Icom IC-M25 Service Manual

Also see for IC-M25: Instruction manual Instruction manual Instruction manual

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Icom IC-M25 Service Manual Manual pdf 9 page image

5-1SECTION 5 CIRCUIT DESCRIPTION5-1 RECEIVER CIRCUITSRF CIRCUITSThe RF circuits consist of RF filters, antenna switch, RFamplifier (RF AMP) and so on. The received signal is extracted and amplified in the RF circuits.• ANTENNA SWITCHING CIRCUITThe RF signal from the antenna is passed through the LPF(L81, L82, C83–C86 and C89), antenna SW (D52, D90, L90,C90 and C91) and 2-staged tuned BPF (D92, D93, L92,L93, C95, C97–C100 and C117), and then applied to the RFAMP (Q90).• RF AMPLIFIERThe filtered RX signal is amplified by the RF AMP (Q90)and passed through another 2-staged BPF (D130, D131,L96, L97, C109, C110 and C112–L115) to remove unwant-ed signals. The filtered signal is applied to the 1st mixer(Q150).2ND IF AND DEMODULATOR CIRCUITSIC170 is an IF IC which contains the 2nd local oscillator,2nd mixer, limiter and quadrature detector in its package.The 21.25 MHz reference frequency signal from the PLLIC (IC1, pin 9) is applied to the IF IC (IC170, pin 2) as the2nd LO signal, and it is mixed with 21.7 MHz 1st IF signalfrom the 1st IF AMP (Q151), resulting in the 450 kHz 2nd IFsignal. The 450 kHz 2nd IF signal is output from the pin 3 ofIC170.The 2nd IF signal is filtered by the external ceramic filter(FI170) which extracts the 450 kHz signal, and then appliedto the internal quadrature detector.The demodulated AF signal is output from the pin 9.The quadrature detector is a frequency demodulator whichuses a discriminator (X170) as a phase delayer.SQUELCH CIRCUITA portion of AF signal from the IF IC is adjusted in level bythe D/A converter (IC190), and then applied to the noiseamplifier (IC170, R174–R176, C179 and 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 voltage,which represents the noise level. The voltage is output fromthe pin 14 of IC170, and then applied to the A/D port of theCPU (IC360, pin 33).The CPU (IC360) compares the applied voltage and squelchreference level to control the audio signal ON/OFF (emits/mutes).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/octave of frequency response, and thenapplied to the D/A converter (IC190) to be adjusted in level.The level-adjusted AF signal is amplified by the AF AMP (IC364)and AF power amplifier (IC362), and then applied to the speak-er (CHASSIS: SP1).• RF CIRCUITS• 2ND IF, DEMODULATOR AND RX AF CIRCUITS• 1ST IF CIRCUITSLPFANTSWANTENNABPFRFAMPBPF BPFBPFTo 1st mixerFrom RF AMPgnalsCOD131 DTX CIRCUITS92D52Q90 D 93D90D130T1CONLPFANTSWANTENNABPFRFAMPBPFBPFIFAMP BPFBPF21.7MHz1st IF signalFI150,Q151To IF ICrom 1st IF AMP To 1st mixerFrom RF AMP1st LO signalsfrom VCOD131 DTX CIRCUITS92D52Q90Q150ELFI151 D 93D90D130T1CONX170450KHzBPFLPFLPFAFAM PX121.25MHzAFMUT EQ1To IF ICFrom 1st IF AMPIC1PLL ICDE -ENPHAIF ICD/A ICSPOAF OUTJ250IC170FI170450KHzIC190IC362IC365VOLINVOLBEEPDe-emphasisR286, C285SQLOSQLI RSSINOISE LEVEL21.25 MHz2ND LO signalDETInt. SPExt. SPIC260DDETMSSP11ST IF CIRCUITSThe RX signal is mixed with the 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).