Icom IC-F21S service manual Transmitter Circuits

The applied AF signals are amplified at the AF power ampli- fier circuit (IC4, pin 4) to obtain the specified audio level. The amplified AF signals output from pin 10 as “AFOUT”signal 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 (IC3, pin 9) as “DET” signal are applied to the D/A convertor IC (IC9, pin 24). The signals from the D/A convertor (IC9, pin 23) as “SQLIN” signals are applied to the active filter section (IC3, 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 output from IC3, pin 13, and is applied to the CPU (IC7, pin 53).

The CPU detects the receiving signal strength from the number of the pulses, and outputs “EXST”, “SO”, “SCK” sig- nals. The signals are applied to the expander IC (IC15, pins

1, 2, 3), and then outputs “RMUT” signal from pin 4. This sig- nal controls the analog switch (IC6, pin 13) to cut the AF sig- nal line.

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 (IC5c, pin 10) after being pass through the high-pass filter (C186, C187). The amplified AF signals are passed through the low-pass filter circuit (IC5d, pins 13, 14) via the mute switch (IC6, pin 4). The filtered AF signals are applied to the modulator circuit after being passed through the mute switch (IC6, pins 8, 9, 10).

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 (D6) to modulate an oscillated signal at the TX-VCO circuit (Q11). The oscillated signal is amplified at the buffer-amplifiers (Q5, Q7), 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 YGR (Q4, 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) via the power detector (D2), and low-pass filter 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 (IC2) protects the drive and the power amplifiers from excessive current drive, and selects HIGH or LOW output power.

The signal output from the power detector circuit (D2) is applied to the differential amplifier (IC2, pin 3), and the “T4” signal from the expander (IC9, pin 11), controlled by the CPU (IC7), is applied to the other input for reference.

When the driving current is increased, input voltage of the differential amplifier (pin 3) will be increased. In such cases, the differential amplifier output voltage (pin 4) is decreased to reduce the driving current.

AF CIRCUIT

	AF	11	Analog	1
	amp.
		2, 3	switch	9, 10
SP	IC4	AF VOL.	IC6
		R1
		SW-A/SW-B	LPF
		unit	IC5D

APC CIRCUIT

VCC

RF signal	Q2	Q1
from PLL

IC5B "DET" signal from IC3, pin 9

HPF

"NWC" signal from IC15, pin 14

Q21

D1

ANT

Pre-driver	Power
amp.	amp.

S5V

APC control circuit

SW

D2

LPF to antenna

T5V	—
Q14	IC2
T4	+

Power detector circuit (D2)

4 - 2