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Section IV
frequency range due to the value of resistors A2R8 and
A2R9 exceeding the input impedance of the emitter follower
at the higher frequencies.
4-11. The complementary symmetry circuit is used to
provide power gain and to increase the dynamic voltage range
of the oscillator; also, the low output impedance of the
complementary symmetry circuit prevents the oscillator
output circuit from being loaded by the RC bridge. The
complementary
s y m m e t r y
forward-biased by diodes A2CR2, A2CR3, and A2CR4, and
with no signal applied, are conducting slightly to reduce
cross-ever distortion in the output signal.
4-12. The output of the oscillator circuit drives the power
amplifier with a constant voltage set by the AMPLITUDE
coarse and VERNIER controls, R2A and R2B. ·Ill. e voltage
level applied to the power amplifier is held constant by the
action of the peak detector circuit.
4·13. AUTOMATIC GAIN CONTROL.
4-14. The output of the oscillator circuit is superimposed on
a negative reference bias at the base of A2Q7. This bias
voltage is determined by the setting of the amplitude
eontrols. The peak detector, A2Q7, will conduct only on the
positive peaks when the negative bias is overcome. Th e
average de voltage across A2C7, A2CS, A2C9 and A2CI0
biases the diodes A2CR6 and A2CR7 to determine the
impedance of the negative feedback side of the Wien bridge.
Thus the amplitude of the oscillations is automatically
controlled.
A2CR5
and
compensation for the bias voltage on A2Q7, and A2CR8
prevents the reverse breakdown of A2Q7. A2RI 7 is
adjustable to compensate for differences in the operating
characteristics of diodes A2CR6 and A2CR7, minimizing
distortion in the negative feedback and subsequently in the
oscillator output.
4·15. POWER AMPLIFIER.
4-16. The power amplifier circuit increases the power gain of
the signal received from the oscillator circuit. The operation
of the differential amplifier A2Q8 and A2Q9, emitter
follower A2Q IO, and complementary symmetry circuit
A2Q I l and A2QI2 is similar to the corresponding stages in
the oscillator circuit. The negative feedback voltage from the
output of the complementary symmetry circuit is applied to
the differential amplifier at a fixed level to stabilize the
power amplifier output signal. The power amplifier output is
continuously monitored by one of the two monitor circuits
before the signal is applied to the output attenuator circuit.
4·17. NORMAL MONITOR CIRCUIT.
4-18. The normal monitor circuit monitors the signal level
applied to the output attenuator circuit and provides a signal
to the output monitor, M I , when the MONITOR switch, S4,
is
in the NORMAL position. The amplifier A l Q9 serves both
4- 2
circuit transistors
A2CR9 provide temperature
as
an impedance converter between the monitor circuit and
the power amplifier output circuit and as a current source to
provide full-scale monitor indications. The high input
impedance of the amplifier prevents the power amplifier
from being loaded with the low impedance of the output
monitor, M l . The emitter follower AIQ8 provides a positive
feedback voltage which is applied between resistors AIRI8
and AIRI9, in the collector lead of amplifier AIQ9. The
application of the feedback voltage at this point is used to
increase the effective resistance of the collector circuit,
which results in the amplifier A l Q9 appearing as a high
are
impedance current source to the monitor. The diode
A I CRI 0 provides a small amount of forward bias to rectifier
diodes A l eRS and AI CR9, which keeps the diodes out of
the non-linear region, thus increasing monitor accuracy at
one-tenth full-scale readings. TI,e 10 MHz adjustment,
A I C15,
compensates
capacitance so the monitor will have a flat frequency
response. The monitor calibration resistor, AI R23, provides
an additional calibration adjustment which is made at 400
Hz.
4-19. EXPAND MONITOR CIRCUIT.
4-20. The expand monitor circuit allows the top 10% of the
output monitor scale to be expanded to full scale. A
reference may then be set on the scale, and deviations up to
+/- 2.5% in the oscillator output will be indicated on the
monitor. The circuit is comprised of an average detector, a
differential amplifier, and a variable reference supply. The
difference between the outputs of the detector and the
reference supply is amplified by the differential amplifier,
and is applied to the monitor, MI, when the MONITOR
switch, S4, is in the EXPAND position. The de output of the
detector is proportional to the amplitude of the oscillator
output, and the amplitude of the reference supply output
may be varied by the REF SET coarse and VERNIER
controls, R3A and R3B, respectively. A reference may be set
at any point on the expanded monitor scale, by varying the
reference supply output.
4-21. The output of the oscillator power amplifier is applied
to the detector through an RCL network (A3C5, A3Ll,
A3RI), capacitor A3C2, and resistor A3R2. The purpose of
the RCL network is to smooth the expand monitor circuit
response On the XIM range. On the XIO range, A3CI is
switched in parallel with the RCL network and the input
capacitor) A3C2, to improve the low frequency response of
the circuit. It is not used on higher frequency ranges, due to
an increase in detector time constant when it is connected.
4-22. The detector is a diode rectifier that provides a dc
output proportional to the average value of the sinusoidal
input from the oscillator power amplifier. The ac input is
rectified by diodes A3CRI and A3CR2, and applied to the
base and emitter, respectively, of A3QI, forward biasing the
transistor. A3QI provides a low impedance path for the
rectified signal, and allows average detection of the applied
input with a sing!e-ended output. A3L2, A3C6, and A3C7
form a fliter network to remove any high frequency
ModeJ652A
for
small
variations in circuit
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