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Figure 3-2. Radio Set Simplified Block Diagram
Up
TM-11-5820-919-12 Radio Set AN/PRC-104(A) (NSN
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Figure 3-3. Control Panel Pictorial Block Diagram


TM 11-5820-919-12
SECTION II
FUNCTIONAL BLOCK DESCRIPTION
3-17. GENERAL.
match standard data systems.
3-18. This section contains a functional block description
of the seven modules within the radio set.
3-23.
Sideband Selection.
Either low or upper
sideband operation can be selected by placing the SB
NOTE
switch in LSB or USB, respectively.  The resultant
Although
the
modules
are
not
Sideband Select signal from the SB switch causes the
replaceable
at
organizational
synthesizer to route either 70 MHz (USB) or 80 MHz
maintenance level, this description is
(LSB) to the modulator/demodulator.
provided to help better understand the
operation of the radio set.
3-24.  Volume Control.  The VOLUME OFF/MAX
control is a conventional on-off switch and potentiometer
3-19.  CONTROL PANEL (Figure 3-3).  The control
combination. The on-off switch connects +28v On to the
panel is a removable module which contains all of the
modules in the radio set. The potentiometer controls the
operational controls, and the audio interface connectors
level of audio signals input to the audio amplifier in the
and filters for the radio set. The operational controls and
modulator/demodulator.
audio filtering are discussed below.
3-25.  Panel Illumination.  The LIGHT switch, when
3-20. Frequency Selection. Selection of the operating
pressed, illuminates the FREQUENCY-KHZ readouts on
frequency  is  accomplished  by  setting  the  six
the control panel with red light.
FREQUENCY-KHZ pushbutton switches to the desired
frequency (2.0000 thru 29.9999 MHz).  The resultant
3-26. Audio Filtering. The Audio to or from the audio
binary-coded decimal (BCD) Frequency Select and Filter
devices is filtered both in receive and transmit modes.
Select signals are used to establish frequency control of
This prevents Xmt RF from being picked up by the
the radio set. Frequency selection is performed in 10-
cables leading to the audio devices and being sent back
MHz, I-MHz, 100-kHz,
10-kHz, l-kHz and 100-Hz
into the radio set.
increments.
3-27.  POWER SUPPLY (Figure 3-4).  The power
3-21. Operational Mode. The MODE switch selects
supply converts +28v On from the control panel to +12.5
one of the following four modes of operation: V-RCV --
and +6.5v.
This is done by two down switchers
Voice and cw receive only
(transistorized regulating circuits).  An overload latch
shuts the down switchers off if an excessive load (a short
V-TB -
Voice and cw key transmit/receive
circuit, for example) is present at the output. The +6.5v
V-TR -
-
Data transmit/receive
goes  to  the  antenna  tuner,  power  amplifier,
V-RCV-
Data receive only
modulator/demodulator, and synthesizer.  The +12.5v
goes to the control panel, synthesizer, and modulator/
3-22. The Mode Select signals from the MODE switch
demodulator. These voltages are regulated.
are routed to the modulator/demodulator to activate solid
state switches which place the radio set in the selected
3-28.  SYNTHESIZER (Figure 3-5).  The synthesizer
operating mode. In V-RCV mode, the radio set operates
generates three sinusoidal LO signals which are used by
as a receiver only with the modulator (transmit) inhibited.
the modulator/demodulator.
All of the frequencies
In the V-TR mode, either transmit or receive operations
generated by the synthesizer are referenced to a
are possible using voice or cw signals. Pushing the PTT
temperature-compensated crystal oscillator (TCXO),
on the handset or pressing the cw telegrapher's key
which is the frequency standard for the radio set.
sends a Xmt Enable to the modulator/demodulator,
Because the TCXO is temperature-compensated over a
which  puts the radio in transmit mode.  The same
-46 to +71 C range, the Xmt RB" frequency will not
features (receive only, transmit/receive) are available for
data D-TR and D-RCV communication, except that the
signal bypasses the volume control, the output level
becomes fixed, and the input impedance is increased to
3-5


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