Wien Bridge Circuit And Rejection; Auto Control Loop Detector - HP 333A Operating And Service Manual

Model 333A/334A
Section IV
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TYPICAL WlEN BRIDGE
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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
e r r o r s o f 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 a t 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 a t
the drain of the field effect transistor A3Q4. For
frequencies other than the fundamental, the reactive
leg of the Wien bridge offers various degrees o f
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 e r r o r amplifier, (A5Q2
andA5Q3) will
be
equal and of opposite phase, and
will
cancel out each other when
t h e
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 e r r o r signals, and
in
a
negative direction (toward -1
V)
for out of phase e r r o r
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 v a r y
the resist-
ance of A6V1 in the direction necessary to balance the
resistive leg of the bridge.
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Figure 4-4. Auto Control Loop Detector
4-3