Icom IC-F3GT, IC-F3GS service manual RF signal From PLL Driver

4-1-5 AF CIRCUIT

AF signals from the FM IF IC (IC2, pin 9) are applied to the mute switch (IC4, pin 1) via the AF filter circuit (IC3b, pins 6, 7). The output signals from pin 11 are applied to the AF power amplifier (IC5, pin 4) after being passed through the [VOL] control (R143).

The applied AF signals are amplified at the AF power ampli- fier circuit (IC5, pin 4) to obtain the specified audio level. The amplified AF signals, output from pin 10, are applied to the internal speaker (SP1) as the “SP” signal via the [SP] jack when no plug is connected to the jack.

4-1-6 SQUELCH CIRCUIT

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

A portion of the AF signals from the FM IF IC (IC2, pin 9) are applied to the active filter section (IC2, pin 8) where noise components are amplified and detected with an internal noise detector.

The active filter section amplifies noise components. The fil- tered signals are rectified at the noise detector section and converted into “NOIS” (pulse type) signals at the noise com- parator section. The “NOIS” signal is applied to the CPU (IC8, pin 19).

The CPU detects the receiving signal strength from the number of the pulses, and outputs an “RMUT” signal from pin 49. This signal controls the mute switch (IC4, pin 13) to cut the AF signal line.

4-1-7 WIDE AND NARROW SWITCHING CIRCUIT

The “NWC” signal from the CPU (IC8, pin 38) is applied to the Q41. Q41 is switched ON or OFF by the signal. Q41 con- trols IC2’s detector output to same level both wide band mode and narrow band mode.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT

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

The AF signals from the microphone are applied to the microphone amplifier circuit (IC3c, pin 10). The amplified AF signals are passed through the low-pass filter circuit (IC3d, pins 13, 14) via the mute switch (IC4, pins 4, 3). The filtered AF signals are applied to the modulator circuit after being passed through the mute switch (IC4, pins 9, 8).

4-2-2 MODULATION CIRCUIT

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

The audio signals change the reactance of a diode (D404) to modulate an oscillated signal at the VCO circuit (Q7, Q8). The oscillated signal is amplified at the buffer-amplifiers (Q4, Q6), then applied to the T/R switching circuit (D3, D4).

4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS

The signal from the VCO circuit passes through the T/R switching circuit (D3) and is amplified at the buffer (Q403, Q3), pre-drive (Q2) and power amplifier (Q1) to obtain 4 W of RF power (at 7.2 V DC). The amplified signal passes through the antenna switching circuit (D1), and low-pass fil- ter and is then applied to the antenna connector.

The bias current of the pre-drive (Q2) and the power ampli- fier (Q1) is controlled by the APC circuit.

4-2-5 APC CIRCUIT

The APC circuit (IC3a, Q37) protects the drive and the power amplifiers from excessive current drive, and selects HIGH or LOW output power.

APC CIRCUIT

VCC

RF signal	Q2	Q1
from PLL
	Driver	Power
	amp.	amp.

S5

TXC		—
	Q37
		IC3a
T4		+

APC control circuit

L4

LPF to antenna

D33 D32

Power detector circuit (D32, D33)

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