EMC Question of the Week: December 7, 2020
Generally, the best way to send a high-speed differential signal, V+ and V-, on an unshielded three-wire ribbon cable is to use
- the two outer wires
- one inner and one outer wire
- V+ on inner and V- on both outer wires
- both V+ and V- on the inner wire
Answer
The best answer is "a". It's important to maintain the balance of any transmission line carrying a high-speed differential signal. In a three-wire ribbon cable, sending the signal on the two outer wires is the only option that is balanced. Using one inner and one outer wire would provide the tightest coupling between the two signal conductors, but the transmission line would typically have an imbalance factor of approximately 0.6. This would create a common-mode voltage between the cable and the circuit boards at either end with an amplitude approximately equal to 10% of the signal voltage. In most situations, this would be more than enough to produce unacceptable radiated emissions.
Option "c" is even worse. Despite its symmetric appearance, the signal would be even more unbalanced than it is with option "b." Note also that option "d" is not really an option, since two wires are needed to carry a TEM signal on a transmission line.
Of course there are many other balanced options if we use a different cable. For example, using a two-wire ribbon cable or the two center wires in a four-wire ribbon cable would be balanced. If the ribbon cable had many wires, then any pair of wires near the center would be approximately balanced, even if the number of wires to each side was not exactly the same.
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