High Frequency Impedance Matching for Twisted Pair Cables

This paper re-examines the 100 Ohm characteristic impedance assumption used in the context of Digital Subscriber Line in the very to ultra high frequency range. A novel method of measuring the characteristic impedance of a transmission line is introduced which uses time-gated measurements from a vector network analyser. The results are utilised to synthesise impedance matching circuits for the excitation of a single twisted pair and a two pair underground cable at centre frequencies of 150 MHz and 500 MHz. These circuits achieved reflections as low as $\Omega$40 dB. Furthermore, we report gains in transmission increasing with frequency, ranging from few decibels below 100 MHz, to several in 100-250 MHz and many above 400 MHz when compared to a commercial reference balun. In fact, compatibility tests beyond the target cable samples demonstrated higher than 10 dB gains in 20, 40 and 60 metre underground cables at around 500 MHz indicating that sample-based matching circuits are not entirely sample-specific. Preliminary tests showed no significant change in crosstalk when using differential matching circuits.