Icom IC-R20 Service Manual

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Icom IC-R20 Service Manual Manual pdf 15 page image

4 - 74-2 PLL CIRCUITS4-2-1 PLL CIRCUITA PLL circuit provides stable oscillation of the 1st LO fre-quencies. The PLL circuit compares the phase of the divid-ed VCO frequency to the reference frequency. The PLL out-put frequency is controlled by the divided ratio (N-data) of aprogrammable divider.• A-BAND CIRCUIT (1STAVCO BOARD)An oscillated signal from one of the 1st VCO circuits(Q1–Q3, D1–D4) passes through the LO switch (D7–D19)and buffer amplifiers (IC2, Q8). The amplified signal isapplied to the PLL IC (IC3, pin 11) and is prescaled in thePLL IC based on the divided ratio (N-data). The PLL ICdetects the out-of-step phase using the reference frequencyand outputs it from pin 5. The output signal is passedthrough the loop filter (Q6, Q7), and is then applied to theone of the 1st VCO circuits as the lock voltage.• B-BAND CIRCUIT (1STBVCO BOARD)An oscillated signal from one of the 1st VCO circuits(Q1–Q5, D1–D5) passes through the LO switch (D7–D10)and buffer amplifiers (IC2, Q8). The amplified signal isapplied to the PLL IC (IC3, pin 11) and is prescaled in thePLL IC based on the divided ratio (N-data). The PLL ICdetects the out-of-step phase using the reference frequencyand outputs it from pin 5. The output signal is passedthrough the loop filter (Q6, Q7), and is then applied to theone of the 1st VCO circuits as the lock voltage.• DOWN CONVERTER PLL CIRCUIT(DOWNCONV BOARD)An oscillated signal from the VCO circuit (Q1, D3) is ampli-fied at the buffer amplifier (Q2). The amplified signal isapplied to the PLL IC (IC4, pin 8) and is prescaled in the PLLIC based on the divided ratio (N-data). The PLL IC detectsthe out-of-step phase using the reference frequency andoutputs it from pin 5. The output signal is passed through thelow-pass filter (L6, C21, C22 and C40), and is then appliedto the one of the 1st VCO circuits as the lock voltage.4-2-2 REFERENCE OSCILLATOR CIRCUIT(RF UNIT)The reference oscillator circuit (X1, Q556, D171) generatesa 19.2 MHz reference frequency which is stabilized withinthe temperature range –10˚C (+14˚F) to +60˚C (+140˚F).The reference frequency is applied to the PLL ICs (refer tothe list below in details) and the FM IF ICs (IC2, pin 2 for A-BAND circuit: IC17, pin 2 for B-BAND circuit) via the LOamplifiers (Q540 for A-BAND circuit; Q555 for B-BAND cir-cuit).4-2-3 1ST VCO CIRCUITS(1STAVCO AND 1STBVCO BOARDS)• A-BAND CIRCUIT (1STAVCO BOARD)The oscillated signal at 1st A-VCO circuits is applied to thebuffer amplifiers (IC2) via one of the LO switches (D7–D9).The amplified signal is applied to the 1st mixer circuit (RFunit; IC1, pin 3) via the LO amplifier (Q9).The 1st A-VCO circuit is composed of 1AVCO, 2AVCO and3AVCO. Components and osillating frequences of each cir-cuits are as follow list.Shift registerPrescalerPhasedetectorLoopfilterREF.OSC.REF.CONT.ProgrammablecounterProgrammabledividerX119.2 MHzto the FM IF ICRF UNIT1AVCOBufferBufferBufferLOswitchQ9Q8IC2161718PLLBSTBIC3 (PLL IC)PLLDATAPLLCKto the 1st mixer circuit195 11Q1, D1, D2 D7D8D92AVCOQ2, D33AVCOQ3, D4Q6,Q7• A-BAND PLL CIRCUITBOARD NAME PLL IC INPUT OUTPUT1STAVCO IC3 pin 19 pin 51STBVCO IC3 pin 19 pin 52NDCVCO IC1 pin 1 pin 62NDDVCO IC1 pin 1 pin 6DOWNCONV IC4 pin 1 pin 51st A-VCO Components Oscillating freq. (MHz)1AVCO Q1, D1 and D2 849.1–899.09992AVCO Q2 and D3 604.1–849.09993AVCO Q3 and D4 429.25–604.0999