8525B/8528 Technical Service Manual Technical Description 4-31
Publication No: 15-02036 Issue 6
4.11.2 Selective Call Detection (04-02250)
The Selective Call PCB fitted for Options SDE and SDEM includes circuits
for the detection of selective calling signals. The received audio is applied
via J1 pin 2 to a band-pass filter formed by IC1a, IC1b and their associated
components. The filter has a 3dB bandwidth of 400Hz centred on 1775Hz.
The filter output passes to the phase locked loop IC2.
The voltage controlled oscillator frequency in IC2 is set by C10, R15 and
R16. In the absence of FSK signals, the lock detect output from pin 6 will
fluctuate and the resulting charge on C8 will be less than half the supply
voltage. This is applied via R11 to the comparator input pin 8, resulting in a
low output from pin 7. Some of the output is fed back to the input via R13
to provide hysteresis. The comparator output is taken via D3 to the non-
inverting input of IC3b. This results in a low output from IC3b which
effectively cuts off V1. When FSK signals are detected the lock detect
output goes high. The resulting high comparator output reverse biases D3,
enabling IC3b.
When FSK signals are applied, the data appears on the loop phase detector
output pin 11. This is applied to the data filter IC3a and associated
components. This is a low-pass filter at approximately 80Hz which passes
the resulting data to the input of IC3b. The data filter output is also peak
rectified by D1, D2, C17 and C18. The voltages on the two capacitors are
summed at the inverting input of IC3b to give a reference voltage equal to
the mean frequency of the FSK signal. Consequently, IC3b regenerates the
data, ignoring any frequency shift in the RF path.
Demodulated FSK signals are applied via P2 pin 6 to the Microprocessor
Controller PCB (04-02451) on P1 pin 1. From there they are applied to the
microcontroller which decodes the addresses contained in the data. If the
selective call is found to be addressed to the transceiver, the microcontroller
sends a revertive message to the caller. At the same time the controller gives
outputs which 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 applies a low on J1 pin 6 (04-02451). This is fed
to Jl pin 4 (on 04-02250) which, through V2 and V3, energises relay K1 for
2 minutes, operating an external alarm (if fitted). K1.1 contacts are rated at
50V 1A. Operation of the PTT cancels the alarm.
Refer to section 7.3.15 for setting up instructions.
To Next Page
Loading...
Need help?
General
