Icom IC-910H Transmitter Circuits, Noise Blanker Circuit Main Unit, AGC Circuit Main Unit

4-1-11 NOISE BLANKER CIRCUIT (MAIN UNIT)

The noise blanker circuit detects pulse-type noises, and stops IF amplifier operation during detection.

A portion of the 10 MHz IF signal from the bandpass filter (FI51 [Main], FI651 [Sub]) is amplified at the noise amplifier circuit (Q102, IC101, Q101 [Main], Q702, IC701, Q701 [Sub]). The amplified signal is rectified at the noise detector (D371 [Main], D701 [Sub]) for conversion into DC voltage. The DC voltage is amplified at the DC amplifier circuit (Q105 [Main], Q705 [Sub]) and then applied to the noise blanker control circuit (Q52, Q107 [Main], Q652, Q707 [Sub]) to stop amplification of the IF amplifier circuit (Q51 [Main], Q651 [Sub]).

4-1-12 AGC CIRCUIT (MAIN UNIT)

The AGC (Auto Gain Control) circuit reduces IF amplifier gain to keep the audio output at a constant level.

A portion of the 10 MHz IF signal from the IF amplifier (Q352 [Main], Q852 [Sub]) is applied to the AGC detector circuit D303 [Main], D902 [Sub]). The detected signal is then ampli- fied at the DC amplifier circuit (Q305 [Main], Q901 [Sub]) and then applied to the IF amplifiers (Q51, Q351, Q352 [Main], Q651, Q851, Q852 [Sub]).

When strong signals are received, the detected voltage increases and the output level of the DC amplifier, as AGC voltage, decreases. The AGC voltage is used for the bias voltage for the IF amplifiers, therefore, the IF amplifier gain is decreased.

AGC response time is controlled by changing the time con- stant at the AGC control line with a resistor and capacitor. While AGC is set to slow, the resistor (R312 [Main], R914 [Sub]) and capacitor (C306 [Main], C911 [Sub]) are con- nected to the AGC control line. While AGC is set to fast, R311 [Main], R913 [Sub] are connected to the AGC control line. Due to Q304 and Q303 [Main]/Q905 and Q904 [Sub] being switched ON that controlled by the “AGSM”, “AGFM” [Main], “AGSS”, “AGFS” [Sub]. Also, R310 [Main]/R912 [Sub] is connected to the AGC control line due to Q302 [Main]/Q903 being switched ON while scanning for faster response than AGC fast mode that controlled by the “AGRM” [Main], “AGRS” [Sub].

4-1-13 S-METER CIRCUIT (MAIN UNIT)

The S-meter circuit indicates the relative received signal strength while receiving and changes depending on the received signal strength.

(1) FM mode

Some of the amplified IF signal is applied to the S-meter detector section in the FM IF IC (IC401 [Main], IC951 [Sub]) to be converted into DC voltage. The converted signal is out- put from pin 12 and applied to the meter amplifier circuit (IC1804c [Main], IC1804a [Sub]). The amplified signal is then applied to the CPU (DISPLAY board; IC1) passing through the analog multiplexer (DISPLAY board; IC4, pins 12 and 1) via the “SMLM [Main]/SMLS [Sub]” line. The CPU then outputs S-meter control signal.

(2) SSB and CW modes

A portion of the AGC control signal is applied to the meter amplifier (IC1804d [Main], IC1804b [Sub]). The amplified signal is then applied to the CPU via the analog multiplexer to control the S-meter.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT (MAIN UNIT)

The microphone amplifier circuit amplifies audio signals from the microphone or ACC connector and then applies them to the FM modulation or balanced modulator circuit. One microphone amplifier circuit is commonly used for both FM/SSB and VHF/UHF.

Audio signals from the [MIC] connector enter the micro- phone amplifier IC (IC1701, pin 22) and are then amplified at the microphone amplifier or speech compressor section. Compression level is adjusted by the setting mode.

The amplified or compressed signals are applied to the VCA section of IC1701. The microphone gain setting from the D/A converter (IC1521, pin 8) is applied to the VCA control ter- minal (IC1701, pin 10). The resulting signals from pin 9 are then applied to the buffer-amplifier (Q1651) via the analog switch (IC1653a). External modulation input from the [ACC] socket (pin 4) is also applied to Q1651.

•AGC CIRCUIT FOR MAIN BAND

D302

RFGM (RF/SQL control)

	AGC det.
	D303
2nd IF	C309
signal
	C310
	–5 V

R316	R317
		Q306
R315	C311	C312	R314

SCAN	FAST	SLOW
								AGC line
R310	R311	R312
		C306	R313	C308	R302 13	IC1804d
						Meter	14
						+
					12	amp.		“SMLM”
Q302	Q303	Q304		R303	R304	–		S-meter signal
			C307

9 V

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