Test Your EQ (Engineering Quotient)

EQ #30

a—Is it possible to transmit on-off (DC) signals between two pieces of equipment in both directions simultaneously on the same wire, in much the same way that telephones do for audio?

b—But doesn’t a true hybrid use transformers, or at least some tricky transformer simulation with op amps to ensure the transmitted signal does not appear on the receive port?

1a—Why not? Hybrids work just as well at DC as they do for
audio; you just need a receiver with balanced inputs, like an RS-422 buffer:

All resistors are the same value (e.g., 4,700 Ω) and the
transmit driver needs to be a voltage source (low impedance).

If the transmitter switches between, say, 0 V and 5 V, the
opposite receiver will see a voltage differential of 0 V and 2.5 V,
respectively, while the local receiver will just see 0V.

For long lines, you’ll probably want to use lower resistances
and you’ll want to limit the slew rate of the transmitter so that the receiver
doesn’t produce glitches on the transitions of the local transmitter.

If the RS-422 receiver is replaced with an op-amp differential amplifier with a gain of 2, then any analog voltage transmitted by one end will be reproduced at the other end.

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.

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EQ #30

by Circuit Cellar Staff time to read: 2 min