User's Manual

Crystal X101,connected to pin 1 of IC101, determines the second local oscillator frequency.
In this case the crystal has a frequency of 20.945MHz. The first IF signal is applied to
the mixer and resultant frequency of 455KHz, is the difference between the IF signal
and second local oscillator.
The 455KHz IF signal is output from pin 3 and is applied to a 455KHz band-pass filter CF1.
The output of CF1is passed via pin 5 to a high gain IF amplifier coupled to the
adjustable quadrature detector T2.Any detected signal is produced at pin 9 of
IC101 and applied to the Receiver Audio Circuit and the Mute (Squelch) Circuit.
Squelch ( MUTE ) Circuit
Any noise signal is amplified by IC101 internal noise amplifier .
Signal is applied to pin10 of IC101. The squelch trigger output (pin 12,IC101) is applied to the
pin 6 of J1.
When noise is present, the voltage at pin 12 of IC101 is exceeds than 0.7V. The squelch trigger
output is open, It’ s make pin 6 of J1 open state.
When no noise is present, the voltage at pin 12 of IC101 less 0.7v and pin 13 of IC101
Is HI voltage. This make pin 6 of J1 short state.
VR101 is set to tuning squelch when 25khz channel space is present.
Carrier Detect
A Carrier Detect ( MUTE DETECT ) output is available on pin 6 of J1 .
AF Output Low Pass Filter
A low pass filter formed by R108 and C110 removes any extraneous 455kHz energy
from the AF output of the FM receiver chip (pin 9 of IC101).
The filtered signal is passed to pin 2 of J1.
Microcontroller
The PIC16C57C04 microcontroller IC controls the programmable features and frequency synthesizer
Data.
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