Icom IC-F3GS, IC-F3GT service manual Circuit Description, Receiver Circuits

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT

Received signals are passed through the low-pass filter (L1, L2, C1–C5, C8). The filtered signals are applied to the λ⁄4 type antenna switching circuit (D2, D8).

The antenna switching circuit functions as a low-pass filter while receiving. However, its impedance becomes very high while D2 and D8 are turned ON. Thus transmit signals are blocked from entering the receiver circuits. The antenna switching circuit employs a λ⁄4 type diode switching system. The passed signals are then applied to the RF amplifier cir- cuit.

4-1-2 RF CIRCUIT

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 are amplified at the RF amplifier (Q12) after passing through the tunable bandpass filter (D9, D10, L16, L17, C78–C80, C83, C86, C277). The amplified signals are applied to the 1st mixer cir- cuit (Q13) after out-of-band signals are suppressed at the tunable bandpass filter (D11, D12, L18, L19, C91, C92, C94, C96–C98).

Varactor diodes are employed at the bandpass filters that track the filters and are controlled by the CPU (IC8) via the expander IC (IC10) using T1–T4 signals. These diodes tune the center frequency of an RF passband for wide bandwidth receiving and good image response rejection.

The signals from the RF circuit are mixed at the 1st mixer (Q13) with a 1st LO signal coming from the VCO circuit to produce a 31.05 MHz 1st IF signal.

The 1st IF signal is applied to two crystal filters (FI1) to sup- press out-of-band signals. The filtered 1st IF signal is applied to the IF amplifier (Q14), then applied to the 2nd mixer circuit (IC2, pin 16).

4-1-4 2ND IF AND DEMODULATOR CIRCUITS

The 2nd mixer circuit converts the 1st IF signal into a 2nd IF signal. A double conversion superheterodyne system (which converts receive signals twice) improves the image rejection ratio and obtains stable receiver gain.

The 1st IF signal from the IF amplifier is applied to the 2nd mixer section of the FM IF IC (IC2, pin 16), and is mixed with the 2nd LO signal to be converted into a 450 kHz 2nd IF sig- nal.

The FM IF IC contains the 2nd mixer, limiter amplifier, quad- rature detector and active filter circuits. A 2nd LO signal (30.6 MHz) is produced at the PLL circuit by doubling it’s ref- erence frequency.

The 2nd IF signal from the 2nd mixer (IC2, pin 3) passes through a ceramic filter (FI2) to remove unwanted hetero- dyned frequencies. It is then amplified at the limiter amplifi- er (IC2, pin 5) and applied to the quadrature detector (IC2, pins 10, 11) to demodulate the 2nd IF signal into AF signals.

4-1-3 1ST MIXER AND 1ST IF CIRCUITS

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

• 2nd IF and demodulator circuits

						2nd IF filter		PLL IC
						450 kHz
		C122		C121				IC1
			R86				17		16
						FI2		⋅2
		8		7	5	3	2
R87	R88
				Noise		Noise
		Active		detector		comp.
		filter		Limiter		Mixer		X1
								15.3 MHz
				amp.

"SQLIN" signal from the		FM
D/A convertor (IC10, pin 23)
		detector					RSSI
									IC2 TA31136F
		9	10	C116 11			12	13	16 1st IF from the IF amplifier (Q14)
AF signal "DET"	R83			R82				"NOIS" signal to the CPU pin 19
	C112	C113				R5	"SD" signal to the CPU pin 98
					R84
			X3

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