Squelch Circuit Noise Squelch, Tone Squelch, Dtmf

The TX/RX HPF filters out 250 Hz and lower audio signals, and the de-emphasis circuit obtains –6 dB/oct of audio characteristics. The expander expands the compressed audio signals and also noise reduction function is provided.

The AF signals are then level adjusted at the volume controller (VR4) and ampliﬁed at the ampliﬁer (RXA2). The ampliﬁed AF signals are output from pin 20 and passed through another de- emphasis circuit (IC13, pins 2, 15), and then applied to the D/ A converter (IC4, pin 16) for level adjustment via the AF mute switch (IC14, pins 3, 4).

The level-adjusted AF signals are applied to the AF ampliﬁ er (FRONT UNIT; IC509, pin 2). The amplified AF signals are output from pin 1, and applied to the AF power amplifier (IC508, pin 17) to obtain 0.5 W of AF output power. The power- ampliﬁ ed AF signals are output from pin 18, and then applied to the internal speaker.

If an external speaker-microphone or headset is attached to the multi-connector (JACK UNIT; MP801), the AF signals from the AF amplifier (IC509, pin 1) are applied to the AF power ampliﬁ er (IC508, pin 14). The power-ampliﬁ ed AF signals are then output from pin 13, and applied to the multi-connector (JACK UNIT; MP801).

5-1-5 SQUELCH CIRCUIT

• NOISE SQUELCH

The squelch mutes the AF output signals when no RF signals are received. By detecting noise components (30 kHz and higher signals) in the demodulated AF signals, the squelch circuit toggles the AF power ampliﬁ er ON and OFF.

A portion of the demodulated AF signals from the FM IF IC (IC3, pin 9) are applied to the D/A converter (IC4, pin 1) for level adjustment (squelch threshold adjustment). The level- adjusted AF signals are output from pin 2 and passed through the noise filter (IC3, pins 7, 8, R121−R124, C216−C218). The ﬁ ltered noise signals are ampliﬁ ed the noise components only.

The amplified noise components are converted into the pulse- type signal at the noise detector section, and output from pin 13 as the “NOIS” signal. The signal is applied to the CPU (IC18, pin 37). Then the CPU outputs serial data to the expand IC (FRON UNIT; IC505, pin 3), and the expand IC outputs “AFON” signal from pin 4 according to the “NOIS” signal level, to the AF power ampliﬁer controller (FRONT UNIT; Q501, Q502, D508). The AF power amplifier controller toggles AF power amplifier (FRONT UNIT; IC508) ON and OFF according to the "AFON" signal.

• TONE SQUELCH

The tone squelch circuit detects tone signals and opens the squelch only when receiving a signal containing a matched sub audible tone. When the tone squelch is in use, and a signal with a mismatched or no sub audible tone is received, the tone squelch circuit mutes the AF signals even when the noise squelch is open.

• BASE BAND IC BLOCK DIAGRAM

• CTCSS/DTCS

A portion of the demodulated AF signals are passed through the active LPF (Q28) to filters CTCSS/DTCS signal. The filtered signal is applied to the CPU (IC18, pin 46). The CPU compares the applied signal and the set CTCSS/DTCS, then output the serial data to the expand IC (FRON UNIT; IC505, pin 3), and the expand IC outputs “AFON” signal from pin 4 to the AF power ampliﬁer controller (Q501, Q502, D508).

• 2/5 TONE

2/5 tone signals in the demodulated AF signals are passed through the LPF in the base band IC (IC5) and output from pin 21, then applied to the CPU (IC18, pin 45) and decoded.

• DTMF

DTMF signals in the demodulated AF signals are passed through the LPF in the base band IC (IC5) and output from pin 21, then applied to the DTMF decoder (IC10, pin 1) and decoded.

5-2 TRANSMITTER CIRCUITS

5-2-1 MICROPHONE AMPLIFIER CIRCUITS (MAIN UNIT)

The AF signals from the microphone (MIC signals) are ﬁltered and level-adjusted at microphone ampliﬁ er circuits.

MIC signals from the microphone are passed through the microphone switch (FRON UNIT; Q515). The microphone switch selects the AF signals from the internal microphone (FRON UNIT; MC1) or from an external microphone.

MIC signals from the microphone switch (FRON UNIT; Q515) are applied to the microphone ampliﬁ er (FRON UNIT; IC509, pin 6), and amplified AF signals are output from pin 7, and passed through the pre-emphasis circuit (IC13, pins 4, 5) to obtain +6 dB/oct of frequency characteristic. The pre- emphasized MIC signals are then applied to the microphone ampliﬁ er (IC9, pin 9). And the ampliﬁed MIC signals are output from pin 8, and applied to the D/A converter (IC4, pin 9) for level adjustment (=microphone sensitivity adjustment). The level-adjusted MIC signals are output from pin 10, and applied to the ALC (Automatic Level Control) circuit (IC15, pin 3) which limits the amplitude of the MIC signals to prevent over deviation. The amplitude-limited MIC signals are output from pin 5, then applied to the base band IC (IC5, pin 3).

The applied MIC signals are ampliﬁed at the ampliﬁer (TXA1), and level adjusted at the volume controller (VR1). The level adjusted MIC signals are applied or bypassed the compressor section, pre-emphasis section, TX/RX HPF, de-scrambler, limiter, splatter, in sequence, then applied to another volume controller.

The compressor compresses the MIC signals to provide high S/N ratio for receive side, and the pre-emphasis obtains +6 dB/oct audio characteristics. The TX/RX HPF filters out 250 Hz and lower audio signals, the limiter limits its level and the splatter ﬁ lters out 3 kHz and higher audio signals.

The ﬁ ltered MIC signals are level adjusted at another volume controller (VR2), and then output from pin 7 via smoothing ﬁ lter (SMF).

BASE BAND IC (IC5)

TXIN	3	VR1	Com-	Pre-						7	MOD
		(HPF)	pressor	emphasis	Limiter	Splatter	VR2	SMF		7	MOD
					Limiter	Splatter	VR2	SMF
	TXA1
				TX/RX	Scrambler/
				HPF	De-scrambler
RXIN	23	VR3	RX		De-					18
		(HPF)	LPF		De-	Expander		VR4		18
		(HPF)	LPF		emphasis	Expander		VR4
	21 RXA1				emphasis
SDEC	21 RXA1									19
									RXA2 20 SIGNAL
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