TM8100 Mobile Radio Service Manual Circuit Descriptions 71
May 2004 © Tait Electronics Limited
Modulation The FCL modulation is implemented within the FPGA and appears at
the output of IC205, and therefore on the VCXO loop voltage.
Consequently, the VCXO is frequency modulated directly by the
relevant modulation information. The latter may be the microphone
audio, an audio tap-in signal, internal modem signals, or any
combination of these.
3.6 CODEC and Audio Circuitry
CODEC Circuitry The CODEC circuitry and audio circuitry are given on sheet 2 of the
circuit diagram for the main board. Regarding the former, analogue-to-
digital conversion and digital-to-analogue 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 tuning, and the output of analogue 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
analogue signal conditioning. The reference voltage (nominally 1.2 V)
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 analogue 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-analogue 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.
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