HP 334A User Manual

Model 333A/334A
Section IV
(5
E IN
0
~
TYPICAL WlEN BRIDGE
I
Figure 4-3. Wien Bridge Circuit and Rejection Characteristics
4-26. In an independent, but similar, control loop,
the bridge input signal is shifted 900 and used as the
reference
signal for the detector. This detector
develops control voltages to null the reactive leg of
the bridge, but
is insensitive to signals of the form
in Figure 4-2b, which are caused by small tuning
errors
of the resistive branch.
4-27. The result is that the two control loops derive
information from
a common source and develop two
independent control signalsfor nulling the two legs
of
the bridge. These control voltages are used to vary
the brilliance of lamps, which
in turn cause resistance
changes
in photocells that form part of the Wien bridge.
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 (SQR1,
3, 5, 7, or 9 and C4A/B) and the shunt reactive leg
(SQRll, 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 fundamental 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 volt-
age between the reactive leg and resistive leg is amp-
lified by
A3Q4, A3Q5, and A3Q6. Figure 4-3 illus-
trates 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 coarse 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 refer-
ence voltage
is taken from R6 at the input to the re-
jection 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 applied to the buffer amplifier A5Q1.
It is amplified by A5Q2 and A5Q3 and coupled to the
detector
A5Q4.
4-30.
Refer to Figure 4-4, partial scheiiiatic for
detector operation. The discussion is applicable
to both resistive and reactive detector circuits.
4-31. The signals from the error amplifier, (A5Q2
andA5Q3)
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 referencc
square wave gates
A5Q4 on, the signal from thc coll-
ector of
A5Q3 will be shorted to ground. Thus the
signal from the collector
of A5Q2 will be couplcd
through the filter network to
the base of A5Q5. If thc
signal from
A5Q2 is in phase with the referencc, thc
positive half of the signal
will be passcd, and if it is
out of phase, the negative half will be passed.
4-32. The normal working voltage at A5TP3 is be-
tween0 and -1 volt. The dc output of the filter nctwork
causes the voltage
at A5TP3 to go in a positive dircc-
tion (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 ampIificd
by
A5Q5 and lamp driver A5Q6. This will change thc
brilliance of lamp
A6DS1, which will vary the resist-
ance of
A6V1 in the direction necessary to balance the
resistive leg of the bridge.
I
I I I I-1
I
Figure 4-4. Auto Control Loop Detector
4-3

HP 334A Specifications

General

Brand	HP
Model	334A
Category	Measuring Instruments
Language	English