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TM 11-5825-271-34
(e.g, voice modulation), does not produce the dc voltage.
2-19. Power Probe A1A6
Therefore, synchronous detector Q4/Q5 prevents the
(fig. FO-9)
external voice input from interfering with the modulation
a. Transformer T1 (winding B) and T2 form the
monitoring circuits.
voltage and current arms of a forward/reflected power
(2) Comparator U1-A compares the dc voltage
bridge from which dc voltages are obtained.  The
from synchronous detector Q4/Q5 with a reference
voltages (proportional to forward power, reflected power,
voltage established by the ratio of R11 and R12. When
rf voltage, and rf current) are used for metering,
the input at pin 5 of U1-A is more positive than the
shutdown control and current limiting A dc voltage
reference voltage at pin 4 of U1-A, the output from U1-A
developed at the collector of Q1 is used to compensate
at pin 2 is high. Conversely, when pin 5 of U1-A is less
for the pedestal of the detector diodes (CR1 through
positive than the voltage at pin 4 of U1-A, the output from
CR4) and improves the measurement accuracy at low-
U1-A (pin 2) Is low. The reference voltage at pin 4 of
power levels.
U1-A is chosen such that during keying the output from
b. Diode CR1 detects the rf carrier level.  Filter
U1-A makes a low-to-high transition each time keying is
L1/C2 transforms the detected signal to an audio signal
gated on. The output from the modulation detector and
superimposed upon a dc voltage proportional to the rf
threshold comparator circuit is applied to the normal and
carrier level The voltage at P1-12 is used by the monitor
special shutdown circuits.
assembly A1A7 to control the shutdown function
b. Carrier Threshold Comparator. The signal from
c.  Secondary winding A of transformer T1 subtracts
the wiper of the CARR THRESHOLD potentiometer R3
the voltage components from the signals at the junction
is filtered by R39 and C13 to remove the audio
of R3 and CR4 to produce, at the cathode of CR2, a dc
component. The resulting dc level, proportional to the
voltage proportional to the rf current The current limit
carrier level, is compared with a reference voltage
signal, at P1-2, is applied to the mark/space modulator in
determined by the ratio of R36 and R37. During normal
the exciter assembly A1A2 The dc outputs at P1-6 and
operation, the dc voltage at pin 9 of U1-C is more
P1-4, derived through CR4 and CR3, are indicative of the
positive than the reference voltage at pin 8 of U1-C
forward and reflected power respectively
Therefore, the output from U1-C (pin 14) remains high as
long as the carrier level is greater than the threshold
2-20. Monitor Assembly A1A7
setting of R3.  If a malfunction or some abnormal
(fig. FO-10)
operating condition causes the carrier level to drop by 3
a. Modulation Detector and Threshold Comparator.
dB or more, the output from comparator U1-C changes
The input signal from the power probe A1A6 is received
state, i e, pin 14 of U1-C goes low from its normally high
at P1-12 through potentiometer R2 The input signal is a
state. The output from U1-C (pin 14) is fed to the normal
dc voltage proportional to the carrier level and contains
shutdown circuit
an ac component indicative of the modulation level. The
c.  Normal  Shutdown  Control.
The  normal
signal from the wiper of R2 is buffered by emitter-follower
shutdown control circuit is controlled by both the carrier
Q1 and applied to two transistors, Q2 and Q3 Transistor
and modulation threshold comparators. Associated with
Q3 and capacitor C4 couple the ac component (audio) to
the circuit are a NORM/BYP (operate/test) switch S1 and
the AUDIO MON jack J2 on the control panel through
a NORM (within limits) indicator DS1.
P1-15, while transistor Q2 couples it to a synchronous
(1)  When the modulation is within limits and
detector formed by Q4 and Q5 When the modulation
keying is present, the output from U1-A is high during
threshold level is correctly set at - 3 dB by R2 and the
keying (mark) and low during a space.  Provided the
modulation depth is at its normal level, the amplitude of
output from U1-C is high (carrier level within limits), the
the signal at the collector of Q2 is approximately 2V p-p.
positive-going transitions from U1-A cause Q6 to
(1)  The 1020 Hz tone input at P1-9 causes Q5
momentarily switch on. If a keying failure occurs, or if
to switch on and off at a 1020 Hz rage Transistor Q4,
the modulation level falls below the preset level, the
which also switches on and off at 1020 Hz, acts as a
transitions at the base of Q6 are no longer present,
gated dc restorer to the signal from C3.  When the
causing it to remain switched off. Similarly, if the carrier
detected modulation signal is at the same frequency and
level falls below the present threshold, the output from
in phase with the 1020 Hz tone input, a positive dc
U1-C  goes  low  and  Q6  remains  switched  off,
voltage is obtained at the junction of C3/R9.  Any
disregarding the positive-going transitions from U1-A.
component of the modulation signal that is not the same
(2)  Whenever Q6 is switched on, capacitor C9
frequency, or out of  phase with the 1020 Hz,
is discharged. When Q6 is off, C9 is constantly charged
through R17. The voltage across C9 is compared with

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