Application Circuit
At the frequencies of interest (128 kHz and 256 kHz), ordinary telephone wire has a characteristic
impedance of about 110 W. In the schematic above, loading resistors R19–22 between LO1 and LO2 and
the Tx windings of the transformer are set to 110 W. The series combination of these resistors is significantly
higher than the 20-W output impedance of the LO1 and LO2 drivers, causing the resistance presented to the
transformer to be set primarily by the resistors alone.
Clamp diodes D1–4 protect the LO1 and LO2 outputs from transient signals on the twisted pair. Line settling
between data bursts is improved by selecting a bandwidth of 20 to 512 kHz for the transformer interface.
The lower corner frequency is set by adjusting the inductance of the transformer's Tx winding to 1.75 mH.
The upper corner frequency is determined by the design of the transformer winding technique.
The impedance matching network on the transmit side of the transformer attenuates the transmitted signal
by 12 dB. This loss is recovered in the receive side of the transformer. A step-up of 4:1 directly compensates
for the 12 dB loss. As with the transmit side, a protection network is required. D5 and D6 clamp the received
signal to a safe level but are sufficiently isolated by R23 so that they do not load the transformer when they
are conducting.
At 192 kHz (the spectral peak of MDPSK), 26 AWG wire attenuates signals about 17 dB/km. The receiver
in the UDLT-3 has sufficient input dynamic range to operate on loops as long as 2 km.