Icom IC-M604A Service Manual - Page 13

4-3-4 VCO CIRCUIT (MAIN UNIT) • TX-VCO/CHANNEL 70-VCO (RX) CIRCUITS The VCO outputs from TX-VCO/CHANNEL 70-VCO (Q18) are amplified at the buffer amplifiers (Q19 and Q27), and are applied to the TX/RX switch circuit (D42, D43). The receiver LO signal is applied to the 1st mixer circuit for CHANNEL 70 (D21, L48, L49) passing through a low-pass filter (L51, L52, C150-C152), and the transmitter signal is applied to the pre-drive amplifier (Q28). A portion of the VCO output signal is re-applied to the PLL IC (IC12, pin 2) via the buffer amplifier (Q15). • OTHER CHANNELS-VCO (RX) CIRCUITS The VCO outputs from OTHER CHANNELS-VCO (Q12) are amplified at the buffer amplifiers (Q13 and Q23). The receiver LO signal is applied to the 1st mixer circuit for OTHER CHANNELS (D11, L18, L19) passing through a low-pass filter (L21, L22, C52-C54). A portion of the VCO output signal is re-applied to the PLL IC (IC12, pin 2 or pin 19) via the buffer amplifier (Q19). 4-4 DSC CIRCUITS 4-4-1 DSC MODULATION CIRCUIT (LOGIC, AF AND MAIN UNITS) The ATIS signal from the CPU (LOGIC unit; IC1, pin 117) is applied to the buffer amplifier (AF unit; Q18) as "DSC" signal. The signal passes through the analog switch (AF unit; IC7, pin 1), and then applied to IDC amplifier (AF unit; IC8a). Then, the amplified signal is applied to the transmitter circuit. The signalis passed through the splatter filter (AF unit; IC8b) to suppress unwanted 3 kHz or higher signals. The filtered signals are then applied to the TX modulation circuit via the D/A converter IC (MAIN unit; IC15, pins 11, 12) as a DSC modulation signal "MOD". 4-5 LOGIC CIRCUITS 4-5-1 LOGIC UNIT • CPU IC1 is a 8 bit single chip micro-computer, which contains LCD driver, serial I/O, timer, A/D converter, programmable I/O, ROM and RAM. • SYSTEM CLOCK CIRCUIT X1 is a crystal oscillator, which oscillates 9.8304 MHz system clock for the main CPU (IC1). • RESET CIRCUIT IC2 is a reset IC, which outputs a reset signal ("LOW" pulse) to main CPU (IC1, pin 79) when turning transceiver power ON. • PLL CIRCUIT VCO Loop filter Q18, D37, D39, D40 Buffer Q19 Buffer Q27 Buffer Q15 D42 to 1st mixer circuit D43 to transmitter circuit IC12 (PLL IC) 8 X2 15.3 MHz 16 30.6 MHz signal 17 to the FM IF IC 2 (IC1, pin 2) Q16 Phase detector Programmable counter Prescaler Programmable divider Shift register/ data latch 2 3 PSTB 4 PCK 5 PDATA • DSC CIRCUIT "DSC" signal from the CPU (LOGIC board; IC1, pin 117) Q18 Buff. amp. IC7 analog switch IC8a IDC amp. LPF IC8b D/A convertor IC15 to VCO circuit 4 - 5