Icom IC-2720H Service Manual

Also see for IC-2720H: Step-by-Step Programming Guide Instruction manual Service manual Operating guide

Contents

4 - 2The signals from the RX band swtiching circuits are thenapplied to the left side displayed 1st mixer circuit (IC1005, pin6).4-1-3 1ST MIXER AND 1ST IF CIRCUIT FOR LEFTSIDE DISPLAY (MAIN UNIT)The 1st mixer circuit converts the received RF signals to afixed frequency of the 1st IF signal with a PLL output fre-quency. By changing the PLL frequency, only the desired fre-quency will pass through the bandpass filter at the next stageof 1st mixer circuit.The RF signals are mixed with 1st LO signals at the 1st mixer(IC1005) to produce a 38.85 MHz 1st IF signal. The 1st IFsignal is output from pin 1, and passed through the crystalbandpass filter (FI1003) to suppress unwanted harmoniccomponents. The filtered signal is amplified at the IF amplifi-er (Q1040) after being passed through the limiter circuit(D1021). The amplified signal is applied to the 2nd mixer cir-cuit (IC1001).4-1-4 2ND IF AND DEMODULATOR CIRCUITS FORLEFT SIDE DISPLAY (MAIN UNIT)The 2nd mixer circuit converts the 1st IF signal to a 2nd IFsignal. A double conversion superheterodyne system (whichconverts receive signal twice) improves the image rejectionratio and obtains stable receiver gain.The FM IC IC (IC1001) contains the 2nd mixer, limiter andnoise amplifiers, quadrature detector, S-meter detector,active filter circuits, etc. A 2nd LO signal (38.4 MHz) is pro-duced at the PLL circuit by dividing it’s reference frequency.The 38.85 MHz 1st IF signal from the IF amplifier (Q1040) isapplied to the 2nd mixer section of the FM IF IC (IC1001, pin16), and is mixed with the 2nd LO signal (38.4 MHz) to beconverted to a 450 kHz 2nd IF signal.The 2nd IF signal is applied to the each demodulator circuitsby AM or FM mode.• FM MODEThe 2nd IF signal is output from the FM IF IC (IC1001, pin 3)and passes through the ceramic bandpass filter (FI1001).The filtered signal is fed back to the IC, and amplified at thelimiter amplifier section (pin 5), then demodulated into AF sig-nals at the quadrature detector section (pins 10, 11). Thedetected AF signals are output from pin 9 and are applied tothe AF circuit via the AM/FM selector circuit (IC2015, pins 7,1).• AM MODEThe 2nd IF signal is output from the FM IF IC (IC1001, pin 3)and passes through the ceramic bandpass filter (FI1000).The filtered signal is applied to the AM detector circuit(Q1017) to convert into AF signals, and then amplified at theQ1014 (pins 5, 1). The amplified AF signals are applied to theAF circuit via the AM/FM selector circuit (IC2015, pins 6, 1).4-1-5 AF AMPLIFIER CIRCUIT FOR LEFT SIDEDISPLAY (MAIN UNIT)The AF amplifier circuit amplifies the demodulated AF signalsto drive a speaker.The AF signals pass through the AF mute switch (Q1010),and are then applied to the electric volume control circuit(IC2011, pin 1) as “VAFO” signal after being passed throughthe low-pass filter (Q1007). The level controlled AF signalsare output from pin 2, and are then applied to the AF poweramplifier (IC2012, pin 2) via the “VOUT1” signal. The poweramplified AF signals are applied to the internal speaker (SP1)via the [EXT SP] jack (J2005).The electronic volume control circuit controls AF gain, there-fore, the AF output level is according to the [VOL] setting andalso the squelch conditions.4-1-6 NOISE SQUELCH CIRCUIT FOR LEFT SIDEDISPLAY (MAIN UNIT)• NOISE SQUELCHA noise squelch circuit cuts out AF signals when no RF sig-nal is received. By detecting noise components in the AF sig-nal, the squelch circuit switches the AF mute switch.16Limiteramp.2nd IF filter450 kHzPLL ICIC1X112.8 MHz38.4 MHzRSSI IC1001 TA31136F131st IF (38.85 MHz)from Q1040"L_RSSI" signal to the CPU(IC2013, pin 33)111098 7 5 3 22VCO UNIT1ActivefilterFI1001FI1000NoisedetectorFMdetector Noisecomp."L_SQL" signal to the CPU(IC2013, pin 40)12R1124R1291C1108C1100R1108C1078C1085 C1084C1097AMDET.IFamp.C1069R1092R1109R1114L_R5X1001FM or WFM AF signalto AM/FM selectorAM AF signalto AM/FM selectorMixer2ndQ1049 from Q10133C1271• 2ND IF AND DEMODULATOR CIRCUIT FOR LEFT SIDE DISPLAY