The 2nd Mixer
The input signal and the 2nd local signal is added or subtracted at mixer IC4, and switched to FM/AM receiving side or WFM receiving side at D24 or D25 .
FM/AM
At FM/AM receiving side, the crystal filter XF1 selects the signal of 38.85 MHz. The signal is amplified at the 1st IF amplifier Q22 after the adjacent signal is eliminated.
WFM
At WFM receiving side, the band pass filter selects the signal of 38.85 MHz. The signal is amplified at the 1st IF amplifier Q22 after the adjacent signal is eliminated.
4. IF FM/AM
The amplified signal at the 1st IF amplifier Q22 is supplied to pin 24 of IC6 for demodulation. The signal of 12.8 MHz from the IC 1 reference buffer output is multiplied by 3 at Q17, then mixed with the signal added to pin 1 of IC6 in the mixer circuit inside IC6 to be converted into the 2nd IF signal of 450 kHz. The converted 2nd IF signal is output from pin 3 of IC6.
FM
The output signal from pin 3 of IC6 is input to pin 7 of IC6 after the adjacent signal is eliminated at the ceramic filter FL2. The 2nd IF signal input to pin 7 of IC6 is demodulated at the limiter amplifier and quadrature detection circuit inside IC. Then the signal is output from pin 12 of IC6 as an AF signall.
AM
The output signal from pin 3 of IC6 is input to pin 5 of IC6 after the adjacent signal is eliminated at the ceramic filter FL2. The 2nd IF signal input to pin 5 of IC6 is output from pin 13 of IC6 as an AF signal after AM is detected inside IC. Also reverse AGC is added to the 1st IF amplifier Q22 by AGC amplifier Q25 and the gain is controlled to get the normal audio output even though the input signal is fluctuated.
WFM
The output signal from pin 3 of IC6 is input to pin 7 of IC6. The 2nd IF signal input to pin 7 of IC6 is demodulated at the limiter amplifier and quadrature detection circuit inside IC, and output from pin 12 of IC6 as an AF signal.
5. Squelch
The AF signal is output from pin 12 of IC6 and input to pin 19 of IC6. The input signal is output from pin 21 of IC6 through the noise filter amplifier and rectifying circuit. The rectified signal is supplied to the A/D port of microcomputer IC1. Then the microcomputer IC1 judges the signal to control ON/OFF of audio output.
