HP 333A, Техническое руководство

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TM 11-6625-1576-15
Model 333A/334A Section IV
paragraphs 4-1 to 4-12
SECTION IV
THEORY OF
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. A block diagram
of the instruments is shown in Figure 4-1. The im -
pedance converter provides a low noise input circuit
with a high input impedance independent of source
impedance placed at the INPUT terminals. The re-
jection amplifier rejects the fundamental frequency of
an 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, M 1. The AM detector (Model 334A only)
detects the modulating signal from the RF carrier and
filters any RF components from the modulating signal
before it is applied to the impedance converter circuit.
4-3. BLOCK DIAGRAM DESCRIPTION.
4-4. DISTORTION MEASURING OPERATION.
4-5. For distortion measurement, the input signal is
applied to the impedance converter, Assembly A2,
through the FUNCTION selector, S1, and the one meg-
ohm attenuator, The one megohm attenuator, a voltage
divider network provides 50 db attenuation in 10 db
steps. The desired level of attenuation is selected by
the SENSITIVITY selector, S2. The impedance con-
verter provides an impedance conversion and unity
gain between the instrument INPUT terminals and the
input of the rejection amplifier. The rejection ampli-
fier consists of a preamplifier, a Wien bridge, and a
bridge amplifier. The SENSITIVITY VERNIER control,
at the input of the preamplifier, provides a set level
signal to obtain a full scale reading on the meter for
any voltage level at the input of the instrument. With
the FUNCTION selector in the SET LEVEL position, a
ground is applied in the Wien bridge circuit to allow
a signal reference level to be set up on the meter.
With the FUNCTION selector in the DISTORTION posi-
tion, the Wien bridge is used as an interstate coupling
network between the preamplifier and bridge amplifier.
The Wien bridge is tuned and balanced to reject the
fundamental frequency of the applied input signal. Two
automatic control loops consisting of two phase detec -
tors, lamp drivers, lamps, and photocells provide fine
tuning and balance in the AUTOMATIC MODE. The
remaining frequency components are applied to the
bridge amplifier and are measured as distortion by
the metering circuit. Negative feedback from the
bridge amplifier to the preamplifier narrows the re-
jection response of the Wien bridge. The output of
the rejection amplifier is applied to the metering cir -
cuit through the post-attenuator. The post-attenuator
is used to limit the input signal level applied to the
metering circuit to 1 mv for full scale deflectlon.
The metering circuit sensitivity is increased to 300
u
V
for full scale deflection on the 300 pv range. The
metering circuit provides a visual indication of the
OPERATION
distortion level of the input signal. In addition to the
visual indication provided by the meter, the OUTPUT
terminals provide a means of monitoring the distortion
components.
4-6. DISTORTION MEASUREMENT IN AM
CARRIERS.
4-7. 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-
put of the AM detector circuit where the modulating
signal is recovered from the RF carrier. The signal
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.
4-8. VOLTMETER OPERATION.
4-9. In the voltmeter mode of operation, the input
signal is applied to the impedance converter circuit
through the 1:1 and 1000:1 attenuator. The 1:1 atten-
uation ratio is used in the .0003 to .3 VOLTS position
of the METER RANGE selector S3, and the 1000:1
attenuation ratio is used in the 1 to 300 VOLTS posi-
t ions. With the FUNCTION selector in the VOLT-
METER position, the output of the impedance converter
bypasses the rejection amplifier and is applied to the
metering circuit through the post-attenuator (METER
RANGE selector).
Metering circuit sensitivity is in-
creased from 1 mv for full scale deflection to 300 uv
on the 300 uv range, as it was in the distortion mea-
suring operation.
The function of the post-attenuator
and metering circuit is the same for voltmeter opera-
tion as for the distortion measuring operation.
4-10. SCHEMATIC THEORY.
4-11. IMPEDANCE CONVERTER CIRCUIT.
4-12. The input signal to the distortion analyzer is
applied to the impedance converter circuit (refer to
Figure 6-2) through the 1:1 and 1000:1 attenuator
S3R12 in the voltmeter mode of operation and through
the one megohm attenuator S2R1 through S2R6 in the
distortion mode of operation. Capacitive dividers
S2C10 through S2C 10 in the attenuator keep the frequency
response flat. The impedance converter is a low
distortion, high input impedance amplifier circuit
wit h gain independent of the source impedance placed
at the INPUT terminals. Instrument induced distor-
tion of the signal being measured is minimized by
keeping the input impedance and the gain of the imped-
ance converter linear. The input impedance is kept
linear by use of local positive feedback from the source
of A2Ql to the gate of A2Q1 and to the protective diodes
A2CR2 and A2CR3. Thus signals with large source
impedance can be measured accurately. Overall in-
duced distortion is further minimized by a high open
loop gain and 100% negative feedback. The high open
loop gain is achieved by local positive feedback from
4-1