TM8000 Mobile Radio Accessories Manual TMAA01-01 Line-Interface Board 27
March 2004 © Tait Electronics Limited
2.8 Circuit Description
2.8.1 Audio Interface
When the line-interface board is used for repeater applications, the audio
passed between the two radios must be of such a level that the message is able
to be repeated intelligibly. The audio interface is therefore capable of
handling a wide range of input and output levels (-20 to +6dBm). The
audio interface is also capable of using either a two- or four-wire isolated
interface formats, which are selectable using S1.
The input to the line driver IC (U5) is the AUDIO TAP OUT line from
the radio. This line is a software-programmabable tap point which can be
chosen from various audio signals available within the radio and is coupled
through a capacitor into the audio line out level control (RV2). This variable
resister 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 600Ω
terminating impedance, but also cause a 50% loss of signal. This is
compensated for by the higher-than-necessary gain of the line driver. Line
out protection is provided by two zener diodes, and the transformer (T1)
provides isolation.
2.7.2 Radio With Line-Interface Board: Receiver + Line Input
Transmitter Frequency Response*
Bandwidth
Response
*relative to 1kHz, 20% deviation, below
limiting
300Hz to 3kHz
+1, -3dB relative to -6dB/octave
Test Signal 0dBm line input*, audio tap T1
*60% deviation at 1kHz
Signal-to-Noise Ratio*
Narrow Band
Wide Band
*demodulated, filtered 300Hz to 3kHz and
de-emphasised 750µs rms
>40dB
>43dB
Mute Ratio >60dB
Distortion*
*demodulated, filtered 15kHz low pass
<3%
To Next Page
Loading...
