Maxon Telecom SP210, SP200 service manual Audio Asic, Audio Signal Path

Maxon SP200/210 Radio SP200/210

5.3 Audio ASIC

The audio ASIC (IC406) processes both the audio signal and the sub-audible tones, including filtering, amplifying, setting attenuations levels etc. It is a programmable device, controlled by the microcontroller, hence alteration of deviation levels is achieved by the ACC-2003 Alignment Software.

The internal block diagram is shown in Figure 5-1. The audio and SAT routes within the ASIC will be described separately.

5.3.1 Audio Signal Path

A1	Buffers discriminator audio input from Pin 30 (INI) and passes it to
	Analogue Switch 1 (ASW1). It has a gain of x1.
A2	Amplifies the analogue signal from the mic input terminal (IN2) and
	passes this signal to Analogue Switch 1. It has a gain of 10dB.
ASW1	This is a two-way analogue switch. It is controlled by the serial control
	interface. Default value is 0, which is Rx (A1).
INTRIM	Sets the deviation for the input analogue signal sensitivity. The control
	range of this device is –3.5 to +4dB controllable in 16 steps. The
	control is a 4 bit signal where the default value is 1000, which
	corresponds to 0dB.
300Hz HPF	Comprises an 8th order filter with a cut-off frequency of 300Hz.
	Attenuation of the unwanted is at least 30dB.
ASW2	This is a four-way analogue switch, controlled by the serial control
	interface. It passes the received audio to the de-emphasis circuit or
	the transmit audio to the pre-emphasis circuit. Alternatively, the pre-
	emphasis or de-emphasis can be bypassed. The control signal is a 2
	bit signal with 00 as the default, which is de-emphasis.
DE-EMPHASIS	Applies de-emphasis to the received audio at 6dB/octave. At 1kHz, the
	gain is x1.
PRE-EMPHASIS	Applies pre-emphasis to the transmit audio at 6dB/octave. At 1kHz,
	the gain is x1.
RXVOL	Controls the magnitude of the Rx audio signal which is passed, via the
	volume control (VR5) to the speaker in 16 steps from 0dB attenuation
	up to 37.5dB attenuation, to Rxout on pin 28.
AMP	This amplifier provides gain, to the Tx audio, in 8 steps from +20dB to
	+41dB.
LIMITER	Applies limiting at 0dB (2.8V p-p). The output of this stage is controlled
	in 4 steps from 0dB to –5.4dB attenuation by a 2 bit control signal.
ASW3	This is a two-way analogue switch, controlled by the serial control
	interface. It passes the Tx audio, or the SAT to the low-pass filter.
VLPF	This is a 6th order low pass filter, which is controlled by the serial
	control interface. The cut-off point is switched between 2.55kHz and
	3kHz.
TXTRIM1	Provides deviation adjustment of Tx audio in 16 steps from +3.5dB
	gain to –4dB attenuation. Uses a 4 bit control signal.
TXSUM	Mixes (Adds) the signal from TXTRIM1 with the SAT signals from
	COMPIN+, or selects one or other of the signals. Can also apply 50dB
	muting. Controlled by the serial control interface.
ATTN	Attenuates the signal from TXSUM by either 0dB or 6dB.
TXTRIM2/3	Signal follows two routes (to VCO & TCXO) for final deviation
	adjustment in 32 stages from +3.75dB to –4dB. Controlled by 5 bit
	signal.
A3/A4/INV	Final output buffer amplifier for Tx analogue signal. Gain is 0dB. A3
	output (modout 1 on pin 1) goes to the VCO and A4 (modout 2 on pin
	2) goes to TCXO, are non-inverting outputs. The inverting output (INV)
	from A4 is not used.

DETAILED FUNCTIONAL DESCRIPTION	Issue 1.0
Page 5-2	03/01