Icom ID-51A Service Manual - Page 12

2ND IF CIRCUITS • Band A The 1st IF signal from the 1st IF circuit is applied to the IF IC (IC13, pin 16), which contains the 2nd IF AMP, 2nd mixer, FM demodulator, and so on. The 1st IF signal is mixed with the 45.9 MHz 2nd LO signal, resulting in the 450 kHz 2nd IF signal. The converted signal is passed through the external 2nd IF filter (FM mode; FI2, FM-N or DV mode; FI1) to remove sideband noise. The filtered signal is amplified by the 2nd IF AMP, and then demodulated by the quadrature detector (X2). The demodulated signal is applied to the RX AF circuit (For FM and FM-N modes) or digital demodulation circuit (For DV mode). • Band B The 1st IF signal from the 1st IF circuit is applied to the IF IC (IC901, pin 16), which contains the 2nd IF AMP, 2nd mixer, FM demodulator, and so on. The 1st IF signal is mixed with the 61.2 MHz 2nd LO signal, resulting in the 450 kHz 2nd IF signal. The converted signal is passed through the external 2nd IF filter (FI900) to remove sideband noise. While receiving in the FM mode, the filtered signal is amplified by the 2nd IF AMP, and then demodulated by the quadrature detector (X900). The demodulated signal is applied to the RX AF circuit, through the AF SW (IC18, pins 6, 1). While receiving in the AM mode, the filtered signal is demodulated by Q904 and amplified by the AGC AMP (Q903). The demodulated signal is applied to the RX AF circuit, through the AF SW (IC18, pins 7, 1). • 2ND IF CIRCUITS X1 15.3MHz Q38 REF X3 45.9MHz BPF To the RX AF circuit (Band A). FM-N/DV CERAMIC BPF FI1 FM CERAMIC BPF -Band A:46.35MHz- FI2 W/N SW FM DET Q36,D15 D16,D18,D19 IC13 From the 1st IF circuit (Band A). To the RX AF circuit (Band B). Q900 REF X4 61.2MHz BPF Q904 AM DET IC18 MODE SW DISCRI X2 FM CERAMIC BPF FI900 -Band B:61.65MHz- IC901 FM From the 1st IF circuit (Band A). DET DISCRI X900 4 - 3