Technical description
4-36 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
The demodulated FSK signals are applied via V302 to the microprocessor IC101
pin 26 (see Sheet 1). The microprocessor decodes the addresses contained in the
data. If the selective call is found to be addressed to the transceiver, the
microprocessor sends a revertive message to the caller. The microprocessor
simultaneously produces outputs to activate the necessary audio and visual
indications that a selective call has been received.
If the transceiver PTT is not pressed on completion of the audio alarm period, the
microprocessor IC101 sends a command for IC109 to open-circuit the output
(pin 6 open collector). This allows the line connected to pin 6 to forward bias
V301 via pull up resistor R328. The transistor V301 energises relay K301 for two
minutes, operating an external alarm (if fitted). The relay K301 contacts are rated
at 50 V at 1 A. The operation of PTT cancels the alarm.
Emergency call (RFDS) and two-tone calling
The two-tone calling system consists of the transmission of two simultaneous
audio tones. The frequency difference of the two tones provides a narrow
bandwidth independent of frequency shifts in the RF transmission path.
The Australian Royal Flying Doctor Service (RFDS) uses the two-tone audio
transmission to alert the RFDS stations of an emergency call. The activation of the
station’s emergency detection system requires the transmission of two audio
frequencies of 1320 Hz and 880 Hz for a minimum period of 10 seconds. This
facility (designated E) must be enabled on the selected channel before it can be
used.
The emergency call function is activated by pressing the Emgcy Call button on
the front panel. The microprocessor IC2 in the control head recognises the request.
It then sends the command via the I
2
C bus to the microprocessor IC101 to initiate
the emergency call [1 04-02976 Sheet 1]. If the selected channel has been
enabled for the emergency call, the microprocessor IC101 programs the tone
generator IC106 to output 880 Hz from pin 13 and 1320 Hz from pin 17. This
generates the PTT command.
The two tones are filtered by a resistor/capacitor network and mixed across the
final filter capacitor C117. They are then sent via R213 and C215 to the input of
the combining amplifier IC204/A (see Sheet 2). As in the Selcall mode, the shunt
transistor V204 is switched off for the period of the transmission.
The mixed tones are also attenuated by R131 and R132 and applied as side tone to
the input of the speaker amplifier IC306 (see Sheet 3).
When the Emgcy Call button is pressed for a minimum of two seconds, an
emergency call will be transmitted for a duration of 45 seconds. The transmission
may be cancelled anytime by:
• pressing Tune
• pressing PTT on the microphone
• switching off the transceiver
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