Icom IC-910H service manual Circuit Description, Receiver Circuit

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUIT

Note: [Main]=Main band, [Sub]=Sub band

4-1-1 VHF TRANSMIT/RECEIVE SWITCHING CIRCUIT (PA UNIT)

Received signals from the antenna connector (CHASSIS; J1) are passed through the low-pass filter (L723–L721, C728–C726, C728) then applied to the transmit/receive switching circuit (RL700, D710).

The transmit/receive switching circuit leads receive signal to the RF circuit from a low-pass filter while receiving. However, the circuit leads the transmit signal from the RF power amplifier to the antenna connector while transmitting.

The passed signals are then applied to the RF amplifier cir- cuit.

4-1-2 VHF RF CIRCUIT (PA UNIT)

Received signals from transmit/receive switching circuit are applied to the RF amplifier circuit (Q507) via the RF attenu- ator (D515), limiter (D514) and tunable band pass filter (D513, L560) circuits.

The amplified signals are then passed through the another three-stage tunable bandpass filters (D512–D510, L13–L15) to suppress unwanted signals. The filtered signals are then applied to the 1st mixer circuit (Q511, Q512).

D510–D513 employ varactor diodes, which are controlled by the CPU (DISPLAY board; IC1) via the D/A converter (MAIN unit; IC1521) and buffer amplifier (MAIN unit; IC1522d), to track the bandpass filter. These varactor diodes tune the center frequency of an RF pass band for wide bandwidth receiving and good image response rejection.

4-1-3 VHF 1ST MIXER CIRCUIT (PA UNIT)

The 1st mixer circuit converts the received signals into a fixed frequency of the 10 MHz IF signal with a PLL output frequency. By changing the PLL frequency, only the desired frequency will pass through a pair of crystal filters at the next stage of the VHF 1st mixer.

The filtered signals from the bandpass filter are mixed with 1st LO signals at the mixer circuit (Q511, Q512) to produce a 1st IF signal (10.85 MHz [Main] or 10.95 MHz [Sub]). The 1st LO signals (125.15 MHz–163.15 MHz) are PLL output frequency, which comes from the VHF VCO circuit (PLL unit; Q191, D191–D194).

The 1st IF signal is then applied to either the Main or Sub band 10 MHz IF circuit in the MAIN unit via P501 [Main] or P510 [Sub].

4-1-4 UHF RF CIRCUIT (PA UNIT)

The received signals from the UHF antenna connector (CHASSIS; J2) are passed through the low-pass filter (L181, L180, C188–C184) and then transmit/receive switching cir- cuit (D182–D185, D265, D266, D227). The signals from the transmit/receive switching circuit are applied to the RF amplifier circuit (Q260) via the RF attenuator circuit (D264) and tunable bandpass filter (D263, L288). The amplified sig- nals are passed through the three-stage tunable bandpass filters (D262–D260, L262–L260), and are then applied to the 1st mixer circuit (Q220, Q221).

4-1-5 UHF 1ST AND 2ND MIXER CIRCUIT (PA UNIT)

The filtered RF signals from the bandpass filter are mixed with a 1st LO signal at the 1st mixer circuit (Q220, Q221) to produce a 1st IF signal (71.25 MHz [Main] or 71.35 MHz [Sub]). The 1st IF signal is passed through a crystal filter (Fl280 [Main], Fl281 [Sub]) to suppress out-of-band signals. The filtered IF signal is applied to the 2nd mixer circuit (Q11) to produce a 10 MHz IF signal (10.85 MHz [Main] or 10.95 MHz [Sub]) with a 2nd LO signal. The IF signal is then applied to the MAIN unit via P1 [Main] or P30 [Sub].

The 1st LO signal (348.75 MHz–408.75 MHz) is generated at the UHF VCO circuit (PLL unit; Q391, D391–D394), and a 2nd LO signal (60.2 MHz) is produced at the PLL circuit by doubling it’s reference frequency (30.2 MHz).

• RECEIVER CONSTRUCTION		PA unit
430 MHz
	1st mixer		2nd mixer
	Q220, Q221		Q11
LPF	BPF	BPF
	1st LO	BPF	2nd LO
	B1LO		B2LO
144 MHz	1st mixer
	Q511, Q512
LPF	BPF

1st LO

A1LO

MAIN unit
	FI51	FI151	IC351	for MAIN band
	BPF	BPF		AF signals to
	10.85 MHz			AF selector circuit
				(IC452)
			BFOM
		to FM IF IC (IC401)		for SUB band
	FI651	FI751	IC851
	BPF	BPF		AF signals to
	10.95 MHz			AF selector circuit
				(IC1002)
			BFOM
		to FM IF IC (IC951)

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