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)

 

4 - 1

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