Icom IC-F51, IC-F50 service manual Transmitter Circuits, AF Amplifier Circuit Main and Front Units

4-1-5 AF AMPLIFIER CIRCUIT (MAIN AND FRONT UNITS)

The AF amplifier circuit amplifies the demodulated AF sig- nals to drive a speaker. This transceiver employs the base band IC which is composed of pre-amplifier, expander, scrambler, MSK de-modulator, etc. at the AF amplifier sec- tion.

The AF signals from the FM IF IC (IC1, pin 9) are amplified at the AF amplifier section of the base band IC (IC10, pin 5) and are then applied to the low-pass filter section of it.

The filtered signals passes through the high-pass filter to suppress unwanted harmonic components. The signals pass through (or bypass) scrambler and expander sections, and are then applied to (or bypass) the scrambler IC (IC14) via the analog switch (IC13). The signals are amplified at the amplifier section of the base band IC (IC10), and pass through the AF mute switch (IC406) and low-pass filter (IC403). The filtered signals pass through the AF volume, and are then applied to the AF power amplifier (IC405) to drive the speaker.

4-1-6 RECEIVE MUTE CIRCUITS (MAIN AND FRONT UNITS)

•NOISE SQUELCH

A 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.

Some noise components in the AF signals from the FM IF IC (IC1, pin 9) are passed through the D/A converter (IC6, pin 1). The signals are applied to the active filter section in the FM IF IC (IC1, pin 8). Noise components about 10 kHz are amplified and output from pin 7.

The filtered signals are converted into the pulse-type signals at the noise detector section and output from pin 13 (NOIS).

The “NOIS” signal from the FM IF IC is applied to the CPU (FRONT unit; IC401, pin 41). Then the CPU analyzes the noise condition and controls the AF mute signal via “AFON” line from expander IC (FRONT unit; IC410, pin 7) to the AF power controller (FRONT unit; Q401, Q402).

•CTCSS AND DTCS

The tone squelch circuit detects AF signals and opens the squelch only when receiving a signal containing a matching subaudible tone (CTCSS or DTCS). When tone squelch is in use, and a signal with a mismatched or no subaudible tone is received, the tone squelch circuit mutes the AF signals even when noise squelch is open.

A portion of the “DET” AF signals from the FM IF IC (IC1, pin

9)passes through the low-pass filter (IC5, pin 5) to remove AF (voice) signals, and are then applied to the amplifier (MAIN unit; IC5, pin 10). The amplified signals are applied to the CTCSS or DTCS decoder inside of the CPU (FRONT; IC401, pin 44) via the “CDEC” line. The CPU outputs AF mute control signal, and is then applied to the I/O expander IC (IC410). The IC outputs AF mute circuit (IC406) and AF power supply circuits (Q401, Q402) control signals via the “AFON” line.

4-2 TRANSMITTER CIRCUITS

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

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

This transceiver employs the base band IC which is com- posed of microphone amplifier, compressor, scrambler, lim- iter, splatter filter, MSK modulator, etc. at the microphone amplifier section.

The AF signals (MIC) from the microphone (MC401) are passed through the microphone mute switch (IC406, pins 2, 1), and are then applied to the amplifier (IC407, pins 2, 6). The amplified signals pass through (or bypass) the scram- bler IC (IC14) via the analog switch (IC13), and are then applied to the microphone amplifier section of the base band IC (MAIN unit; IC10, pins 3, 4). The amplified signals are passed through or bypass the compressor, scrambler sec- tions of IC10 (MAIN unit), and are then passed through the high-pass, limiter amplifier, splatter filter sections of IC10 (MAIN unit).

The filtered AF signals are applied to the FM/PM switch (MAIN unit; IC11, pin 6), and pass through the low-pass fil- ter (MAIN unit; IC5, pin 1). The amplified signals are applied to the D/A converter (MAIN unit; IC6, pin 4)

The output signals from the D/A converter (MAIN unit; IC6, pin 3) are applied to the modulation circuit (MAIN unit; D18).

•AF AMPLIFIER CIRCUIT

"DET" AF signal

from FM IF IC (IC1, pin 9)

5

Base band IC

(IC10)

		IC403	AF
			volume	IC405
20	AF mute
		LPF		AF	Speaker
	(IC406)				(SP1)
18	18 Scrambler IC 15 8			10
	(IC14)
		11		9

Analog switch

(IC13)

19

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