Motorola IC-T81E, IC-T81A service manual 7 1ST Mixer Circuit RF Unit

• RF signals WFM band • 50, 144, 440 and 1200 MHz band

The WFM band signals are applied to the RF amplifier (Q201) via the tunable band-pass filter (D202). The ampli- fied signals pass through the tunable bandpass filter (D203), and are then applied to the 1st mixer circuit (IC601, pin 1).

Varactor diodes (D106, D107, D202, D203, D306, D309, D310) are employed by the tunable bandpass filter to tune the center frequency of the bandpass filter. These diodes are controlled by the PLL lock voltage and obtain good image response rejection.

4-1-5 440 MHz BAND RF CIRCUIT (RF UNIT)

The signals from the antenna switching circuit (D402 and D403) are amplified at the RF amplifier (Q401). The ampli- fied signals pass through the bandpass filter (FI401), and are then applied to the 1st mixer circuit (IC601, pin 1) after being amplified at another RF amplifier (IC501).

4-1-6 1200 MHz BAND RF CIRCUIT (RF UNIT)

The signals from the antenna switching circuit (D51 and D52) are amplified at the RF amplifier (Q51). The amplified signals pass through the bandpass filter (FI51), and are then applied to the RF amplifier (IC51). The amplified signal is applied to the 1st mixer circuit (IC601, pin 1) via the band- pass filter (FI52).

4-1-7 1ST MIXER CIRCUIT (RF UNIT)

The 1st mixer circuit converts the received RF signals into a fixed frequency of the 1st IF signal with a 1st LO output fre- quency. By changing the PLL frequency, only the desired frequency will pass through at the next stage of the 1st mixer. 1st mixer circuit produces the different 1st IF signal for WFM and other band signals.

• 2nd IF AND DEMODULATOR CIRCUITS (LOGIC unit)

The applied RF signals are mixed with 1st LO signals at the 1st mixer (IC601) to produce a 69.45 MHz 1st IF signal. The 1st IF signal is output from the 1st mixer (IC601, pin 6), and then passed through the crystal bandpass filter (FI601) to suppress unwanted harmonic components. The filtered 1st IF signal is applied to the IF amplifier (IC701). The amplified signal is applied to the 2nd mixer circuit (LOGIC unit; IC701, pin 16).

• WFM band

The RF signals are mixed with 1st LO signals at the 1st mixer (IC601) to produce a 13.35 MHz 1st IF signal. The 1st IF signal is output from the 1st mixer (IC601, pin 6), and then passed through the 1st IF filter (FI602) to suppress unwant- ed harmonic components. The filtered signal is applied to the 2nd mixer circuit (LOGIC unit; IC701, pin 16).

The 1st LO signals are gererated at the VCO circuit which consists of Q301, Q302, D301, Q311, Q312, D302, D311 for 50 MHz, 144 MHz and WFM, Q321, Q322, D321, D322 for 440 MHz, Q350, D351, D352 for 1200 MHz on the VCO unit.

• The 1st LO signal for 50, 144, 440, WFM band

The 1st LO signals which are generated on the VCO unit are applied to the buffer-amplifier (Q313 and D312 for 50 MHz, 144 MHz and WFM, Q323 for 440 MHz). The buffer-ampli- fied signals are applied to the LO-amplifier (Q822 for 50 MHz, 144 MHz and WFM, Q823 for 440 MHz), and are then applied to the 1st mixer circuit via the TX/RX switch (D802 and D803 for 50 MHz, 144 MHz and WFM, D804 and D805 for 440 MHz) on the RF unit.

• The 1st LO signal for 1200 MHz band

The 1st LO signals which are generated on the VCO unit are applied to the buffer-amplifier (Q351). The buffer-amplified signals are applied to the doubler circuit (Q353), and pass- es through the high-pass and low-pass filter. The filtered sig- nals are applied to the 1st mixer circuit (Q601) on the LOGIC unit after being amplified at the LO-amplifier (Q824).

R58

				2nd IF filter
C733		C734		450 kHz
				FI701
8	R730	7	5	3

2

"2nd LO" signal from IC801, pin 13 (RF unit)

R731	Noise
Active	detector
filter	Limiter
C736

	amp.
R735	FM
	detector

2nd Mixer

Noise

C733

AF signal to AF filter (LOGIC unit; IC241)

9	10 C712	RSSI
		11 R727	12

R706

C701

R729

C720Q702

comp.	16	IC701 TA31136FN
13		2nd IF (19.65 MHz)
		from Q701 (RF unit)
		"NOISE" signal to the CPU
		pin 12
WFM		"SD" signal to the CPU
		pin 3

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