Twisted Pair Testing
What are twisted pairs?
- Two wires twisted together form a twisted pair.
- Usually, there are several twisted-pairs in a twisted pair cable.
- In higher performance cables each twisted pair will be shielded.
Why do they put twisted pairs in cables?
It is an extremely effective way to send high-speed signals down a cable because:
- Most electrical noise entering into and/or radiating from the cable can be eliminated.
- Cross-talk (signals leaking between wires in a cable) is minimized.
Why do signals leak between wires?
In addition to energy flowing down a wire, it can flow between wires due to the electrostatic and magnetic effects that occur when voltages or currents in the wire change. In understanding electrostatic effects, consider that "insulation between two conductors" is the definition of a capacitor. More surface area (longer cables) means more inter-wire capacitance. Adjacent wires in the same cable behave as though capacitors are connecting them together, thus higher frequency signals can leak, or "cross talk", from wire to wire through this capacitance.
How does twisted pair cabling minimize the effects of capacitance?
A. Cross-talk (leaking signals)
Signals are sent down twisted-pair wires such that when one wire in the pair becomes positive the other wire becomes negative by the same amount. Any other wires close to this pair will be affected by cross-talk equal to the sum of the two signals, so if this sum is zero (or nearly zero) then the effects of cross-talk are eliminated.
Notice in period 1 in the illustration below that the signal sent through the orange wire shows up on the black wire. In period 2 the signal sent through the white wire shows up on the black wire. In period 3 the signal sent through the orange wire and its opposite polarity signal on the white wire cancel each other out, leaving no effect on the black wire.
B. Immunity to Electrical Noise
The receiving electronics is intended to detect only the difference in polarity between the two wires in the twisted pair. Since electrical noise affects both wires of a twisted pair equally, the receiving electronics gets a true signal by rejecting signals on the twisted pair that move toward the same polarity.
If I am using a good, certified, twisted pair cable and I test for opens, shorts and even perform high voltage insulation tests, is this enough to assure the quality of my assemblies?
Continuity, resistance, and insulation tests can tell you if the connections are correct but they can NOT tell you anything about the noise canceling effects provided by twisted pairing. A common and serious error in a twisted pair cable is a "split pair."
This error occurs when one wire from each of two different pairs gets swapped on both ends of the cable. In our example above, you can see that the white wire of the blue & white twisted pair (pin 4) and the white wire of the black & white twisted pair (pin 6) have been swapped on both ends. The result is a cable that will pass a standard continuity test, but will have serious cross-talk problems. Split pair errors can easily happen in twisted pair cables where one wire of each pair is the same color. Some twisted pair cables have wires of all the same color, making this type of error even easier to produce and harder to find. Even when all the colors are different, it is not that hard to make a mis-wire on one end that is caught during a continuity test and then "fix it" at the other end so that the continuity test passes but a split pair error now exists.
What testers detect split pairs?
Some of our testers can detect split pairs by checking that specific wires behave electrically like they are twisted together thus canceling the capacitance coupling effects. Twisted pair testing is available on the following Cirris testers: Signature 500, Signature 1000H+, 1100H+, Touch 1, easy-wire CH, EasyTouch, and easy-wire CH2.
However, there is more to twisted pair testing than just using the right tester!
Tips to successfully detect Split Pairs
Even with a tester capable of testing for split pairs, there are other challenges when testing twisted pair cables. Test fixturing, twisted pair cable quality and cable length all play an important role in testing for split pairs. Keep in mind the following issues when testing your twisted pair assemblies:
- Do not let the test fixturing degrade your test. Keep your interface wire lengths as short as possible (testing directly on Cirris adapter cards plugged into the tester is preferable). If the interface cable length is more than an inch or two, it must be constructed with the same twisted pairing as the cable to be tested. Also, the total length of interface cables (both sides) must be less than 1/3 of the total length of the cable under test. If you cannot avoid longer interface cables, or if you cannot maintain the same twisted pair pattern in your interface cable, a coaxial cable can be used instead. In this case, all points that must be tested for twisted pairs must be connected through coax cable and the shields of all the coax wires must be connected together and grounded to the tester chassis.
- You may not be able to test poor-quality twisted pair cabling shorter than 20 feet. In poor-quality twisted pair cabling, each pair is not shielded, the wires have less than 2 twists per inch and there is a general twisting lay to the bundle every foot or so for the length of the cable. For best results with this type of poor-quality cable it is even more important to keep all adapting cables short and to use high quality twisted pair wiring, or shielded wiring in your fixturing.
- Very short cables (less than 2 feet) have little chance of being properly tested for split pairs. If you have a cable under 6 feet long or a poor quality twisted pair cable as outlined above, you may have success testing this type of cable by adding an extender cable. By using a couple of feet of high-quality twisted pair cabling matching the twist pattern of the cable to be tested, you can increase the length of the whole assembly being tested thus allowing the tester to get a "better read" on the cable. Keep in mind, however, if you exceed the length of the cable under test with the extender cable, you will be testing the extender cable for split pairs rather than the cable you really want to test.
- Some other techniques you can use to test short cables are to build the cables twice as long as required, test them, then cut the cable in half and terminate the cut ends. Remember, the twisted pair test can be performed on a cable from one end only! Another simple approach is to terminate one end of your cable while the other end is still attached to the reel. After testing for twisted pairs, cut the cable to length right from the reel and terminate the second end.
4 Wire Fixturing Issues
If you will be testing using 4-wire fixturing adapter cables that are longer than a few inches:
Each pair of wires that goes to a single test point must be shielded, either individually or as pairs within one shield. These shields in the adapting cable must be tied to the tester chassis.