Engineers often construct test fixtures that include high-speed differential signals. Although miniature coaxial cable is widely available, there's no commercial off-the-shelf source for small-gauge twisted-pair cable that's suitable for differential signals. Although Category 5 Ethernet cable contains four twisted pairs, it's too large for crowded fixtures and for attachment to the Amp Z-Pack connectors some fixtures require. Many engineers are unaware that they can twist together two lengths of AWG #30 Kynar-insulated wire—garden-variety wire-wrap and prototype cut-and-jumper wire—to make a 102Ω differential-transmission line. If you use Kynar's dielectric constant and the insulation's thickness to compute its properties, the line's calculated differential impedance works out to 110Ω. In practice, differential TDR (time-domain-reflectometer) measurements show that the line's actual impedance consistently measures 102Ω—only 2% away from the target impedance and thus close enough for most practical purposes.

To make your own twisted pair, start with a long AWG #30 Kynar-insulated wire and fold it in half. Enlist a co-worker to hold the cable's closed end by slipping the loop around a screwdriver's blade. If you're working alone, slip the loop around a doorknob. Tightly twist the two wires' free ends together and insert the twisted ends into the chuck of a Dremel (www.dremel.com) rotary tool. Tighten the chuck and hold the Dremel tool so that the wires are stretched tightly, are of the same length, and lie parallel with each other.

Apply a slight amount of tension to the wires and start the tool. As the wires twist together, the pair shortens and pulls the tool's operator toward the loop support. A variable-speed Dremel tool works best when you operate it at its slowest setting. If you have only a fixed-speed Dremel tool, avoid overtwisting the wires by preparing a length of 10 to 20 ft of cable at a time. The extra length allows time to switch the tool off and avoid overtwisting the wires. Cut off and discard the cable's nonuniformly twisted end sections.

The amount of twist in the wires is not critical, but the wires should be firmly twisted together. Using approximately eight to 10 twists/in. works well. To count the twists, hold a portion of the cable against a ruler or measuring scale under a magnifier and count 16 to 20 "bumps," or half-twists, per inch. Using too many twists per inch uses excess wire and increases losses and propagation delay. For the lengths in a test fixture, losses are insignificant except at extremely high frequencies.

You can also use a variable-speed hand drill with a ¼- or 3/8-in. chuck to twist the wires, but you need to fold the wires' free ends several times and wrap them in duct tape to ensure a snug fit in the drill's chuck. When using any power tool, wear safety glasses or other eye protection during the procedure.