Icom IC-M36 Service Manual - Page 20

4-2 TRANSMITTER CIRCUITS TX AF CIRCUITS AF signals from the microphone (MIC signals) are passed through the MIC line SW (IC260A) and amplified by MIC AMP (IC200B), then passed through the MIC mute SW (IC260C). The MIC line SW (IC260A) selects the MIC signal path (from int. MIC or from ext. MIC), and the MIC mute SW (IC260C) disconnects the MIC line in receiving. The MIC signals from the MIC mute SW (IC260C) are adjusted its amplitude (=deviation) by the D/A converter (IC190), and pre-emphasized by R201 and C385 to obtain +6 dB/oct of frequency response. Pre-emphasized MIC signals are amplitude-limited by the limiter (IC200A) to prevent over deviation, and filtered by the splatter filter (IC200D) to remove 3 kHz and higher signals, then applied to the VCO for frequency modulation. • TX AF AND MODULATION CIRCUITS Q 21,Q22 D22,D23 Q23 BUFF Q24 BUFF To LO switch MOD MUT E MOD Q20 D 20 D 21 VCO [SP MIC] J250 MC250 Int. MIC S250 [PTT] MIC IC200D LPF IC200A LIMIT AM P EXTMIC INMIC IC260A MIC LINE SW IC200B MIC AMP IC190 D/A PRE ENPHA IC260C MIC MUT E TX AMPLIFIERS The frequency-modulated signal from the VCO (TX signal) is passed through the LO SW (D50), and amplified by the buffer AMP (Q50), driver AMP (Q53) and power AMP (Q54) in sequence, to obtain TX output power. APC CIRCUIT A portion of TX output signal is rectified by D91 to be converted into the DC voltage which corresponds to TX power level. IC50 compares the detected voltage and TX power setting voltage from D/A converter (IC190). The resulting voltage from IC50 controls the gain of both the driver (Q53) and the power (Q54) AMPs to keep the TX output power stable. • TX AMPLIFIERS AND APC CIRCUIT D50 From VCO LO SW Q50 BUFF Q53 PRE DRIVE Q54 PWR AM P D52 D90 ANT SW IC50 Q357 APC CTRL D91 PWR DET PCON T5V ANTENNA J1 LPF 4 - 2