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|  TM
11-5820-919-40-1
(if.) and then into Xmt RF. The Xmt RF
Tune Start.
The Tune Start
269.
signal form
the
modulator/demodulator
is then routed to the power amplifier
momentarily grounds the 0.69-3.49 MHz
for further amplification.
Line.
The divided frequency at the high
frequency loop no longer equals 10,000
Receive
Operation.
In receive
The VCO tune voltage increases,
Hz.
operation, the absence of the Keyline
causing the LO output to shift.
Eventu-
signal places the frequency converters
ally, a new steady-state LO frequency is
into receive mode.
During receive
operation, the frequency converters
reached.
extract the Rcv Audio frequency from the
The 1kHz Standard
incoming RF Signal (demodulation). The
1-kHz Tone.
developed at the frequency divider U9 is
audio control hybrid processes the Rcv
Audio and routes it to the control
The low frequency phase lock U5 filters
panel.
the 1-kHz Standard , and routes the 1-kHz
Audio
Control
Hybrid
Standard to the modulator/demodulator.
Power Filters. The +12.5 and
NOTE
+6.5v from the power supply is given
additional filtering at power regulator
The audio control hybrid is
AR2-1 , AR22, Q1 and Q2.
The +12.0 and
not repairable but the block
+6.Ov are distributed within the synthe-
diagram (figure 2-7) is neces-
sizer.
The +6.Ov is also applied to the
sary to understand the func-
modulator/demodulator.
tion.
Therefore, no circuit
The chip supply
voltage for operational amplifiers AR2-1
designators are given.
and AR2-2 is provided by transistor Q4,
which is controlled by operational ampli-
2-77.
General.
The audio control
fier AR1-1.
hybrid contains the control circuits
required by the modulator/demodulator
MODULATOR/DEMODULATOR
to control and process the Xmt and Rcv
Audio.
The hybrid also generates Tune
Start , Power Amplifier (PA) On/Off (not
General.
The modulator/demodula-
tor performs audio-to-radio frequency
used in PRC-104; used with AM-6879/URC)
conversion.
and Keyline signals.
It also coordinates other
functions, such as level control, gain
controls,
2-78.
Transmit
Operation.
fault signals and tuning ini-
Xmt Audio
is received from the control panel and
The
modulator/demodulator
tialization.
routed through the microphone/data
contains the frequency converters and
attenuator switch to the transmit audio
The frequency
the audio control hybrid.
converters are controlled by the audio
When in voice mode, the Xmt
amplifier.
Audio is not attenuated.
control hybrid.
In data mode,
The Keyline signal from
the data Xmt Audio is attenuated by
the audio control hybrid, and the +28v
the microphone/data attenuator switch.
Xmt Enable from the control panel, deter-
The transmit audio amplifier is a three-
mine whether the radio set is in transmit
stage differential amplifier with an
or receive operation.
agc feedback loop consisting of the
transmit output level adjust and a gain
Transmit
Operation.
In transmit
The gain control
control
amplifier.
operation, the Keyline signal and +28v
amplifier tries to maintain a constant
Xmt Enable are routed to the frequency
output with a varying input.
converters, enabling the transmit cir-
The output
of the transmit audio amplifier is
cuits.
The Xmt Audio is then processed
routed to the third frequency converter
by the audio control hybrid and sent to
for further processing and to the side-
the frequency converters.
The frequency
tone level adjust for use in sidetone
conversers translate the Xmt Audio fre-
operation.
u e n c y into two intermediate frequencies
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