Answer 7

No. A hybrid is just a bridge circuit, with one arm of the bridge replaced by the line and the termination at the far end. The transmit signal is applied to two opposite corners of the bridge and the receive signal is taken from the other two corners.

In order to provide the Tx/Rx isolation, the bridge must be balanced, which in the example above, means that the lower resistor on each side must match the impedance of the line/far end combination. For DC and short lines, a simple resistor suffices. At audio frequencies and with the long unshielded twisted pairs used in telephony, a more complex matching impedance is required. Transformers are used only because it's the easiest way (and the only passive way) to get a balanced drive and/or receive signal—the transmit driver and receiver cannot share a ground. In order to mass produce phones that were dirt cheap, yet simple and reliable, the phone company figured out how to use a multi-winding transformer to provide the both the isolation and the balanced/unbalanced conversion in both directions, usually with a single resistor and capacitor to provide the line matching. As noted, modern electronic phones use active electronics to achieve the same things. As always, the theory is simple, but the practical implementations can get complicated.

Contributor: Dave Tweed

Published March 2003