Tait TM8000 Series Accessories Manual

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Tait TM8000 Series Accessories Manual Manual pdf 27 page image

TM8000 Mobile Radio Accessories Manual TMAA01-01 Line-Interface Board 27March 2004 © Tait Electronics Limited2.8 Circuit Description2.8.1 Audio InterfaceWhen the line-interface board is used for repeater applications, the audiopassed between the two radios must be of such a level that the message is ableto be repeated intelligibly. The audio interface is therefore capable ofhandling a wide range of input and output levels (-20 to +6dBm). Theaudio interface is also capable of using either a two- or four-wire isolatedinterface formats, which are selectable using S1.The input to the line driver IC (U5) is the AUDIO TAP OUT line fromthe radio. This line is a software-programmabable tap point which can bechosen from various audio signals available within the radio and is coupledthrough a capacitor into the audio line out level control (RV2). This variableresister is AC coupled into the line driver (U5) which is used in a bridged-output format, with gain set to provide the necessary 21dB gain.The resistors on the output of the line driver provides the necessary 600Ωterminating impedance, but also cause a 50% loss of signal. This iscompensated for by the higher-than-necessary gain of the line driver. Lineout protection is provided by two zener diodes, and the transformer (T1)provides isolation.2.7.2 Radio With Line-Interface Board: Receiver + Line InputTransmitter Frequency Response*BandwidthResponse*relative to 1kHz, 20% deviation, belowlimiting300Hz to 3kHz+1, -3dB relative to -6dB/octaveTest Signal 0dBm line input*, audio tap T1*60% deviation at 1kHzSignal-to-Noise Ratio*Narrow BandWide Band*demodulated, filtered 300Hz to 3kHz andde-emphasised 750μs rms>40dB>43dBMute Ratio >60dBDistortion**demodulated, filtered 15kHz low pass<3%