from
the
application of high voltage
to
the input
by
a
zener
referenced
protection
circuit
which
limits
the input
voltage
to
approximately
1
1
volts
peak.
The
input
is
further protected
by
a
fuse
which
limits
the input current
to
approximately
60
mA.
During normal
operation,
FET
A3Q100
supplies
a
feedback
signal
which
is
equal
in
phase
and
amplitude
to
the input signal
to
eliminate
leakage caused by
the
capacitance of
the
protection
diodes.
8-30.
Analyzer
Circuitry.
8-31.
Notch
Filter.
8-32.
The
purpose
of the
Notch
Filter
is
to
eliminate
the
fundamental
frequency
of the signal
being measured.
The
basic
notch
filter
circuit,
as
shown
in
Figure
8-3,
is
a
"bridged
T"
RC
filter
network.
The
filter
is
tuned
to
approximately
the
fundamental
frequency of
the input
signal
by
the front
panel
FREQUENCY
controls
and
is
fine
tuned
to
the
exact
frequency by
the
phase
control
circuit.
The
notch
filter
by
itself
attenuates
the
fundamental
frequency only about
-16
dB.
To
improve
the
"notch"
characteristics,
a
portion of
the input signal
is
"fed-forward"
and
algebraically
summed
with
the
output
of
the
notch
filter
by
notch
amplifier
A3U3. the
Application
of
the
feed-forward
signal
cancels
the
remaining fundamental
signal.
The
correct
level
of
feed-forward
signal
necessary
to
cancel the
fundamental
frequency
is
regulated
by
the
amplitude
control
circuit.
The
combination
of
feed-forward
signal
and
the
automatic frequency
tuning
provided
by
the
phase
control
circuit
improves
the
"notch depth"
to
8-3
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