Icom IC-T90A manual 4 1ST Mixer and 1ST if Circuits RF, AF and VCO Units, RF Circuit

1st mixer

(IC351)

(3) 76.0 MHz–229.995 MHz RF CIRCUIT

The signals from the antenna switching circuit pass through the attenuator (D351), and then applied to the bandpass fil- ter (D352, D353, L351, L352) to suppress unwanted signals. The signals pass through the bandpass filter (D354–D356, D359, L354, L355, L357, L358, C357–C364) after being amplified at the RF amplifier (Q351).

(4) 230.0 MHz–629.995 MHz RF CIRCUIT

The signals from the antenna switching circuit are applied to the band switch (D457), and then pass through the attenua- tor (D401). The signals pass through the bandpass filter (D402, D403, L401, L402, C402, C404–C406) to suppress unwanted signals, then applied to the RF amplifier (Q401). The amplified signals pass through the bandpass filter (D405, D407, L409, C414, C415), and are then amplified at the RF amplifier (Q402).

(5) 630.0 MHz–999.990 MHz RF CIRCUIT

The signals from the antenna pass through the high-pass fil- ter (L3, L4, C6–C8), and then applied to the attenuator (D451) after being passed through the band switch (D10). The signals pass through the bandpass filter (D452, L451, C451–C454), then applied to the RF amplifier (Q451). The amplified signals pass through the bandpass filter (D453, D454, L452, L453, C456, C460, C462–C464) and RF ampli- fier (Q452).

The amplified or filtered signals pass through one of the band switch (D303, D313, D358, D408, D455), and then applied to the 1st mixer circuit (IC351, pin 1).

4-1-4 1ST MIXER AND 1ST IF CIRCUITS (RF, AF AND VCO UNITS)

The 1st mixer circuit converts the received RF signals to a fixed frequency of the 1st IF signal with a PLL output fre- quency. By changing the PLL frequency, only the desired fre- quency will pass through the bandpass filter at the next stage of the 1st mixer.

The amplified or filtered RF signals are mixed with 1st LO signals at the 1st mixer (IC351) to produce a 69.45 MHz 1st IF signal. The 1st IF signal is output from pin 6, and passed through the bandpass filter (Narrow: AF unit; FI101, Wide: AF unit; FI102) to suppress unwanted harmonic components via the mode switch (AF unit; D101, D102). The filtered 1st IF signal is applied to the IF amplifier (AF unit; Q102). The amplified signal is applied to the 2nd mixer circuit.

The 1st LO signals (53.350 MHz–99.345 MHz, 99.350 MHz–560.545 MHz or 281.675 MHz–534.720 MHz) are gen- erated at the 144 MHz VCO (VCO unit; Q3–Q5, D3, D4) or 430 MHz VCO (VCO unit; Q1, Q2, D1) circuits. The oscillat- ed signal is applied to the 1st mixer via the doubler circuit (Q354, D357) or directly.

4-1-5 2ND IF AND DEMODULATOR CIRCUITS (AF 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 obtain stable receiver gain.

The FM IF IC (IC151) contains 2nd local oscillator, 2nd mixer, limiter amplifier, quadrature detector and S-meter detector circuits.

The amplified 69.45 MHz 1st IF signal from the IF amplifier (Q102) is mixed with the 2nd LO signal at the 2nd mixer (IC151) to produce a 455 kHz 2nd IF signal. The 2nd IF sig- nal from the IC151, pin 3 passes through (AM and FM mode) or bypasses (WFM mode) the 2nd IF filter (FI151) where unwanted heterodyne signals are suppressed via the mode switch (D151, D152). The filtered signals are applied to the AM detector circuit or FM detector circuit respectively.

• RF CIRCUIT

1st LO signal

from the VCO circuit

1st IF signal to the AF unit (AM, FM: 69.45 MHz, WFM: 13.35 MHz)

		Q302
SW		RF	BPF	SW
D303				D301,
		Q303		D306
SW	BPF	RF	BPF
D313
			Q351
SW		BPF	RF	BPF
D358
	Q402		Q401
SW	RF	BPF	RF	BPF
D408
	Q452		Q451
SW	RF	BPF	RF	BPF
D455

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

"AGC" signal from

the AF unit (Q202, pin 4)

4 - 2