Getting the most out of your twisted pair cable

Know its Twists
Setting performance bounds (e.g. how fast and how far one can transmit binary digital signals using these cables) for twisted pair cables requires thorough understanding of their key electrical characteristics. One of them is cable insertion loss or cable attenuation. Above approximately 1 MHz, its most dominant component is the skin effect loss which is directly proportional to the square root of the frequency. Per ANSI/TIA/EIA-568-B.2 Category 5e standard, which defines currently the most common twisted pair cable, the insertion loss is mathematically modeled with the following equation:

Insertion Loss = -{1.967 * SQRT (f) + 0.023 * f +0.05 / SQRT (f)} [dB / 100m]

With this equation, you can approximate insertion loss for any CAT5e cable length at any frequency (Figure 1). It becomes very handy when you quickly want to assess what kind of loss your signal can expect.

You can use the data from Figure 1 to estimate maximum data rate as a function of cable length. The estimates require knowing the criteria your transmitter and receiver mandate for error free operation. For example, when building RS-485 transmission networks, attenuation of 9 dB at the frequency of 1/tUI in hertz, where tUI is a unit interval at a given signaling rate, is an accepted industry guideline for determining maximum signaling rate. For networks that use interface ICs with reduced voltage swings for higher signaling rates (i.e. LVDS, CML), attenuation of 6 dB at 1/tUI hertz may be used as a general guideline when determining maximum signaling rate for a given cable length. These guidelines assume dc-balanced data, point-to-point links, zero crosstalk and pair-to-pair skew, and no external interference.

Figure 2 illustrates the maximum signaling rate as a function of CAT5e cable length for RS-485 and LVDS interfaces based on the 9 dB and 6 dB guidelines respectively. In Figure 2, the flat segment of each curve is determined based on the cable's ohmic losses (typically 9 ohms per 100 meters for 24 AWG twisted pair) and the assumption that the signal driver requires 100 ohm differential cable termination. The sloped segment of each curve is determined using the attenuation values given in Figure 1.

Note the dashed portion of the LVDS curve. In theory, LVDS interfaces can transmit sub-Mbps signals over hundreds of meters of CAT5e, however, LVDS receivers can only handle +/- 1V of ground noise and as such are not suitable for a long-haul dc-coupled interfaces.

Other important twisted pair cable characteristics are near-end crosstalk (NEXT) and pair-to-pair skew. The NEXT is critical when considering bidirectional transmission. For unshielded twisted pair (UTP) cables, the NEXT increases quickly with frequency and leaves little room for cable attenuation given the allotted signal-to-noise ration, therefore the maximum cable length for bi-directional transmission is much shorter than it is for unidirectional transmission. The cables with individually shielded or foiled twisted pairs (FTP) have much better NEXT performance (Figure 3) and allow bidirectional transmission at frequencies beyond 1 GHz.

The pair-to-pair skew is a parameter that should not be overlooked when timing relationship between signals being transmitted on the same twisted cable is critical. Novice CAT5e users find themselves surprised when they discover that the pair-to-pair skew can be as high as 45 ns (typically 25 ns) per 100 meters (TIA/EIA-568-B.2). It is a high number of nanoseconds when dealing with signals that run at hundreds of megabits per second.