By Dan Roach
This month I propose to finish off my diatribe from last time, about the importance of proper source impedance when driving telephone lines, with a roundup of information about analog lease circuits. Last time I was describing an electrically long (i.e., >300 m) twisted pair, in terms of its series inductance and resistance and shunt capacitance, as being similar to an analog low-pass filter. The equivalent circuit is shown in Figure 1A. I was attempting to show how the response at the output of the line varied with R, the impedance of the audio source. Why is that?
Well, it’s because it varies the Q of the series RLC circuit, of course, as we know from our AC theory. In Figure 1B you can see how the upper frequency response varies with R. As the source impedance is reduced, the Q of the filter is increased, and the attenuation of high frequencies is delayed until the inevitable plunge to zero output at infinite frequency.
The traditional approach to flattening the response is to equalize it with a parallel resonant RLC circuit, arranged as in Figure 2A. If you look carefully, the bottom half of the response curve (below resonance frequency) complements the response curve of the uncorrected telephone line—add ‘em together and you should get unity! By adjusting the resonance frequency of the equalizer to just above the frequency response desired, and varying the damping resistor to adjust the loaded Q, one can get a fairly flat resultant response from the program circuit. With this type of equalizer, frequency response above resonance drops like a rock, reducing out-of-band noise as an added benefit. But since the equalizer is completely passive, it can’t boost frequencies that have been lost in the line, it can only attenuate frequencies that have less loss to balance the response. A long line can have a lot of inherent high frequency loss, so that at the output of one of these equalizers levels will need amplifying, sometimes a lot! Which is why we’ve progressed from the simple RLC equalizer shown, to more modern equalizers from folks like Tellabs and McCurdy Telecom that can provide gain, and other features like phase equalization. Because, you see, these old RLC circuits can kind of ruin the phase response of a line. This subjectively doesn’t sound too bad with a moderately-equalized circuit, but can show up as an odd kind of hollow sound when extreme amounts of equalization have been used.
At least we’ve chosen an unloaded pair. Normally, telco adds loading coils every fraction of a mile, which add series inductance, with the net effect that line attenuation is much reduced in the voice-band (300-3kHz), but drops precipitously above that. Once the audio’s been through this kind of mess, it’s impossible to smooth out to get better high end response. Instead of a smooth attenuation curve, you end up with bumpy in-band response followed by a sharp drop-off.
One trick that old-timers have been known to use can come in handy when you have a fairly short loop and no budget for equalizing: you can use a pair of repeat coils to drop the impedance of the source from 600 to 150 ohms, which more closely matches the actual impedance of the line. At the receive end, you pop in another repeat coil to get from 150 back up to 600 ohms. The circuits need to be properly terminated with 600 ohms at each end. The result is flatter response and less attenuation than if you had left the circuit at 600 ohms throughout.