Icom ID-51A Service Manual - Page 13

DIGITAL DEMODULATION CIRCUIT (LOGIC UNIT) While operating in the DV mode, the demodulated signal (digital audio signal) is passed through the LPF (R305, R336, R339, C334 and C337) and amplified by the AF AMP (IC300, pins 2 and 1). The amplified signal is passed through the LPF (Q300 and Q302) and amplified by another AF AMP (IC300, pins 13 and 14), and then applied to the A/D converter (IC19, pin 3) to be converted into the digital audio signal. The digital audio signal is demodulated by the DSP (IC18, pins M10 and L11), and then applied to the linear codec (IC25) to be decoded into an analog audio signal. The decoded AF signal is applied to the RX AF circuit. RX AF CIRCUIT (LOGIC AND MAIN UNITS) • Band A The demodulated AF signal is passed through the mode SW (IC390, pins 4 and 3) and AF filter (IC300, pins 9 and 8, 5 and 7), and then applied to the D/A converter (MAIN UNIT: IC11, pin 18) to be adjusted in volume level. RADIO RECEIVE CIRCUITS • FM BAND (76-108 MHz) The RF signal from the antenna or earphone is passed through two LPFs (L513, L515, L516, C547, C549, C553, C555 and C556), ANT SW (D506 and D903), RF SW (D781), ANT SW (D782 and D783) and LPF (L781, L782, C783 to C786), and then applied to the RF AMP (Q780). The amplified signal is applied to the radio IC (IC441, pin 2), through the ATT (D908) and limiter (D904). • AM BAND (0.52-1.71 MHz) The RF signal from the antenna is passed through two LPFs (L513, L515, L516, C547, C549, C553, C555 and C556), ANT SW (D506 and D903), and LPF (L402, L403, C407- C411), and then applied to the RF AMP (Q908). The amplified signal is applied to the radio IC (IC441, pin 4), through the ATT (Q906 and Q907) and limiter (D905). The radio IC (IC441) demodulates input signals and output to the RX AF circuit. • Band B The demodulated AF signal is passed through the AF filter (IC350, pins 2 and 1, 5 and 7) and mute SW (IC351, pins 2 and 1), and then applied to the D/A converter (MAIN UNIT: IC11, pin 1) to be adjusted in volume level. The level-adjusted AF signal is amplified by the buffer (MAIN UNIT: IC20, pins 3 and 4), and then applied to the AF power AMP (MAIN UNIT: IC2, pin 1), through the ATT (MAIN UNIT: Q20). The amplified AF signal is output to the internal speaker (CHASSIS: SP1) or external speaker, through the [SP] jack (MAIN UNIT: J4). • RADIO RECEIVE CIRCUITS ANT SW LPF LPF From the TX circuit IC441