Model 333A/334A
Section IV
SECTION
IV
THEORY
OF
OPERATION
4-1.
OVERALL DESCRIPTION.
4-2. Models 333A and 334A Distortion Analyzers in-
clude
an
impedance converter,
a
rejection amplifier,
a
metering circuit, and
a
power supply. The Model
334A also
contains
an
AM
detector. The impedance
converter provides a
low
noise input circuit with
a
high input impedance independent
of
source impedance
placed
at
the INPUT terminals. The rejection ampli-
fier
rejects the fundamental frequency of the
input
signal
and
passes
the remaining frequency components
on to the metering circuit for measuring distortion.
The metering circuit provides visual indications of
distortion
and
voltage
levels
on the front panel meter,
M1. The
AM
detector (Model 334A only) detects the
modulating
signal
from the
RF
carrier
and
filters
all
RF
components from the modulating
signal
before
it
is
applied
to
the impedance converter circuit,
(Refer to Figure 4-1)
4-3.
BLOCK DIAGRAM DESCRIPTION.
level
applied to
the
metering circuit to
1
mV for full
scale deflection. Sensitivity
of
the metering circuit
is
increased to 300
pV
for full scale deflection onthe
.0003
Vrange.
The metering circuit provides a visual
indication of the distortion level
of
the input signal.
In
addition to the indication provided
by
the
meter, the
OUTPUT terminals provide
a
means
of
monitoring
the distortion components.
4-8. DISTORTION MEASUREMENT
IN
AM
CARRIERS.
4-9. The
Model
334A Distortion Analyzer contains an
AM
detector circuit for measuring envelope distortion
in
AM carriers. The input
signal
is applied to the in-
pt
of
the AM detector circuit where the modulating
signal
is
recovered from
the
RF carrier. The sipal
is
then applied to the impedance converter circuit
through
the one megohm attenuator and then through
the same circuits previously described in the distor-
tion measuring mode operation.
4-4. DISTORTION MEASURING OPERATION. 4-10. VOLTMETER OPERATION.
4-5. For distortion measurement, the input signalis
applied to
the
impedance converter (Assembly
A2)
through the FUNCTION selector, S1, and the one
megohm attenuator. The one megohm attenuator
is
a
voltage
divider
network which provides 50 dB atten-
uation
in
10
dB
steps. The desired level of attenuation
is
selected
by
the SENSITIVITY selector,
S2.
The
impedance
converter provides impedance matching and
unity
gain between the INPUT terminals and the input
of the rejection amplifier.
4-6. The rejection amplifier consists of
a
preamplifier,
a
Wien
bridge, andabridge amplifier. The SENSITIVITY
VERNER control,
at
the input
of
the preamplifier,
provides
a
set
level
signal
to
obtain
a
full scale read-
ing on the meter for any voltage level at the input
of
the
instrument.
With the FUNCTION switch inthe SET
LEVEL position,
a
ground
is
applied to the Wienbridge
circuit to allow
a
signal
reference level to be set up
on the meter.
With
the FUNCTION switch
in
theDIS-
TORTION position, the Wien bridge
is
used as an in-
terstage coupling network between the preamplifier
and bridge amplifier. The Wien bridge
is
then tuned
and balanced to reject the fundamental frequency
of
the
applied input
signal.
Two automatic control loops con-
sisting of
two
phase detectors, lamp drivers, lamps,
and photocells provide fine
tuning
and balance in the
AUTOMATIC MODE. The remaining frequency com-
ponents are applied to the bridge amplifier and
are
measured
as
distortion by the metering circuit.
Nega-
tive feedback from the bridge amplifier to the preamp-
lifier narrows the rejectionresponse of the Wien bridge.
4-7. The output of the rejection amplifier
is
applied
to the metering circuit through the post-attenuator.
The post-attenuator
is
used to limit the input signal
4-11. In the voltmeter mode of operation, the input
signal
is
applied to the impedance converter circuit
through the
1:l
and
1OOO:l
attenuator. The
1:l
atten-
uation ratio isusedonthe0.0003 too.
3
VOLTSrangcs,
and
the
1OOO:l
attenuationratiois used in the
1
to 300
VOLTS ranges.
With
the FUNCTION switch
in
the
VOLTMETER position, the output of
the
impedance
converter bypasses the rejection amplifier and
is
applied to the metering circuit through the
post-
attenuator (METER RANGE switch). Metering circuit
sensitivity
is
increased from
1
mV for full scale
deflection to
300
pV on the ,0003V range,
as
it was in
the distortion measuring operation. The function
of
the post-attenuator and metering circuit
is
the same
for
voltmeter operation as for the distortion measuring
operation.
4-12.
DETAILED CIRCUIT DESCRIPTION.
4-13. IMPEDANCE CONVERTER CIRCUIT.
;
4-14. The input
signal
is
applied to the impedance
converter circuit through the
1: 1
and 1000:
1
attenuator
S3R12
in voltmeter operation, and through the one
megohm attenuator S2R1 through
S2R6
in distortion
operation.
Capacitive dividers SC1 through SC10
in
the attenuator keep
the
frequency response flat.
The impedance converter
is
a
low distortion, high
input impedance amplifier circuit with gain indepen-
dent of the source impedance placed at the INPUT
t erm inals.
4-15. Instrument induced distortion of the signal
being measured
is
minimized
hy
keeping the input
impedance and the gain of
the
impedance converter
4-1
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