Icom IC-M710 Service Manual

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Icom IC-M710 Service Manual Manual pdf 6 page image

3-1 RECEIVER CIRCUITS3-1-1 RF FILTER CIRCUIT (MAIN UNIT)Received signals from the antenna connector pass throughthe transmit/receive switching relay (FILTER board RL4317)and are then applied to the MAIN unit via J2.The signals pass through the protection relay (RL2),1.6 MHz cut off high-pass filter (L2–L4, C4–C8, C629) andare then applied to one of nine bandpass filters (includingone low-pass filter for below 2.0 MHz). These filters areselected by the filter control signals (B0–B8) as describedin the table below.The filtered signals pass through the 30 MHz cut-off low-pass filter (L71, L72, C130–C134, C618), and are thenapplied to the 1st mixer circuit (Q6, Q7).• RF FILTERS USED3-1-2 1ST MIXER AND IF CIRCUITS (MAIN UNIT)The 1st mixer circuit converts the received signals into afixed frequency, 69.0115 MHz 1st IF signal using the PLLoutput frequency. By changing the PLL frequency, only thedesired frequency is picked up at the pair of crystal filters(FI1a, FI1b) at the next stage.The IF amplifier (Q8) and resonator circuits are designedbetween the filter pair. The PLL output signal (1LO) entersthe MAIN unit via J3 and is amplified at the 1st LO ampli-fier (Q5) and then applied to the 1st mixer (Q6, Q7)3-1-3 2ND MIXER AND IF CIRCUITS (MAIN UNIT)The 1st IF signal from the crystal filter (FI1b) is convertedagain into a 9.0115 MHz 2nd IF signal at the 2nd mixercircuit (D52, L66, L67). The 60 MHz 2nd local signal (2LO)from the PLL unit enters the MAIN unit via J4 to be appliedto the 2nd mixer.The 2nd IF signal is passed through the noise blanker gate(D15, D16) and amplified at the 2nd IF amplifier (Q16) andthen applied to one of the 9 MHz IF filters as describedbelow. The passed signal is amplified at the two stage 2ndIF amplifiers (Q32, Q33) and is applied to a demodulatorcircuit (D39 for H3E or IC10 for J3E and others).• 2ND IF FILTERS USED3-1-4 NOISE BLANKER CIRCUIT (MAIN UNIT)The noise blanker circuit cuts off the IF circuit line at themoment of receiving a pulse-type noise.A portion of the 2nd IF signal between resonator circuits(L83, L84 after stage of the 2nd mixer, D52) is amplifiedat the noise amplifiers (Q9, IC8, Q11). The signal is thendetected at the noise detector (D17) to convert the noisecomponents to DC voltages.The signals are then applied to the noise blanker switch(Q13, Q14). At the moment the detected voltage exceedsthe Q13’s threshold level, Q14 outputs a blanking signalto close the noise blanker gate (D15, D16) by applyingreverse-biased voltage. Q15 turns the noise blanker circuitON and OFF.3 - 1SECTION 3 CIRCUIT DESCRIPTIONLPF orBPF0.5–29.999 MHz1st mixerQ6, Q71st LO:69.5115–99.0115 MHzFl1a/Fl1bCrystalfilterCrystalfilter69.0115 MHz2nd mixerD522nd LO: 60.0 MHzFl2 orFl3/Fl49.0115 MHz Other modesH3E DetectorD39DemodulatorIC10Audio outputBFOJ3E, J2B, R3E, FSK:FSK narrow, J2B narrow:A1A:9.013 MHz9.0123 MHz9.0116 MHzFrequency(MHz)0.5–1.9992–2.9993–4.9995–6.9997–9.999Frequency(MHz)10–13.99914–17.99918–23.99924–29.999ControlsignalB5B6B7B8ControlsignalB0B1B2B3B4EntrancecoilL49L8L13L18L23EntrancecoilL28L33L38L43MODEJ3E, R3E, FSKH3EFSK narrow,A1A narrowUsed filterFI2FI3/FI4Optional narrowfilter*Control signalSEL8: low, H3E8: lowSEL8: low, H3E8: highSEL8: high, H3E8: low• RECEIVE FREQUENCY CONSTRUCTION*Built-in to the GMDSS versions