24 Functional Description TB7100 Service Manual© Tait Electronics Limited October 20053.2 Transmitter OperationParts of TransmitterBoard The main circuit parts of the transmitter board are:■ transmitter■ frequency synthesizer■ CODEC (coder-decoder) and audio circuitry■ power supply■ interface circuitrySoftware plays a prominent role in the functioning of the board.When describing the operation of the radio the software must be includedwith the above. This is considered further below.These functional parts are described in detail below.3.2.1 Audio Processing and SignallingMicrophone Input The input to the transmitter path begins at either the SI board or theP ROG /MIC connector of the UI board. Only electret-type microphones aresupported. The audio input is then applied to tap point T8 on thetransmitter board (the tap point is user-selectable).Analog Processingof MicrophoneSignalThe CODEC (AD6521) performs microphone selection and amplification.The microphone amplifier consists of an amplifier with a fixed gain of 16dBfollowed by a programmable-gain amplifier with 0dB to 22dB gain.The amplified microphone signal is converted to a digital stream by a 16-bitADC with integral anti-alias filtering (0.1 to 3.2kHz). The digital stream istransported to the DSP for further audio processing.Figure 3.3 Transmitter high-level block diagramAnalog-to-digital conversionLPFANTPLLLoopfilterLoopfilterFrequencycontrolTriple-pointEqualisationAudiofilteringPre-emphasisOptionalprocessingData and signallingencodersALC MicPGADir.Coup. Fin Drv/ExPwrCrtlRampcontrolTCXO:13.000 MHzVCOVCXOBiasCUSTOMLOGICBLOCKHARDWAREBLOCKHARDWARE BLOCKDigital-to-analog conversionKEYPROG/MIC connectorDSPBLOCKSYSTEM connectorTap point T8
