User's Manual

Crystal X1, connected to pin 1 of IC6, determines the second local oscillator frequency. In this case
the crystal has a frequency of 44.545MHz. 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 (12.25
kHz channel spacing) or CF2 (6.25 kHz channel spacing). The selection of the filters is
accomplished by diodes D13 (input) and D14 (output) whose bias is controlled by software and
applied to the diodes from pin 21 of IC501.
The output of CF1/CF2 is passed via pin 5 to a high gain IF amplifier coupled to the adjustable
quadrature detector T2 (pin 8). Any detected signal is produced at pin 9 of IC6 and applied to the
Receiver Audio Circuit and the Mute (Squelch) Circuit.
Squelch (MUTE) Circuit
The noise detect circuit in conjunction with IC6 consists of diode D11 and RV2.
Any noise signal is amplified by IC6 internal noise amplifier rectified by D11 .D11 Signal is applied
to pin12 of IC6. The squelch trigger output (pin 14,IC6) is applied to the pin 6 of J501.
When noise is present, the voltage at pin 12 of IC6 is less than 0.7V. The squelch trigger output is
0V(logic 0) It’s make pin 6 of J501 open state.
When no noise is present, the voltage at pin 12 of IC6 exceeds 0.7v and pin 14 of IC6 IS AT
5v(logic 1). This make pin 6 of J501 short state.
Carrier Detect
A Carrier Detect (MUTE DETECT) output is available on pin 6 of J501.
AF Output Low Pass Filter
A low pass filter formed by C134, C135 and R82 removes any extraneous 455kHz energy from the
AF output of the FM receiver chip (pin 9 of IC6).
The filtered signal is passed to pin 2 of J501.
Micro controller
The PIC 16C57 microcontroller IC controls the programmable features and frequency synthesizer
data.
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