Tait TP9100 Service Manual

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Contents

TP9100 Service Manual Circuit Descriptions 61© Tait Electronics Limited May 20053.5 CODEC and Audio CircuitryIntroduction For a block diagram of the CODEC and audio circuitry, refer to Figure 3.5.A/D and D/AConversion The analog-to-digital conversion and digital-to-analog conversion isperformed by the devices IC204 and IC205.Device IC205 IC205 contains two CODECs. One is used by the FCL. The second is usedfor auxiliary audio (input) and VCO modulation (output). The digitalsection communicates with this device via a four-wire synchronous serialinterface.Device IC204 IC204 contains base-band, voice-band and auxiliary CODECs, transmitterbiasing, and some analog signal conditioning. The reference voltage(nominally 1.2V) for these CODECs is provided internally by IC204 but isdecoupled externally by C217.Device IC204 (Base-Band CODEC) The base-band CODEC handles the I and Q outputs (IRXP, IRXN, QRXP andQRXN balls) of the receiver’s second IF stage. The analog signals aredifferential and biased at 1.2V nominally. The digital section communicateswith this CODEC via a two-wire synchronous serial interface (BSDO andBSOFS balls). The digital-to-analog conversion section of the base-bandCODEC is not used.Device IC204 (Voice-band CODEC) The voice-band CODEC handles the microphone and speaker signals.The digital section communicates with this CODEC via a three-wiresynchronous serial interface (VSFS, VSDO and VSDI balls). IC204 also containsvoice-band filtering, pre-amplification and volume control.Device IC204(Auxiliary CODEC) The auxiliary CODEC handles transmitter biasing and power control,receiver tuning and gain control, accessory audio output and general analogmonitoring functions. The digital section communicates with this CODECvia a three-wire synchronous serial interface (ASFS, ASDI and ASDO balls).The DAC used for transmitter bias and receiver gain control (IDACOUT ball) isa current output type. Current-to-voltage conversion is performed byR238. The full-scale output of 1.2V is amplified by IC201 to approximately3V as required by the receiver and transmitter.Audio Circuitry The audio circuitry performs four functions:■ output of audio signal for speaker■ input of microphone audio signal■ input of accessory audio signal■ output of accessory audio signalThe sections of the circuitry concerned with these functions are describedbelow.