Icom IC-M45 Service Manual

Also see for IC-M45: Service manual

Contents

Icom IC-M45 Service Manual Manual pdf 7 page image

SECTION 3 CIRCUIT DESCRIPTION3 - 13-1 RECEIVER CIRCUITS3-1-1 ANTENNA SWITCHING CIRCUITThe antenna switching circuit functions as a low-pass filterwhile receiving and as resonator circuit while transmitting.The circuit does not allow transmit signals to enter receivercircuits.Received signals enter the MAIN unit from the antenna con-nector and pass through the low-pass filter (L1–L3, C1–C5,C7). The signals are then applied to the RF circuit via theantenna switching circuit (D1, L4, L25).3-1-2 RF CIRCUITThe RF circuit amplifies signals within the range of frequen-cy coverage and filters out-of-band signals.The signals from the antenna switching circuit pass througha tunable bandpass filter (D2, L5) where the object signalsare led to the RF amplifier circuit (Q1).The amplified signals at Q1 are applied to the 2-stage tun-able bandpass filter (D3, D4, L6, L7) to suppress unwantedsignals and improve the selectivity. The signals are thenapplied to the 1st mixer circuit.D2–D4 employ varactor diodes, that are controlled by thePLL lock voltage, to track the band pass filters.3-1-3 1ST MIXER AND 1ST IF CIRCUITSThe 1st mixer circuit converts the received signal to a fixedfrequency of the 1st IF signal with a 1st LO (VCO output) fre-quency. By changing the 1st LO frequency, only the desiredfrequency will be passed through a pair of crystal filters atthe next stage of the mixer.The signals from the RF circuit are mixed with the VCO sig-nals at the 1st mixer circuit (Q2) to produce a 30.85 MHz 1stIF signal.The 1st IF signal is applied to a pair of crystal filters (FI1) tosuppress out-of-band signals and is then amplified at the IFamplifier (Q3). The amplified signal is applied to the 2ndmixer circuit (IC1).3-1-4 2ND IF AND DEMODULATOR CIRCUITSThe 2nd mixer circuit converts the 1st IF signal to a 2nd IFsignal. A double superheterodyne system (which convertsreceive signals twice) improves the image rejection ratio andobtains stable receiver gain.The FM IF IC (IC1) contains the 2nd local oscillator, 2ndmixer, limiter amplifier, quadrature detector, and noisedetector circuits, etc.The 1st IF signal from Q3 is applied to the 2nd mixer sectionof IC1 (pin 16), and is mixed with a 30.4 MHz 2nd LO signalgenerated at the PLL circuit by doubling the reference fre-quency (15.2 MHz) to produce a 450 kHz 2nd IF signal.The 2nd IF signal from IC1 (pin 3) is passed through theceramic filter (FI2), where unwanted signals are sup-pressed, and is then applied to the 2nd IF and limiter ampli-fiers in IC1 (pin 5). The signal is applied to the FM detectorsection in IC1 for demodulation into AF signals.The FM detector circuit employs a quadrature detectionmethod (linear phase detection), which uses a ceramic dis-criminator (X2) for phase delay to obtain a non-adjusting cir-cuit. The detected signal from IC1 (pin 9) is applied to the AFcircuit.3-1-5 AF AMPLIFIER CIRCUITThe AF amplifier circuit amplifies the detected signals todrive a speaker. The AF circuit includes an AF mute circuitfor the squelch.AF signals from IC1 (pin 9) are applied to the de-emphasiscircuit (R92, C152). The de-emphasis circuit is an integratedcircuit with frequency characteristic of –6 dB/octave.The integrated signals are applied to the active filters (Q21,Q22). Q21 functions as a high-pass filter to suppressunwanted lower noise signals and Q22 functions as a low-pass filter to suppress higher noise signals.The filtered signals are passed through the [VOLUME] con-trol, and are then applied to the AF power amplifier (IC9, pin1) via the analog switch (IC5, pins 4, 3). The output signalfrom IC9 (pin 4) drives the internal (external) speaker.Mixer16Limiteramp.2nd IF filter450 kHzPLL ICIC3X115.2 MHzX2(30.4 MHz)RSSIIC1 TA31136F131st IF (30.85 MHz)from Q3"SQL" signal to the CPU pin 22111098 7 5 3AF signal "DET"R5R22C53 R26 C54C42R23C50R25C51Squelch leveladjustment potR1 (VR-B unit)217 16ActivefilterFI2NoisedetectorFMdetector2• 2ND IF AND DEMODULATOR CIRCUITS