Tait TB7100, Руководство по обслуживанию

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52 Circuit Descriptions TB7100 Service Manual
© Tait Electronics Limited October 2005
4.5 CODEC and Audio Circuitry
Introduction
For a block diagram of the CODEC and audio circuitry, refer to Figure 4.5 .
A/D and D/A
Conversion
The analog-to-digital conversion and digital-to-analog conversion is
performed by the devices IC203, IC204 and IC205.
Device IC203
IC203 is an eight-channel DAC that provides control of transmitter biasing,
front-end AGC, front-end tuning, and the output of analog RSSI signals.
The digital input data are fed to IC203 in synchronous serial form. Three of
the DAC channels are not used.
Device IC205
IC205 contains two CODECs. One is used by the FCL. The second is used
for auxiliary audio (input) and VCO modulation (output). The digital
section communicates with this device via a four-wire synchronous serial
interface.
Device IC204
IC204 contains base-band, voice-band and auxiliary CODECs and some
analog signal conditioning. The reference voltage (nominally 1.2V) for
these CODECs is provided internally by IC204 but is decoupled externally
by C228.
Base-band CODEC
The base-band CODEC handles the I and Q outputs (
IRXP
,
IRXN
,
QRXP
and
QRXN
balls) of the receiver’s second IF stage. The analog signals are
differential and biased at 1.2 V nominally. The digital section communicates
with this CODEC via a two-wire synchronous serial interface (
BSDO
and
BSOFS
balls). The digital-to-analog conversion section of the base-band
CODEC is not used.
Voice-band CODEC
The voice-band CODEC handles the microphone and speaker signals.
The digital section communicates with this CODEC via a three-wire
synchronous serial interface (
VSFS
,
VSDO
and
VSDI
balls). IC204 also contains
voice-band filtering, pre-amplification and volume control.
Auxiliary CODEC
The auxiliary CODEC handles transmitter power control, receiver gain
control, auxiliary audio output and general analog monitoring functions.
The digital section communicates with this CODEC via a three-wire
synchronous serial interface (
ASFS
,
ASDI
and
ASDO
balls). The DAC used for
receiver gain control (
IDACOUT
ball) is a current output type. Current-to-
voltage conversion is performed by R238. The full-scale output of 1.2V is
amplified by IC201 to approximately 3V as required by the receiver.
Audio Circuitry
The audio circuitry performs four functions:
■
output of audio signal for speaker
■
input of microphone audio signal
■
input of auxiliary audio signal
■
output of auxiliary audio signal