Icom IC-F3022S Circuit Description, Receiver Circuits, Antenna Switching Circuit PA Unit

SECTION 5 CIRCUIT DESCRIPTION

5-1 RECEIVER CIRCUITS

5-1-1 ANTENNA SWITCHING CIRCUIT (PA UNIT)

The antenna switching circuit toggles the receive (RX) line and transmit (TX) line.

The received signals from the antenna are passed through the low-pass filter (ANT UNIT; L801, L802, C802, C803, C807) and antenna switch (D701, D704, D706).

While transmitting, the voltage on the T5V line is applied to D701, D704 and D706, and these are ON. Thus the TX line is connected to the antenna. Simultaneously, the RX line is connected to the ground (GND) to prevent transmit signal entering.

While receiving, no voltage is applied to the D701, D704 and D706, and these are OFF. Thus the TX line and the antenna are disconnected to prevent received signals entering. Simultaneously, the RX line is disconnected from the GND and the received signals are passed through the low-pass filter (L712, L714, C750, C751). The fi ltered signals are applied to the RF circuits.

5-1-2 RF CIRCUITS (MAIN UNIT)

RF circuits filter and amplify the received signals within the frequency coverage.

The received signals from the antenna switching circuit are passed though the two-staged bandpass filter (BPF; D19, D24, L7, L8, C22, C25, C27–C29, C369) to fi lter-out unwanted signals, and the fi ltered signals are applied to the RF amplifi er (Q5). The amplifi ed received signals are then applied to the 1st mixer (Q6) via another BPF (L47–L49, C19, C40, C437–C439, C443).

5-1-3 1st IF CIRCUITS (MAIN UNIT)

The received signals are converted into the 1st IF signal, fi ltered and amplifi ed at the 1st IF circuits.

The received signals from the BPF (L47–L49, C19, C40, C437 –C439, C443) are applied to the 1st mixer (Q6) and converted into the 46.35 MHz 1st IF signal by being mixed with the local oscillator (LO) signal from the RX VCO (Q17, D9, D11, D33).

The converted 1st IF signal is passed through the 1st IF fi lter (FI1) to filter-out adjacent signals, then applied to the 1st IF amplifi er (Q7). The amplifi ed 1st IF signal is then applied to the FM IF IC (IC9, pin 16).

5-1-4 2nd IF AND DEMODULATOR CIRCUITS (MAIN UNIT)

The 1st IF signal is converted into the 2nd IF signal, and demodulated.

The 1st IF signal from the 1st IF amplifi er is applied to the 2nd mixer in the FM IF IC (IC9, pin 16), and converted into the 450 kHz 2nd IF signal by being mixed with the 45.9 MHz 2nd LO signal from the reference frequency oscillator (X2) via the tripler (Q22) and BPF (L33, C163, C164, C166). The converted 2nd IF signal is output from pin 3, and passed through the 2nd IF fi lters to remove sideband noise.

The 2nd IF signal is passed through the 2nd IF fi lter (FI2) and applied to the FM IF IC (IC9, pin 5) again. The fi ltered 2nd IF signal is amplifi ed at the limiter amplifi er, and FM-demodulated by the quadrature detector (IC9, pins 10, 11, X1). The demodulated AF signals are output from pin 9, then applied to the AF amplifi er circuits.

5-1-5 AF AMPLIFIER CIRCUITS (MAIN UNIT)

The demodulated AF signals from the FM IF IC are amplifi ed and fi ltered at AF circuits.

The demodulated AF signals from the FM IF IC (IC9, pin 9) are passed through high-pass fi lter (HPF; IC5, pins 2, 1) to remove tone signals. The filtered AF signals are passed through the de-emphasis circuit (R142, C249) to obtain the 6 dB/oct of frequency characteristic. The de-emphasized AF signals are passed through the RX mute switch (Q32, Q33), AF switch (Q36, Q37), HPF (IC5, pins 13, 14), analog switch (IC3, pins 1, 2), AF mixer (IC5, pins 6, 7) and analog switch (IC3, pins 10, 11) in sequence.

• 2nd IF AND DEMODULATOR CIRCUITS									Q22
							45.9 MHz
						FI2			BPF	3
		8		7	5	3	2		X2
									TCXO	15.3 MHz
23			Filter		Noise	Noise
D/A converter			amp.		amp.	detector
				Limiter
(IC12)	Quadrature
24				amp.		Mixer
	detector
					RSSI			FM IF IC (IC9)
	9	10	11		12	13	16

		1st IF signal from the 1st IF amplifier (Q7)
Demodulated signals		“NOIS” signal to the CPU (IC22: pin 75)
	R5V	“RSSI” signal to the CPU (IC22: pin 50)
to the AF circuits
	X1

5 - 1