Radio Glen TBU

Posted on Jan 26, 2013

The heart(s) of the unit are the ETAL P3400 line modules available from Farnell. These modules mop-up all of the line interface functions like ring detection, physical hybrid, on-off hook handling etc. This means that the TBU as a whole would probably pass PSTN type approval testing if it was subjected to it. This sheet shows the differential amplifiers which take the signal from the RX side of the hybrid, and the differential amps which accept the mixer TX output for sending to the TX side of the hybrid. The TX signal is bandpass filtered with -3dB points of 200Hz and 3500Hz at -12dB/octave. This avoids sending unecessarily wide audio to the telephone line and makes cancellation of the TX crosstalk easier.

Radio Glen TBU
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RX refers to the signal being received from the far end of the phone line and sent to the studio mixer. TX refers to the signal going from the studio mixer to the phone line. Because of the imperfect nature of hybrids and phone lines, the RX signal typically has a roughly equal amount of the TX signal present due to crosstalk. This TX crosstalk is cancelled by subtracting the known TX signal from the RX signal. Measurements have shown that the TX crosstalk signal coming out of the RX side of the hybrid is also slightly phase shifted from the original TX signal. So an equivalent phase shift is added to the signal that is used to cancel the TX crosstalk signal. The cancellation amplitude and phase shift are adjustable on preset pots. To adjust these pots, the TBU has to be put into a call. The receiver of the phone that is dialled up may be simply left on a cushion to block the mouthpiece. A 1KHz sine is sent to the TX from the mixer and the amount of this sine on the RX is measured with an oscilloscope. The amplitude pot is adjusted first, to minimise the level of the crosstalk signal. Then the phase pot is adjusted to minimise the crosstalk signal. There is not much interaction between the two pots but it may be of benefit to readjust the amplitude pot and then the phase pot again. This adjustment should ideally be done when the TBU is connected to the actual line to be used in the field. In practice however, the...

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