4-32 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.12 Two-Tone Calling System
The two tone calling system consists of the transmission of two
simultaneous audio tones. The difference frequency is detected to provide a
narrow bandwidth independent of frequency shifts in the RF path.
4.12.1 Two-Tone Calling
The call is initiated by pressing the Call pad on the front panel. On
recognising the request for a call, the microcontroller first checks that the
channel selected has two-tone calling programmed. If enabled, the
microcontroller activates the tone generator IC12 and the required tones are
generated, one from pin 10 and one from pin 13. These pass through the
filters formed by C18, R21, C14, R14, etc. to be mixed at the junction of
R21 and R22. The mixed tones are applied via J3 pin 9 to the auxiliary
compressor input of the Audio and IF 1650kHz PCB. The tones are also
attenuated by R23 and applied as a side tone to the auxiliary LS input of the
Audio and IF 1650kHz PCB. At the same time the microcontroller generates
a PTT signal so that the tones are transmitted.
An RFDS emergency call (as determined by the programming) is
transmitted for 15 seconds after the Emgcy Call pad has been held for at
least 2 seconds. For other calls, the transceiver reverts to the receive mode
as soon as the pad is released.
Note: If selective call and 2-tone calls are required on the same frequency,
the frequency must be allocated to two channels; one with selective
calling and one with 2-tone calling.
4.12.2 Two-Tone Detection
Received audio signals are applied to the Two-tone Decoder PCB (refer to
04-02231) J2 pin 1. IC1b and its associated components form a high-pass
filter, while IC1a and its components form a low-pass filter. Together, these
form a band-pass filter which accepts frequencies between 850 and 1500Hz,
i.e. the band used for 2-tone calling.
The accepted frequencies are applied to an audio compressor circuit formed
by IC2a, V1, V2 and their associated components. Transistor V2 conducts
during positive half cycles of the output of IC2a to charge capacitor C9 so
that the potential on the capacitor is proportional to the amplitude of the
audio signals. This potential controls the gate of V1 thus controlling the
drain-to-source resistance of the FET and consequently the gain of the
circuit. Thus the audio signals at the output of IC2a are maintained at a
constant amplitude.
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