Icom IC-M23 Service Manual

Also see for IC-M23: Instruction manual

Contents

Icom IC-M23 Service Manual Manual pdf 8 page image

4 - 1SECTION 4. CIRCUIT DESCRIPTION4-1 RECEIVER CIRCUITSRF CIRCUITSThe RF circuits consist of RF filters, antenna switch (ANTSW), RF amplifier (RF AMP), etc., and extract and amplifythe signal of frequency which is desired to receive.• ANTENNA SWITCHING CIRCUITThe RF signal from the antenna is passed through the LPF(L81, L82, C83–C86, C89) and antenna SW (D52, D90,L90, C90, C91), and then applied to the RF AMP (Q90)through the 2-staged tuned BPF (D92, D93, L92, L93, C95,C97–C100, C117).• RF AMPLIFIERThe filtered RX signal is amplified by the RF AMP (Q90)and passed through another 2-staged BPF (D130, D131,L96, L97, L110, L112–L115) to remove unwanted signals.The filtered signal is applied to the 1st mixer (Q150).• 2ND IF AND DEMODULATOR CIRCUITSIC170 is an IF demodulate IC which contains the 2nd localoscillator, 2nd mixer, limiter and quadrature detector in itspackage.The 21.25 MHz reference frequency signal from the PLL IC(IC1, pin 2) is applied to the IF demodulate IC (IC170, pin 2)as the 2nd LO signal, and then mixed with 21.7 MHz 1st IFsignal from the 1st AMP (Q151). The resulting signal of 450kHz 2nd IF signal is output from the pin 3 of IC170.The 2nd IF signal is filtered by the external ceramic filter(FI170) to extract 450 kHz signal only, and then appliedto the internal quadrature detector to be frequency-demodulated. The demodulated AF signals are output fromthe pin 9.The quadrature detector is a frequency demodulator whichuses a discriminator (X170) as a phase delayer, andprovides demodulation without any adjustment.SQUELCH CIRCUITA portion of AF signal from the IF demodulate IC is adjustedin level by the D/A converter (IC190), and then applied tothe noise amplifer (IC170, R174–R176, C179, C180).Only noise components (approximately 30 kHz signal) areamplified by the noise amplifier, and then rectified by theinternal noise detector to be converted into DC voltagecorresponding to noise level; the squelch voltage. Thesquelch voltage is output from the pin 14 of IC170, and thenapplied to the A/D port of the CPU (IC360, pin 33).The CPU (IC360) compares input voltage and presetsquelch level to control audio signals ON/OFF (emit/mute).RX AF CIRCUITSThe demodulated AF signal from the IF demodulate IC (IC170,pin 9) is passed through the AF mute SW (IC260D) and LPF(IC365). The filtered AF signal is de-emphasized by R286 andC285 to obtain –6 dB/oct. of frequency response, and thenapplied to the D/A converter (IC190). The level-adjusted AFsignal is amplified by the AF power amplifier (IC362), and thenapplied to the speaker (CHASSIS: SP1).• RF CIRCUITS• 2ND IF, DEMODULATOR AND RX AF CIRCUITS• 1ST IF CIRCUITSLPFANTSWANTENNABPFRFAMPBPF BPFBPFTo 1st mixerD131 DTX CIRCUITS92D52Q90 D 93D90D130T1CONBPFIFAMP21.7MHz1st IF signalFI150,Q151To IF IC From RF AMP1st LO signalsfrom VCOQ150FI151X170450KHzBPFLPFLPFAFAM PX121.25MHzAFMUT EFrom 1st IF AMPIC1PLL ICDE -ENPHAIF ICD/A ICSPOAF OUTJ250IC170FI170450KHzIC190IC362IC365VOLINVOLBEEPDe-emphasisR286, C285SQLOSQLI RSSINOISE LEVEL21.25 MHz2ND LO signalDETInt. SPExt. SPIC260DDETMSSP11ST IF, 2ND IF AND DEMODULATOR CIRCUITS• 1ST IF CIRCUITSThe RX signal is mixed with 1st LO signal at the 1stmixer (Q150), resulting in the 21.7 MHz 1st IF signal. Theconverted 1st IF signal is filtered by the crystal filters (FI150and F151) to remove unwanted out-of-band signals.The filtered 1st IF signal is amplified by the 1st IF AMP(Q151), and then applied to the IF demodulate IC (IC170,pin 16).