Motorola IC-T81A Circuit Description, Receiver Circuits, Duplexer Circuit RF Unit, RF Circuit

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 DUPLEXER CIRCUIT (RF UNIT)

The transceiver has a duplexer (low-pass and high-pass fil- ters) on the frist stage from the antenna connector to sepa- rate the signals into below UHF and SHF signals. The high- pass filter (L51–L55, C41–C46 and C48–C50) is for SHF (1200 MHz) signal and the low-pass filter (C9, C10, C12–C14 and L7–L9) is for below UHF (50 MHz, 144 MHz, 440 MHz and WFM) signals. The filtered SHF signal is applied to the low-pass fileter (C51–C54, L56 and L57).

The RF signals below UHF pass through the duplexer circuit and are separated into VHF (50 MHz, 144 MHz and WFM band) and UHF (440 MHz band) signals. The high-pass fil- ter (C4–C8, L5, L6) is for UHF (440 MHz band) signal and the low-pass filter (C15–C20, L10–L12) is for VHF (50 MHz, 144 MHz and WFM band) signals.

The VHF signals are applied to the another duplexer circuit for separation into 50 MHz and above WFM band signals. The high-pass filter (C21–C24, C84 and L13–L15) is for 144 MHz and WFM band signals and the low-pass filter (C27–C33 and L16–L18) is for 50 MHz band signal.

The separated signals are applied to each RF circuits.

4-1-2 ANTENNA SWITCHING CIRCUITS (RF UNIT)

The antenna switching circuit functions as a low-pass filter while receiving. However, its impeadance becomes very high while transmitting by applying a current to D101 and D102 (50 MHz), D302 and D303 (144MHz and WFM), D402 and D403 (440 MHz), D51 and D52 (1200 MHz).

Thus, transmit signals are blocked from entering the receiv- er circuits. The antenna switching circuit employs a 1/4λ type diode switching system. The passed signals are then applied to each RF amplifier circuit.

4-1-3 50 MHz BAND 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 (D101 and D102) are amplified at the RF amplifier (Q101). The ampli- fied signals pass through the tunable bandpass filter (L108–L110, C115, C117, C120, D106, D107) to suppress out-of-band signals, and are then applied to the 1st mixer circuit (IC601, pin 1).

4-1-4 144 MHz AND WFM BANDS RF CIRCUITS (RF UNIT)

The signal from the antenna switching circuit (D302, D303) are applied to the each band-pass fileters and RF amplifier.

• RF signals 144 MHz band

The 144 MHz band signals are applied to the RF amplifier (Q301) via the tunable bandpass filter (L303, L304, C306, D306). The amplified signals pass through the tunable band- pass filter (C318–C321, D309, D310, L305, L306), and are then applied to the 1st mixer circuit (IC601, pin 1).

• RF CIRCUIT

SHF band (1200 MHz)

						ANT
FI52	IC51	FI51	Q51	D51, D52
BPF	RF	BPF	RF	ANT	LPF	HPF
				SW
IC501	FI401	Q401		D402, D403

	FM mode		UHF band	RF	BPF			RF	ANT	HPF	LPF
			(440 MHz)						SW
D602	FI601	D601
			VHF band
Mode	69.45 NHz Mode			D309, D310	Q301		D306	D302, D303
SW	XTAL	SW	(144 MHz)					ANT
				BPF	RF		BPF		HPF	LPF
to IF amp.	BPF		IC601					SW
(Q701)			1st mixer
			1st LO
	CERAMIC			D203	Q201		D202
	BPF		WFM band
	FI602			BPF	RF		BPF
			(RX only)
	13.35 NHz
WFM mode				D106,	Q101		D101,
				D107			D102
			VHF band	BPF	RF		ANT	LPF
			(50 MHz)				SW
				LEVEL		Q9
				CONV.
							"TUNE" signal from IC5, pin 11 (LOGIC unit)

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