Icom IC-4008MKII Circuit Description, Receiver Circuits, Antenna Switching Circuit RF Unit

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)

Received signals from the antenna connector are passed through the low-pass filter (L5, L6, C8–C12). The filtered sig- nals are applied to the λ⁄4 type antenna switching circuit (D7, D101, D102, L4, L206, C209, C210).

The antenna switching circuit functions as a low-pass filter while receiving. However, its impedance becomes very high while D101 and D102 are turned ON (while transmitting). Thus transmit signals are blocked from entering the receiver circuits. The passed signals are then applied to the RF ampli- fier circuit.

4-1-2 RF CIRCUIT (RF 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 are amplified at the RF amplifier (Q3) and passed through the bandpass fil- ter (FI1) to suppress out-of-band signals. The filtered signals are applied to the 1st mixer circuit (Q2).

4-1-4 2nd MIXER AND DEMODULATOR CIRCUITS (RF UNIT)

The 2nd mixer circuit converts the 1st IF signal to 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 (Q1) is applied to the 2nd mixer section in the FM IF IC (IC2, pin 16), and is mixed with the 2nd LO signal to be converted into a 450 kHz 2nd IF signal.

The FM IF IC contains a 2nd mixer, quadrature detector, noise amplifier and a limiter amplifier, etc. The PLL reference oscillator (X1) is used for the 2nd LO signal via the PLL IC (IC1, pins 11, 9), and is applied to pin 1 of the FM IF IC (IC2).

The mixed 2nd IF signal is output from pin 3 and passed through the ceramic bandpass filter (FI3) to remove unwant- ed heterodyne frequencies. It is then amplified at the limiter amplifier section (IC2, pin 5) and applied to the quadrature detector section (IC2, pins 10, 11) to demodulate the 2nd IF signal into AF signals.

4-1-3 1st MIXER AND 1st IF CIRCUITS (RF UNIT)

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

The signals from the bandpass filter are mixed at the 1st mixer circuit (Q2) with a 1st LO signal coming from the VCO circuit to produce a 21.7 MHz 1st IF signal.The 1st IF signal is applied to a crystal filter (FI2) to suppress out-of-band sig- nals. The filtered 1st IF signal is applied to the IF amplifier (Q1), then applied to the 2nd mixer circuit (IC2, pin 16).

4-1-5 AF CIRCUIT (RF AND MAIN UNITS)

AF signals from the FM IF IC (RF unit; IC2, pin 9) are passed through the high-pass filter (RF unit; Q15, Q16) to remove CTCSS signals then applied to the MAIN unit via J2 pin 10 as the “VOL” signal.

The “VOL” signal (AF signals) from the RF unit is applied to the [VOL] control (MAIN unit; R58) to control the audio level via the volume mute switch (Q23). The level controlled AF signals are applied to the AF power amplifier (IC9, pin 2) to drive an internal speaker (SP1) via the [SP] jack (J1).

• 2nd IF AND DEMODULATOR CIRCUITS

	C75	C74
	8	R51 7	5
R52	R53	Noise
	Active	detector
	filter	Limiter
	C76

amp.

FM

detector

9 10 11

AF signal "AF"

	R58	C77
C67		C78
	R57
		R+3
		X2

2nd IF filter
450 kHz		(21.25 MHz)				PLL IC
FI3					9 IC1
3			1		11
	Mixer
				X1
RSSI			21.25 MHz
			IC2 TA31136FN
14			16

1st IF (21.7 MHz) from Q1

"SQL" signal to the CPU pin 59

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