Tait TM8000 Series, Руководство по обслуживанию

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TM8100 Mobile Radio Service Manual Circuit Descriptions 73
May 2004 © Tait Electronics Limited
Microphone Signals
There are two microphone source signals:
■
ITF AUX MIC AUD
from auxiliary or internal options connector
■
ITF CH MIC AUD
from control head
The biasing for electret microphones is provided by a filtered 3.0 V
supply via R226 and R227. The components R209 and C202 provide
the supply filtering. The microphone inputs to IC204 (
VINAUXP
,
VINAUXN
,
VINNORP
and
VINNORN
balls) are differential. The negative inputs are
decoupled to the filtered 3.0 V supply by C215 and C216. The positive
inputs are biased to approximately 1.5 V by R229, R232, R230 and
R233. AC coupling and DC input protection is provided by C213 and
C214.
Auxiliary Audio Input
The auxiliary audio input signal
ITF AUD TAP IN
is DC-coupled to the ADC
input of IC205. R241 combined with internal clamping diodes in
IC205 provide DC protection for the ADC input. IC205 provides the
input biasing of approximately 1.5 V.
Auxiliary Audio Output
The source for the auxiliary audio output signal
CDC AUD TAP OUT
is
provided by IC204 (
RAMPDAC
ball). The DAC output of IC204 is low-
pass filtered to remove high-frequency artefacts. The low-pass filter,
formed by IC201 (pins 1 to 3), R219, R220, R221, R224, C206, C208
and C210, is a third-order Butterworth type with a cut frequency of
approximately 12 kHz. The output of the low-pass filter is amplified by
6 dB by a buffer amplifier, IC201 (pins 5 to 7), and fed via R207 and
R208 to drive the
CDC AUD TAP OUT
interface line. The DC bias for this
signal path is provided by IC204 and is approximately 1.2 V when
operational. The offset at
CDC AUD TAP OUT
is approximately 2.4 V owing
to the gain of the buffer amplifier.
3.7 Power Supply
Introduction
The power-supply circuitry consists of the following main sections:
■
supply protection
■
supervisory circuit
■
internal power supplies
■
control of internal power supplies
The power-supply circuitry is given on sheet 6 of the circuit diagram for
the main board, and is described below.