TM 11-5820-890-20-1
1.1O.
RECEIVE SIGNAL PATH.
a.
RF Signal. The RF signal is picked up by an antenna.
Manpack antennas connect directly to the RT ANT connector J1. The RT contains an impedance
network that matches the RT to the AS-3683 antenna. The AS-4266A contains its own matching
network.
Vehicular antennas use an RF cable to transfer RF energy to the RT. The antenna base and RT have
impedance networks to match the 50 ohm RF cable.
b.
Analog Voice Receive Signals. RT voice signals are limited to the bandwidth of 300 Hz to 3 kHz. The
analog signal is amplified and sent to the AUD/DATA, AUD/FILL, and rear system connector P1.
AM-7239, AM-7239A, AW-7239B only
Audio from AUD/DATA is taken by the W4 cable to the VAA. In the VAA, audio is amplified and sent to
connector J6 (SPKR) and VAA system connector P1 as variable audio. The RT-A and RT-B audio are
summed in the VAA for the speaker connector J6. A muting line is provided at MB connector J2 for
external muting of RT-A or RT-B audio at J6.
From RT-A or RT-B rear system connector P1, audio is sent to the VAA system connector P1 as fixed
level audio.
High level audio lines are used by the intercom. The amplification of the audio for these lines is provided
by thr radio system. For AM-7239, AM-7239A, and AM-7239B this is the amplified video mentioned in the
box above. For other VAAs this is from the fixed level audio at RT P1, amplified by the VAA. The “A audio
enters the MB system connector J5, pin F and exits at connector J3, pin H. The “B” audio enters MB
system connector J5, pin N and exits at connector J4, pin H.
Low level audio lines are used by the intercom and the LS-671. The amplification of this audio is
provided by the peripheral equipment. The audio is from RT P1. The “A” audio enters the MB system
connector J5, pin H and exits at connector J3, pin K. The “B” audio enters the MB system connector J5,
pin P and exits at connector J4, pin K. Detailed information on the receive paths of the intercom and
LS-671 is provided in TM 20-2.
C.
Analog Digital (AD1 or TF) Signals. Analog data can be processed using either AD1 or TF. When the
RT is set to AD1, the analog data signal follows the audio receive path. When TF is used, the analog
data is converted into 16 kbps digital data by the transmitter. The receiver converts it back into analog
data.
d.
Squelch Circuit. A tone squelch circuit is used to prevent the reception of noise when a signal is not
being received. This allows the RT-1523 series and AN/VRC-12 (NATO FM radio) series radios to
communicate with each other.
1.11.
TRANSMIT SIGNAL PATH.
a.
Keying Circuit. The transmitter is “keyed” when the handset PTT switch is pressed. This places a
ground on the handset audio connector pin C. For the AM-7239C and later VAAs and all manpack
radios, the handset is connected to the RT-A or RT-B AUD/DATA pin C. For AM-7239B and earlier, the
handset is connected to the VAA and is sent to the RT AUD/DATA connector, pin C, via the W4 cable.
A PTT ground can also be generated by the LS-671. MB J3 controls RT-A by passing the ground from
pin S to VAA P1, pin K. For AM-7239C and later VAAs the ground is routed to RT system connector P1,
pin S on RT-a. For AM-7239B and earlier, VAA P1, pin K is connected to the 1 watt audio amplifier in the
VAA. The 1 watt audio amplifier places the ground on pin C of AUD/DATA connector J5. The ground is
sent through the W4 cable, pin C, and then to the RT-A AUD/DATA, pin C.
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