TRANSMIT RF SIGNAL PATH.
The transmit mode is initiated by a PTT input from outside the RT. The RADIO PTT-N line is set to logic 0 when the
RT is keyed. If the request is valid (frequency loaded and front panel switches set correctly), the control module
responds by setting the T/R line to logic 1. See figure FO-2.
The exciter/power amplifier, synthesizer, and the impedance matching network are digitally tuned by the SERIAL
DATA signal from the control module.
TUNE GATE-N and TUNE CLK are used to decode the SERIAL DATA.
The exciter/power amplifier modulates the carrier with the FM MOD signal from the switching module. The RF
REFERENCE signal provided by the synthesizer is 7 MHz higher than the carrier frequency. A 3.2 MHz reference
frequency is also provided by the synthesizer.
The exciter/power amplifier generates the FM signal using two phase-locked loops (PLL). In the first, the FM
MOD signal controls a 3.9 MHz voltage controlled crystal oscillator (VCXO). This produces an FM 3.9 MHz signal.
Mixing it with the 3.2 MHz reference frequency generates a 700 kHz FM output. The phase detector samples the
700 kHz output and the feedback regulates the VCXO frequency. The second PLL operates the same. It phase
locks the 700 kHz output of the first loop with the RF REFERENCE signal to generate the modulated RF signal. The
modulated RF signal is amplified and filtered and then sent to the impedance matching network.
The RF output level is selected by the RF switch on the front panel.
The control module reads the RF switch
position and then sets the RF PWR A and RF PWR B lines as follows:
RF PWR A
RF PWR B
A HI PWR XMT line is set to logic 1 when the RF switch is set to PA. This signal is routed to the system connector
(P1) and is used to enable the power amplifier.
The impedance matching network routes the RF signal to the ANT connector. A 50-ohm network is used when a
BNC is connected to the RT. When a manpack antenna is connected to the RT, one of three impedance
matching networks is used depending on the frequency selected.
A VSWR detector in the impedance matching network checks the output versus reflected power. If the VSWR
goes above 5:1, the SIDETONE DISABLE line is set to logic 1. The SIDETONE DISABLE line can also be set to logic
1 by the exciter/power amplifier if the temperature of the module exceeds 105°C. The RF power output level will
be reduced 10 to 15 dB if an over-temperature condition occurs.
During FH operation, the control module sets the FH MODE line to logic 1.
2-10. RECEIVE AUDIO SIGNAL PATH.
The received audio signal enters the ICOM l/O section as the FM DEMOD signal output by the lF/demodulator. Its
path to the audio connectors is shown in figure FO-3.
The switching module detects the presence of the 150-Hz squelch tone. If present, the BIT SYNC/TONE
SQUELCH line is set to logic 1.
The switching module routes the FM DEMOD signal through the processing
circuitry. For single channel (SC), plain text (PT) operation, the signal continues as RCV PT AUDIO. It is routed
through the ICOM power supply and the ICOM control module. The ICOM data I/O module sums the audio signal
(RCV AUDIO/SIDETONE) with the 600-Hz alarm tone, if present, and amplifies it. It is routed through a high-pass
filter, low-pass filter, and the VOL control. It is amplified using a voltage-to-current converter and routed to the
AUD/FILL and AUD/DATA connectors at pin B.