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Model 654A
Section IV
4-19.
AMPLITUDE
CONTROL
AND
AUTOMATIC
LEVELING CONTROL.
4-20. The amplitude of the balanced sine wave signal from
the Balanced Amplifier is independent of the Attenuator
settings but can be varied over a 2 dB range by the front
panel AMPLITUDE control (Schematic 2). The Automatic
Leveling Circuit (ALC) consists of the Average Detector,
the Amplitude Control Integrator, and the Amplitude
Current Reference (varied by the AMPLITUDE control).
The Average Detector monitors the output of the Balanced
Amplifier and produces a dc current proportional to the
amplitude of the Balanced Amplifier signal. The Amplitude
Control Integrator compares this dc current with a current
of
opposite
polarity
from
the
Amplitude
Current
Reference; any difference in magnitude between the
current from the Average Detector and the reference
current
is
used
to
apply
negative
feedback to the
photosensitive resistor at the input of the Buffer Amplifier
until the output of the Balanced Amplifier is at the level
where the two currents are equal. The output of the
Balanced Amplifier is at the required level when the current
from the Average Detector is equal to the reference current.
4-21. AVERAGE DETECTOR. (Schematic 2)
4-22. The detector monitors the output of the Balanced
Amplifier. A2Q24 ad A2Q25 form a high gain amplifier
which is a current source for the detector (A2CR21 and
A2CR22). A2CR21 supplies the metering circuits with a
positive dc current and A2CR22 supplies the ALC circuits
with a negative dc current. These currents are equal in
amplitude to each other and proportional to the amplitude
of the Balanced Amplifier output signal. A2Q24 and
A2Q25 (together with the components connected to the
base of A2Q25) form essentially one transistor with high
gain,
high
output
impedance
and
very
low
output
capacitance; these factors together with the 'bootstrap'
capacitor A2C42 account for the amplifier's effectiveness as
a current source over a wide frequency and temperature
range.
A2C43 ( Freq.
Response) is adjusted
for flat
frequency response of the detector circuit.
4-23. AMPLITUDE CURRENT REFERENCE.
(Schematic 2)
4-24. Zener Diode AlCR8 maintains a constant voltage
across R3 (front-panel AMPLITUDE control) and A2R91
in series, the Amplitude Control Integrator maintains
essentially 0 Vdc at the output of the current reference (at
the base of A3Q6): thus, for any given setting of R3, there
is a fixed voltage drop across A3R19 and A3R20 and a
fixed amount of current flows from the Amplitude Current
Reference. When the setting of the AMPLITUDE control
(R3) is changed, the voltage drop across A3R19 and A3R20
is changed; this sets a new fixed value of current flowing
towards the base of A3Q6.
el 4-25. AMPLITUDE CONTROL INTEGRATOR.
(Schematic 2)
4-26. The circuit consists of A3Q6 through A3Q9 and
associated circuitry, including the lamp of the photo-
sensitive resistor A2DSV1 (Schematic 1). A3Q6 and A3Q7
form a differential amplifier; any change in output from the
collector of A3Q7 is amplified by A3Q8 and applied to the
base of A3Q9 to change the current through the lamp of
the photosensitive resistor (A2DSV1). Changes in lamp
current change the impedance of the voltage divider at the
input of the Buffer Amplifier (Paragraph 4-12), thus
changing the sine wave signal level through the Buffer
Amplifier and the Balanced Amplifier. The dc current from
the Average Detector is compared, at the base of A3Q6,
with the current from the Amplitude Current Reference. In
the differential pair (A3Q6, A3Q7) the base of A3Q7 is
connected directly to ground; therefore, as long as the base
of A3Q6 is held at 0 Vdc, there will be no change in output
from the collector of A3Q7. The amplitude of the positive
current flowing from the Amplitude Current Reference is
fixed; the amplitude of the negative current flowing from
the Average Detector depends on the level of signal at the
output of the Balanced Amplifier. These currents are
summed at the base of A3Q6. The difference current flows
into the base of A3Q6. The amplitude of the base current is
set by the AMPLITUDE CAL adjustment and the AMPLI-
TUDE control. As long as this current level is not changed
by a variation in the Average Detector output, there will be
no change in current through the lamp of the photo-
sensitive resistor; thus the ac signal level at the output of
the Balanced Amplifier will be constant. In this condition,
the ALC loop is in the "quiescent" state and the output of
the 654A is at the required level.
4-27. Suppose that the output level of the Balanced
Amplifier now changes for some reason (e. g. the frequency
of the Wien Bridge Oscillator is changed); then the
Amplitude Control Integrator will act to return the signal
back to its original level in the following manner:
a. The negative current from the Average Detector
(A2CR22) will change proportionally with the
change in ac signal level. This current flowing into
the summing node at the base of A3Q6 diminishes
the positive current amplitude thus decreasing the
base current to A3Q6. This, in turn, begins to shut
off the transistor.
b. The output of the Amplitude Control Integrator
changes in response to the new input so as to
change the ac signal level into the Buffer Ampli-
fier. This will be in such a direction as to return
the Balanced Amplifier ac output back to the level
where the negative dc current from the Average
Detector is again equal to its quiescent value. In
this way the 654A output signal is maintained at a
constant level.
4-28. To manually control the 654A output level over the
2 dBm range, the AMPLITUDE control setting is changed;
this changes the current from the Amplitude Current
Reference thus changing the base current to A3Q6. The
Amplitude Control Integrator now acts as before to change
the Balanced Amplfier ac output level until the current
4-3
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