4-1-6 SQUELCH CIRCUITSquelch circuit mutes AF output signal when no signals are received.

A portion of the AF signals from the FM IF IC (IC231, pin 9) are applied to the IC251 to control the level, and the active filter (R239–R241, C237, C238). The filtered signals are applied to the noise amplifier section in the FM IF IC (IC231, pin 8) to amplify the noise components only.

The amplified noise components are detected at the noise detector section, and output from pin 14 as the "NOISV" signal to the CPU (IC661, pin 32). Then the CPU outputs "AFMS" signal from pin 84 according to the "NOISV" signal level to control the AF mute switch (Q411).

4-2 TRANSMITTER CIRCUITS

4-2-1 MIC AMPLIFIER, SPLATTER FILTER CIRCUITS

The MIC amplifier circuit amplifies audio signals from the microphone within +6 dB/oct pre-emphasis characteristics and amplifies to the level needed for modulation.

The AF signals from the microphone are passed through the microphone switch (Q461) and the microphone mute switches (IC682 A, pins 1, 2; IC682 B, pins 5, 6). The AF signals are then passed through the pre-emphasis circuit (R463, C463) to obtain frequency characteristics of +6 dB/oct.

The pre-emphasized AF signals are amplified at the microphone amplifier (IC261 B), and adjusted its level at the microphone gain control circuit (Q653, Q654, R474, R705, R707, R708). The level adjusted signals are applied to the limiter amplifier (IC491 A; pins 1, 2) to limit its level via the AF mute switch (IC481 A; pins 1, 2).

The AF signals are then passed through the splatter filter (IC491 B; pins 6, 7) to suppress unwanted 3 kHz and higher audio signals, then applied to the modulation circuit via the D/A converter (IC251; pins 3, 4).

4-2-2 MODULATOR CIRCUIT

The modulation circuit modulates the VCO oscillating signal with the audio signals from the microphone.

AF signals from the D/A converter (IC251, pin 3) are applied to the modulation circuit (D39) to modulate the oscillated signal by changing the reactance of D39 at the TX VCO (Q51, D35–38).

4-2-3 TRANSMIT AMPLIFIERSTransmit amplifiers amplify the TX VCO output to transmit power level.

The TX VCO output signal is buffer-amplified by the buffer amplifiers (Q61, Q62) and passes through the TX/RX switch (D91). The signals from the TX/RX switch are applied to the another buffer amplifier (Q91), pre-drive (Q101) and power amplifier (Q111) and amplified to the transmit level.

The power amplifier output is applied to the antenna connector (J41: CHASSIS UNIT) via the antenna switching circuit and the LPFs.

4-2-4 APC CIRCUIT

The APC (Automatic Power Control) circuit stabilizes transmit output power and controls transmit output power High, Middle and Low.

The RF output signal from the power amplifier (Q111) is detected at the power detector (D121). The detector converts the RF signals into DC voltage, and the detected voltage is applied to the APC amplifier (IC141, pin 3).

The transmit output power setting voltage is applied to another input terminal of the APC amplifier (IC141, pin 1) as the reference voltage. The APC amplifier controls the bias of the buffer, pre-drive and power amplifiers by comparing the detected voltage and reference voltage. Thus the APC circuit maintains a constant output power.

Another power detector (D132) detects the RF output level and outputs "TDETV" signal to the CPU (IC661, pin 31). The CPU outputs "TLED" signal to the LED driver (Q655) to light TX LED (DS655).

• APC CIRCUIT

T5V

 

 

 

 

 

 

 

VCC

 

 

Q91

Q101

Q111

 

 

 

 

RF signals

 

Buffer

Pre-

Power

LPF

ANT

LPF

To ANT

 

AMP

drive

AMP

SW

from TX/RX SW

 

 

 

 

 

 

 

 

 

 

T1CON

1 + IC141

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

APC

4

 

 

D121

D132

 

 

 

AMP

 

 

 

 

 

 

3 –

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TDETV

4 - 2