4-1-5 SQUELCH CIRCUIT (MAIN AND LOGIC UNITS)

The noise squelch circuit cuts out AF signals when no RF signals are received. By detecting noise components in the AF signals, the squelch circuit switches the AF mute switch.

A portion of the AF signals divided by C242 from the FM IF IC (IC231, pin 9) are applied to the D/A converter (IC251, pin 2) to adjust amplitude. The signals from the D/A converter (IC251, pin 1) are applied to the active filter section (IC231, pin 8, R239–R241, C237, C238). The active filter section amplifies and filters noise components. The filtered signals are applied to the noise detector section and output from IC231 (pin 14) as the “NOISV” signal.

The “NOISV” signal from IC231 (pin 14) is applied to the CPU (LOGIC unit; IC611, pin 32). The CPU compares the set squelch level voltage and “NOISV” signal voltage to control the squelch output.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT (LOGIC AND MAIN UNITS)

The microphone amplifier circuit amplifies audio signals with +6 dB/octave pre-emphasis characteristics from the microphone to a level needed for the modulation circuit.

• In case of the internal microphone

The AF signals from the internal microphone (MC461) is applied to the microphone amplifier (IC471a, pin 6) via the “INMIC” signal.

• In case of external microphone

The AF signals from the external microphone (CP458) is applied to the microphone amplifier (IC471a, pin 6) via the “EXTMIC” signal.

The amplified signals are passed through the pre-emphasis circuit (R463, C463) and are then applied to the AF mute switch (IC481a, pin 1) via the “MICO” signal after being passed through the another microphone amplifier (IC471b, pins 2 and 1).

The AF signals are amplified again at the limiter-amplifier (IC491a, pin 2) and then passed through the low-pass filter (IC491b, pins 6 and 7). The filtered audio passes through the analog swtich (MAIN unit; IC251, pins 4 and 3), and is then applied to the MAIN unit as the “MOCON” signal.

4-2-2 MODULATION CIRCUIT (MAIN UNIT)

The modulation circuit modulates the VCO oscillating signal (RF signal) using the microphone audio signals.

The audio signals “MOCON” change the reactance of D39 to modulate an oscillated signal at the TX VCO circuit (Q51, D35–D38, L34, L51, C32, C33, C52–C54). The oscillated signal is amplified at the buffer-amplifiers (Q61, Q62).

4-2-3 PRE-DRIVE/POWER AMPLIFIER CIRCUITS (MAIN UNIT)

The signal from the VCO circuit passes through the transmit/receive switching circuit (D91, D92) and is applied to the buffer-amplifier (Q91). The amplified signal is amplified by the pre-driver (Q101) and the power amplifier (Q101) to obtain 5 W of RF power (at 7.2 V). The amplified signal passes through the antenna switching circuit (D131), and low-pass filter (L131, L132, C131–C136) and is then applied to the antenna connector.

The bias current of the buffer amplifier (Q91), pre-driver (Q101) and power amplifier (Q111) is controlled by the APC circuit to stabilize the output power.

4-2-4 APC CIRCUIT (MAIN UNIT)

The APC circuit provides stable output power from the power amplifier even when the input voltage or temperature changes, and, selects HIGH, MIDDLE, LOW or EXTRA LOW output power. The APC circuit consists of a power detector and APC control circuits.

• POWER DETECTOR CIRCUIT (MAIN UNIT)

The power detector circuit (D132) detects the transmit output power level and converts it to DC voltage as the “TDETV” signal. The detected signal is applied to the APC control circuit.

•APC CONTROL CIRCUIT (MAIN AND LOGIC UNITS) The “TDETV” signal from the power detector circuit is applied to the CPU (LOGIC unit; IC661, pin 31) to control the input voltage of the buffer amplifier (Q91), pre-driver (Q101) and power amplifier (Q111). When the output power changes, the CPU (LOGIC unit; IC661) outputs APC control signal to the D/A converter (IC190). And then “T1CON” signal from the D/A converter controls the APC controller (IC141) to provide stable output power.

• APC CIRCUIT

VCC

 

 

T5V

 

S5V

"TXMS" signal from the expander IC (IC341, pin 12)

antenna

RF signal

 

Q91

 

Q101

 

 

Q111

 

 

 

 

 

 

D131

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

from PLL

Buffer

 

Pre-drive

 

Power

 

 

 

 

 

 

 

ANT

 

 

 

 

 

 

 

 

 

 

 

LPF

 

 

 

 

 

amp.

 

amp.

 

amp.

 

 

 

 

 

 

 

SW

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Power detector

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

circuit (D132)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q141

IC141

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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"T1CON" signal from the D/A convertor IC (IC251, pin 14)

APC control circuit

"TDETV" to the CPU (LOGIC unit; IC661, pin 31)

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