SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN UNIT)

The antenna switching circuit functions as a low-pass filter while receiving and as resonator circuit while transmitting. The circuit does not allow transmit signals to enter receiver circuits.

Received signals from the antenna connector pass through the low-pass filter (L131, L132, C131–C136) and antenna switching circuit (D151, D152). The filtered signals are then applied to the RF amplifier circuit (Q165).

4-1-2 RF AND 1ST MIXER CIRCUITS (MAIN UNIT)

The 1st mixer circuit converts the received signals to a fixed frequency of the 1st IF signal with a PLL output frequency. By changing the PLL frequency, only the desired frequency will be passed through a pair of crystal filters at the next stage of the 1st mixer.

The signals from the antenna switching circuit are passed through the 2-stage bandpass filters (D154, D155, L154, L155) and amplified at the RF amplifier (Q165). The amplified signals are passed through another 2-stage bandpass filters (D181, D182, L181, L182), and then applied to the 1st mixer circuit (Q199).

The filtered signals are mixed at the 1st mixer (Q199) with a 1st LO signal coming from the PLL circuit to produce a 31.05 MHz 1st IF signal. The 1st IF signal is passed through a pair of crystal filter (FI211) and is then amplified at the IF amplifier (Q211).

The 1st IF signal is applied to a 2nd mixer section of the FM IF IC (IC231, pin 16). The signal is then mixed with a 2nd LO signal for conversion into a 450 kHz 2nd IF signal.

IC231 contains the 2nd mixer, limiter amplifier, quadrature detector and active filter circuits. A 30.6 MHz 2nd LO signal is produced at the PLL circuit using the reference frequency.

The 2nd IF signal from the 2nd mixer (IC231, pin 3) passes through ceramic filters (FI231, FI232) to remove unwanted heterodyned frequencies. It is then amplified at the limiter amplifier section (IC231, pin 5) and applied to the quadrature detector section (IC231, pins 10 and 11) to demodulate the 2nd IF signal into AF signals.

4-1-4 AF CIRCUIT (MAIN AND LOGIC UNITS)

AF signals from the FM IF IC (IC231, pin 9) are fed to the analog switch (IC282).

The AF signals (detected signals) passes through the AF mute switch (IC281A, pins 2 and 1) via “DET” signal, and are then applied to the analog switch (IC282, pin 1). The signals are then applied to the low-pass filter (IC261B, C266, C267, R270–R272).

The filtered AF signals are applied to and adjusted audio level at the [VOL] control (VR unit; R801) via the “VOLIN” signal. The level controlled signals are passed through the AF mute switch (LOGIC unit; Q411) which is controlled by “AFMS” signal from the CPU (IC661, pin 84). The passed signals are applied to the AF power amplifier (IC421, pin 4), and then output to the internal speaker or [EXT SP] jack after being passed through the de-emphasis circuit (R411, C413) to obtain the –6 dB/octave frequency characteristics

4-1-3 2ND IF AND DEMODULATOR CIRCUITS (MAIN UNIT)

The 2nd mixer circuit converts the 1st IF signal to a 2nd IF signal. A double conversion superheterodyne system (which converts receive signal twice) improves the image rejection and obtain stable receiver gain.

• 2ND IF AND DEMODULATOR CIRCUITS

R240 R241

"SQCON" signal to the D/A convertor IC (IC251, pin 2)

 

 

 

 

Q231

2nd IF filters

 

 

 

 

Q232

 

 

 

 

 

450 kHz

C238

C239

 

 

W/N

 

 

 

 

 

 

 

 

 

SW

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8

R239

7

5

 

Fl232 Fl231

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Noise

 

 

 

 

 

 

 

 

Active

 

 

detector

 

 

 

 

 

 

 

 

filter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

IF amp.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

30.6 MHz

 

 

 

 

2

17

16

 

 

3

2 Q221

PLL IC

 

 

 

 

IC1

2nd

Mixer X1

 

 

 

 

 

FM

 

 

 

 

 

 

 

 

"SQLOUT" signal to the D/A detector

 

 

 

 

 

 

Noise

convertor IC (IC251, pin 1)

 

 

 

RSSI

 

 

 

 

 

comp.

 

 

 

 

 

 

 

 

 

9

10

11

 

 

 

12

 

 

C242

 

 

 

 

14 16

15.3 MHz

IC231 TA31136FN(D)

AF signal "DET"

R242

 

C232

C244

 

C243

 

R231

"IF" (1st IF signal: 31.05 MHz) from RF unit, Q211

"NOISV" signal to the CPU (pin 32)

R232

"RSSIV" signal to the CPU (pin 33)

X231

R5V

450 kHz

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