Icom IC-T90A manual Circuit Description, Receiver Circuits

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 TRIPLEXER CIRCUIT (RF UNIT)

The transceiver has a triplexer (low-pass and bandpass fil- ters) on the first stage from the antenna switching diode to separate the signals.

• RF signals 0.495 MHz–75.995 MHz

The 0.495 MHz–75.995 MHz RF signals are passed through the low-pass filters (L5, L6, L24, C43, C483–C486, L11–L13, L17–L19, C21, C481, C482) and are applied to the antenna switching circuit.

• RF signals 76.0 MHz–299.995 MHz

The 76.0 MHz–299.995 MHz RF signals are passed through the low-pass (L5, L6, L24, C43, C483–C486) and high-pass (L9, C9–C11) filters and are applied to the antenna switching circuit.

• RF signals 230.0 MHz–629.995 MHz

The 230.0 MHz–629.995 MHz RF signals are passed through the high-pass (L1, C1–C3) and low-pass (FI1) filters and are applied to the antenna switching circuit.

• RF signals 630.0 MHz–999.990 MHz

The 630.0 MHz–999.990 MHz RF signals are passed through the high-pass (L3, L4, C6–C8) filter and are applied to the RF circuit.

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

The antenna switching circuit functions as a low-pass filter while receiving. However, its impedance becomes very high while transmitting by applying a current to D4, D6, D9, D12, D13, D19.

Thus, tramsmit signals are blocked from the entering the receiver circuits. The antenna switching circuit employs a 1/4λ type diode switching system. The signals are applied to the each antenna switching circuit.

• RF signals 0.495 MHz–75.995 MHz

The signals pass through the antenna switching circuit (D9), and then applied to the RF circuit.

• RF signals 76.0 MHz–299.995 MHz

The signals pass through the antenna switching circuit (D8), and then applied to the RF circuit.

• RF signals 230.0.0 MHz–629.995 MHz

The signals pass through the antenna switching circuit (D7), and then applied to the RF circuit.

4-1-3 RF CIRCUIT (RF UNIT)

The RF circuit amplifies the received signals within the range of frequency coverage and filters out-of-band signals.

(1) 0.495 MHz–29.995 MHz RF CIRCUIT

The signals from the antenna switching circuit pass through the attenuator (D305) and band switch (D301). The signals applied to the bandpass filter (L301, L302, C301–C307) to suppress unwanted signals, then amplified at the RF amplifi- er (Q302).

(2) 30.0 MHz–75.995 MHz RF CIRCUIT

The signals from the antenna switching circuit pass through the attenuator (D305) and band switch (D306). The signals applied to the bandpass filter (D307, D308, L303, L304, C317–C320) to suppress unwanted signals, then pass through the bandpass filter (D309, D310, L306, L307, L323–C325, C331) after being amplified at the RF amplifier (Q303).

• TRIPLEXER AND ANTENNA SWITCHING CIRCUITS

0.495 MHz 75.995 MHz RF signals

76.0MHz 299.995 MHz RF signals

230.0MHz 629.995 MHz RF signals

630.0MHz 999.990 MHz RF signals

Q355		Antenna		ANT
ATT	R3V			Triplexer
		switching
CTRL	ATT	circuit		circuit
D305		D6, D9
LIMIT		ANT	LPF	LPF
		SW
D351		D4, D19
LIMIT		ANT	HPF
		SW
D401	D457	D12, D13
LIMIT	SW	ANT	LPF	HPF
		SW
D451	D10
LIMIT	SW			HPF
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