2-9, TRANSMIT RF SIGNAL PATH. Continued
The amplifier-oscillator modulates the carrier with FM MOD signal from the switching module. The RF REFERENCE
signal from the synthesizer is 7 MHz higher than the carrier frequency. A 3.2 MHz reference frequency is also
provided by the synthesizer.
The amplifier-oscillator generates the FM signal using two phase-locked loops (PLL). In the first, FM MOD signal
modulates a 3.9 MHz voltage controlled crystal oscillator (VCXO). The VCXO output is a frequency modulated
3.9 MHz signal. Mixing it with 3.2 MHz reference signal results in 700 kHz FM. A phase detector samples the 700
kHz to regulate VCXO frequency. The second PLL has the 30-87.975 MHz oscillator. The 30-87.975 MHz signal
is mixed with RF REFERENCE from the synthesizer. The 7 MHz difference is divided by 10 and compared with the
700 kHz from the first PLL. This results in an FM output rf signal that is 7 MHz below the RF REFERENCE fre-
quency. The rf signal is then amplified, filtered, and sent to the communications antenna.
The modulated rf passes through VSWR and power sensors. The power sensors are used for automatic level
control (ALC). The ALC controls power amplifier gain. The VSWR signal causes the power to be reduced and
disables sidetone when antenna VSWR exceeds 5:1. The same power reduction and sidetone disable occur if an
overtemperature condition is detected by the temperature sensor. The temperature sensor is mounted in the
amplifier-oscillator adjacent to the power amplifier stage.
A 52 MHz detect circuit sends a control signal to the optional IFM power amplifier. The output is logic 1 when
transmit frequency is 52 MHz or greater.
2-10. RECEIVE AUDIO/DATA SIGNAL PATH
The received signal enters the audio/data section as FM DEMOD from the lF/demodulator. Its audio path is shown
in Figure FO-3.
The switching module detects the 150-Hz squelch tone. If present, the BIT SYNC/TONE SQUELCH line is set to
logic 1. The switching module and interface module processes the FM DEMOD signal. For single channel (SC),
plain text (PT) audio operation, the signal leaves the switching module as RCV PT AUDIO. In the interface module,
processing includes filtering, volume control, and amplification. When a local control rt is remote controlled,
TAKE CONTROL input bypasses the volume control. The PT audio logic disconnects receive audio and sidetone
for certain rt faults (SIDETONE/SQUELCH) and IFM power amplifier faults (IFM FAULT). Also the audio is discon-
nected during transmit as is the sidetone during receive by the inverted T/R signal. An audio transformer
matches the RCV RT AUDIO output to the line going to the intercom.
In the frequency hopping (FH), plain text (PT) voice mode, the FM DEMOD signal is a digital data stream. It is
routed to the ECCM module as BIT SYNC DATA to be deinterleaved. (See para 2-15.) The signal is sent through a
digital-to-analog converter to recover the original audio. The audio signal is returned to the switching module as
RCV FH PT AUDIO. In the switching module it is connected to the RCV PT AUDIO output and is processed in the
interface module like the SC/PT audio.
In the single channel (SC), cipher text (CT) voice mode and in the SC data mode (PT or CT), the FM DEMOD
signal is again a digital data stream. It is routed to the interface module as RCV X-MODE. In the interface module
it is buffered and amplified before being routed to external equipment via the rt system connector. The X-mode
logic in the interface module disconnects RCV X-MODE during transmit and during retransmit (RXMT).
In FH/CT voice mode and in FH data mode (PT or CT), the ECCM module deinterleaves the data stream. The
digital signal is returned to the switching module as FH DATA. In the switching module it is connected to the RCV
X-MODE output and is processed in the interface module like the SC/CT voice and SC data signals.