Icom IC-M604A Service Manual - Page 10

4-1-4 2ND IF AND DEMODULATOR CIRCUITS (MAIN UNIT) The 2nd mixer circuit converts the 1st IF signal into a 2nd IF signal. A double superheterodyne system (which converts receive signals twice) improves the image rejection ratio and obtains stable receiver gain. The FM IF IC (IC6 for channel 70, IC1 for other channels) contains the 2nd local oscillator, 2nd mixer, limiter amplifier, quadrature detector, and noise detector circuits, etc. • CHANNEL 70 CIRCUIT The 1st IF signal from the 2nd IF amplifier (Q10) is applied to the 2nd mixer section of FM IF IC (IC6, pin 16), and is mixed with a 21.25 MHz 2nd LO signal, which is generated at the 2nd oscillator section in IC6 and X3, to produce a 450 kHz 2nd IF signal. The 2nd IF signal from IC6 (pin 3) is passed through the ceramic filter (FI6), which unwanted signals are suppressed, and is then applied to the 2nd IF (limiter) amplifier in IC6 (pin 5). The signal is applied to the FM detector section in IC6 for demodulating into AF signals. The FM detector circuit employs a quadrature detection method (linear phase detection), which uses a ceramic discriminator (X2) for phase delay to obtain a non-adjusting circuit. The detected signal from IC6 (pin 9) is applied to the AF circuit. • OTHER CHANNELS CIRCUIT The 1st IF signal from the 2nd IF amplifier (Q2) is applied to the 2nd mixer section of FM IF IC (IC1, pin 16), and is mixed with a 30.6 MHz 2nd LO signal, which is generated at the PLL circuit using the reference frequency (15.3 MHz), to produce a 450 kHz 2nd IF signal. The 2nd IF signal from IC1 (pin 3) is passed through the ceramic filter (FI3), which unwanted signals are suppressed, and is then applied to the 2nd IF (limiter) amplifier in IC1 (pin 5). The signal is applied to the FM detector section in IC1 for demodulating into AF signals. The FM detector circuit employs a quadrature detection method (linear phase detection), which uses a ceramic discriminator (X1) for phase delay to obtain a non-adjusting circuit. The detected signal from IC1 (pin 9) is applied to the AF circuit. • 2ND IF AND DEMODULATOR CIRCUITS (For CHANNEL 70 ONLY) IC5a, "DEC2" signal IC5b IC9 DSC decoder IC3c, IC3d AF signal "DET" 1 6 IC11 Limiter amp. FM detector 9 2nd IF filter 450 kHz FI6 5 3 X3 2 21.25 MHz 2nd mixer 10 11 IC6 TA31136FNG 16 1st IF (21.7 MHz) from Q10 X2 8V • 2ND IF AND DEMODULATOR CIRCUITS (For OTHER CHANNELS) D/A convertor IC (IC15, pin 23) 8 7 2nd IF filter 450 kHz FI3 5 3 (30.6 MHz) 2 Q16 2 17 16 PLL IC IC12 "WXDEC" signal IC3a Active filter FM detector Noise detector Limiter amp. 2nd Mixer X4 15.3 MHz RSSI "DEC1" signal DSC IC3b decoder 9 10 11 14 IC1 TA31136FNG 16 1st IF (30.15 MHz) from Q2 AF signal "DET" 1 7 IC11 "NOISE" signal to the SQL amplifier (IC2, pin 1) X1 8V 4 - 2