Icom IC-F410 Service Manual

Also see for IC-F410: Instruction manual Instruction manual

Contents

Icom IC-F410 Service Manual Manual pdf 7 page image

SECTION 4 CIRCUIT DESCRIPTION4 - 14-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN unit)The 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 antenna connector and passthrough the low-pass filter (L1–L3, C1, C2, C9–12). The fil-tered signals are then applied to the RF circuit passedthrough the λ⁄ 4 type antenna switching circuit (D4, D5, L5).4-1-2 RF CIRCUIT (MAIN unit)The 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 throughthe attenuator circuit (D4, D5) and the two-stage tunablebandpass filters (D7, D8). The filtered signals are amplifiedat the RF amplifier (Q2) and then enter other two-stagebandpass filters (D9, D10) to suppress unwanted signals.The filtered signals are applied to the 1st mixer circuit (Q3).The tunable bandpass filters (D7–D10) employ varactordiodes to tune the center frequency of the RF passband forwide bandwidth receiving and good image response rejec-tion. These diodes are controlled by the CPU (FRONT unit;IC1) via the level controller (IC12).The attenuator circuit (D4, D5) functions only when theattenuator function is assigned to a programable key andturnes on to protect the RF amplifier from distortion causedby receiving excessively strong signals.When the attenuator function is turned on, the CPU (FRONTunit; IC1, pin 32) switches the voltage level of the “RF ATT”line from high to low and then controls the attenuator switch(Q1). In this case, the current of D4, D5 is increased and D4,D5 act as an attenuator.4-1-3 1ST MIXER AND 1ST IF CIRCUITS(MAIN unit)The 1st mixer circuit converts the received signals to a fixedfrequency of the 1st IF signal with the PLL output frequency.By changing the PLL frequency, only the desired frequencywill pass through a pair of crystal filters at the next stage ofthe 1st mixer.The RF signals from the bandpass filter are applied to the1st mixer circuit (Q3). The applied signals are mixed with the1st LO signal coming from the RX VCO circuit (Q23) to pro-duce a 46.35 MHz 1st IF signal. The 1st IF signal passesthrough a pair of crystal filters (FI1a/b) to suppress out-of-band signals. The filtered signal is amplified at the 1st IFamplifier (Q4) and applied to the 2nd IF circuit.4-1-4 2ND IF AND DEMODULATOR CIRCUITS(MAIN unit)The 2nd mixer circuit converts the 1st IF signal to a 2nd IFsignal. A double-conversion superheterodyne systemimproves the image rejection ratio and obtains stable receiv-er gain.The 1st IF signal from the IF amplifier (Q4) is applied to the2nd mixer section of the FM IF IC (IC1, pin 16) and is thenmixed with the 2nd LO signal for conversion to a 450 kHz2nd IF signal.IC1 contains the 2nd mixer, limiter amplifier, quadraturedetector, active filter and noise amplifier circuits, etc. Atripled frequency from the PLL reference oscillator is usedfor the 2nd LO signal (45.9 MHz).The 2nd IF signal from the 2nd mixer (IC1, pin 3) passesthrough ceramic filters (FI2 and FI3) during narrow channelspacing selection or passes through FI2 (bypassing FI3)only during wide channel spacing selection. It is then ampli-fied at the limiter amplifier section (IC1, pin 5) and applied tothe quadrature detector section (IC1, pins 10, 11 and X1) todemodulate the 2nd IF signal into AF signals.The AF signals are output from pin 9 (IC1) and are thenapplied to the AF amplifier circuit.• 2nd IF and demodulator circuitsFI22nd IF filterNoisedetectorPLL ICIC10Limiteramp.FMdetectorActivefilterAF signals5VX1 DiscriminatorIC12RSSIMixerX215.3 MHz45.9 MHz1st IF from the IF amplifier (Q4)NOIS signal to the CPU (FRONT unit; IC1)8 7 5FI3 BPF3 2317 16161311109IC1TA31136FN