SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT

The antenna switching circuit toggles receive line and transmit line. This circuit does not allow transmit signals to enter the receiver circuits.

Received signals from the antenna are passed through the low-pass filter (LPF: L131, L132, C131–C136) and applied to the antenna switching circuit (D151, D152).

While receiving, no voltage is applied to D151 and D152. Thus, the receive line and the ground are disconnected and L151, L152, C151, C152 and C153 function as an LPF which leads received signals to the RF circuits.

4-1-2 RF CIRCUITS

The RF circuits amplify signals within the range of frequency coverage and filters off out-of-band signals.

The signals from the antenna switching circuit are passed through the two-stage tunable bandpass filter (BPF: D154, D155, L154–L156, C156, C157, C159–C161, C163, C164, C168), then applied to the RF amplifier (Q165).

The amplified signals are passed through another two-stage BPF (D181, D182, L166, L181 C181, C182, C184–C186, C188) to suppress unwanted signals. The filtered signals are then applied to the 1st mixer circuit.

The 1st IF signal from the 1st mixer is passed through the crystal filter (FI211) to suppress unwanted signals, and amplified at the 1st IF amplifier (Q211).

The amplified 1st IF signal is applied to the FM IF IC (IC231).4-1-4 2nd IF AND DEMODULATOR CIRCUITS

The 1st IF signal is converted into the 2nd IF signal and de-modulated by the FM IF IC. The FM IF IC contains 2nd mixer, limiter amplifier, quadrature detector, etc. in its package.

The 1st IF signal from the 1st IF amplifier is applied to pin 16 of IC231, and mixed with the 30.6 MHz 2nd LO signal coming from the doubler (Q221), to convert into the 450 kHz 2nd IF signal and output from pin 3. The 2nd IF signal is filtered by the ceramic filters (FI231, FI232) to suppress the heterodyne noise, then applied to IC231 (pin 5) again and amplified at the limiter amplifier section and demodulated by the quadrature detector.

The quadrature detector is a detection method which uses a ceramic discriminator (X231). The demodulated AF signals are output from pin 9.

4-1-5 AF CIRCUITSDemodulated signals are filtered and amplified at the AF circuits.
4-1-3 1st MIXER AND 1st IF AMPLIFIER CIRCUITS

The 1st mixer circuit converts received signals into the 1st intermediate frequency (IF) signal by mixing with local oscillator (LO) signal. The converted 1st IF signal is filtered at the 1st IF filter, then amplified at the 1st IF amplifier.

The signals from the two-stage BPF are converted into the

31.05MHz 1st IF signal at the 1st mixer (Q191) by being mixed with the 1st LO signals generated at RX VCO (Q41, D31–D34).

• 2nd IF AND DEMODULATER CIRCUITS

AF signals from IC231 (pin 9) are passed through the AF mute switch (IC281 A; pins 1, 2), analog switch (IC282; pins 1, 7), LPF (IC261 C; pins 8–10) and R801 (VR BOARD) to control the AF output level.

The level controlled AF signals are passed through the AF mute switch (Q411) and the de-emphasis circuit (R411, C413) to obtain the frequency characteristic of –6 dB/oct.

The de-emphasized signals are applied to the AF power amplifier (IC421, pin 4). The AF power amplifier provides more than 0.3 W of audio power.

450 kHz 2nd IF filters FI232

"SQCON" signal

 

 

FI231

 

 

 

 

from the D/A converter (IC251)

 

 

 

30.6 MHz 2nd LO signal

 

 

 

W/N

W/N

 

 

 

 

SW

SW

 

 

 

 

 

 

D232

D231

 

 

 

 

8

7

5

 

3

2

PLL

 

 

 

 

 

 

 

IC

 

 

Noise

Noise

 

×2

(IC1)

X1

Active

AMP

detector

 

2

 

1

 

Q221

 

 

filter

Limiter

 

 

 

 

15.3 MHz

 

 

 

 

 

 

 

 

 

 

 

 

 

AMP

 

Noise

Mixer

 

 

 

FM

 

 

comparator

 

 

 

 

detector

 

 

 

 

 

 

 

 

 

 

 

IC231 TA31136FN

9

10

11

14

16 1st IF signal

 

 

 

 

from the IF amplifier (Q211)

AF "DET" signal

 

 

"NOISV" signal to the CPU (IC661, pin 32)

 

 

R5V

 

to the AF circuits

 

 

 

 

 

 

 

 

X231

 

 

 

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