HP 333A, Техническое руководство

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TM 11-6625-1576-15
Section IV
Paragraphs 4-28 to 4-35 and Figures 4-4 to 4-5
Figure 4-4. Wien Bridge Circuit and Rejection Characteristics
4-28. When the bridge circuit is tuned and balanced,
the voltage and phase of the fundamental, which appears
at junction of the series reactive leg (S4R1, 3, 5, 7,
or 9 and C4A/B) and the shunt reactive leg (S4R11,
13, 15, 17, or 19, and C4C/D), is the same as at the
midpoint of the resistive leg (A3R12 and A3R14). When
these two voltages are equal and in phase, the funda-
mental frequency will not appear at the drain of the
field effect transistor A3Q4. For frequencies other
than the fundamental,
the reactive leg of the Wien
bridge offers various degrees of attenuation and phase
shift which cause a voltage at the output points of the
bridge. This difference voltage between the reactive
leg and resistive leg is amplified by A3Q4, A3Q5, and
A3Q6. Figure 4-4 illustrates a typical Wien bridge
circuit and the rejection characteristics for it.
4-29. The Wien bridge circuit is designed to cover a
continuous frequency range of over a decade for each
position of the FREQUENCY RANGE selector, S4. S4
provides course tuning of the reactive leg by changing
the bridge circuit constants in five steps at 1 decade
per step. For the automatic control loop, the reference
voltage is taken from R6 at the input to the rejection
amplifier and applied to the buffer amplifier A5Q7.
The reference voltage is amplified and clipped by A5Q8
and A5Q9 and coupled to the detector A5Q4. The output
of the metering circuit, which contains the fundamental
frequency if either leg of the bridge is untuned, is ap-
plied to the buffer amplifier A5Q1. It is amplified by
A5Q2 and A5Q3 and coupled to the detector A5Q4.
4-30. Refer to Figure 4-5 simplified partial schematic
for detector operation. The discussion is applicable
to both resistive and reactive detector circuits.
4-31. The signals from the error amplifier, (A5Q2
and A5Q3) will be equal and of opposite phase, and will
cancel out each other when the detector, A5Q4 is off.
However, when the positive half of the reference
square wave gates A5Q4 on, the signal from the col-
lector of A5Q3 will be shorted to ground. Thus the
signal from the collector of A5Q2 will be coupled
through the filter network to the base of AQ5. If the
signal from A5Q2 is in phase with the reference, the
4-4
positive half of the signal will be passed, and if it is
out of phase, the negative half will be passed.
4-32. The normal working voltage at TP3 is between
0 and -1 volt. The dc output of the filter network
causes the voltage at TP3 to go in a positive direction
(toward zero) for in phase error signals, and in a
negative direction (toward -1 v) for out of phase error
signals. The change in base voltage is then amplified
by A5Q5 and lamp driver A5Q6. This will change the
brilliance of lamp A6DS1, which will vary the resis-
tance of A6V1 in the direction necessary to balance
the resistive leg of the bridge.
Figure 4-5. Auto Control Loop Detector
4-33. When the FUNCTION selector is set to the VOLT-
METER or SET LEVEL position, the junction of the
series and shunt reactive branches of the Wien bridge
is connected to circuit ground through R19 by S1BF
which disables the frequency rejection characteristic
of the bridge circuit. With the bridge circuit disabled,
the rejection amplifier circuit provides one db of gain
for the fundamental frequency and the harmonics. In
the SET LEVEL operation, this signal is used to es-
tablish the SET LEVEL reference.
4-34. BRIDGE AMPLIFIER CIRCUIT.
4-35. The bridge amplifier circuit consists of three
stages of amplification, A3Q4 through A3Q6. The