8525B/8528 Technical Service Manual Technical Description 4-33
Publication No: 15-02036 Issue 6
The two-tone audio signal is further filtered by a high pass filter formed by
IC2b, IC3a and their associated components. The filter output is applied to a
phase splitter V3, so that equal anti-phase waveforms are produced. Each of
these is applied to one of the two transistors V4 and V5 which operate as a
full-wave rectifier. The output from the emitters of these is a half-wave
sinusoidal waveform containing the sum and difference of the two tones.
This output is applied to the high-Q low-pass filter formed by IC3b and its
associated components removing unwanted frequencies. R25 and R26 set
the peak of the response.
The difference frequency is then applied to the input of tone decoder IC5.
The internal oscillator of the tone decoder is set to 360Hz or 440Hz by the
value of C24, R30 and R31. When the difference frequency matches the
setting of the internal oscillator within a narrow bandwidth (approximately
±15Hz), pin 8 of the tone decoder goes low. Thus the receipt of any two
tones with a difference in frequency corresponding to the frequency to which
the tone-decoder is set (normally 360 or 440Hz) causes this output from the
Two-tone Decoder PCB.
The output is taken via J3 pin 4 to P2 on the Microprocessor Controller
PCB. This activates the tone generator IC12 to generate three short pips to
indicate the receipt of a two-tone call.
Refer to section 7.3.16 for setting up instructions.
Note: The S’call mute facility can be used with the two-tone decoder in
the same way as it is used with selective calling.
To Next Page
Loading...
Need help?
General
