Tait TM8200 Accessories Manual

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Tait TM8200 Accessories Manual Manual pdf 27 page image

TM8100/TM8200 Accessories Manual TMAA01-01 Line-Interface Board 27© Tait Electronics Limited August 20052.8 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.The audio interface is capable of using a two- or four-wire interface, so atap is taken from one side of the balanced line out and is feed directly intothe line input level control (RV3). When using a four-wire interface, thesignal comes in through a second isolation transformer, T2. T2 is terminatedwith 600Ω and also acts as a voltage divider. This means that the signal levelat RV3 will be identical to the level at RV3 when using a two-wire interface.To achieve the required output level the non-inverting AC amplifier (U7)has a gain of 10, which provides the necessary 13dB of gain. The output ofthe amplifier is AC coupled into the AUDIO_TAP_IN line (pin 6 of SK2).2.8.2 Logic InterfaceThe line-interface board is able to provide simple interface solutions withother radios. Logic is used to control keying of both radios as well asproviding time delays to prevent squelch or cycling problems. The logic usesgates rather than discrete components.The choice of which input controls BUSY/GATE can be selected usingswitches 3 and 4 of S1, while the sense of BUSY/GATE (pin 12 of SK1) canbe selected using switches 5 and 6. Switch 7 accommodates either a two-line keying system or a single bi-directional keying line.