Signal Skew on RGBHV / VGA Cables / CAT Cables


The standard EIA/JESD65 defines skew as “the magnitude of the time difference between two events that ideally would occur simultaneously”.
Signal skew is a very important factor for analogue and digital signal transmission. In digital transmissions skew results in an increase in jitter.

Digital jitter, see Signal Quality, Eye Pattern, Re-Clocking, Insertion Loss

Skew in analogue Signal Transmissions

The individual signals of any signal consisting of combined components as RGBHV, VGA or YUV are prone to signal skew that will degrade the resulting information. For instance, RGB transmission should be received with all five components exactly at the same time, in unison, as they were generated: red, green, blue and the sync information.

Differences of the arrival time of the individual parts of an RGBHV signal will result in color fringing, this condition is called skew. Different cable lengths will result in different signal arrival times and therefor in signal skew between the components.

The physical length of each individual cable of an RGBHV signal connection must be closely matched to ensure equal arrival times for the individual signals.
In a 1280 x 1024 resolution VGA with an 85Hz refresh rate, the pixel duration is typically 9.7 nanoseconds. The propagation velocity in coaxial cable is approximately 66 % of the speed of light, which equates to a cable propagation rate of 198 mm per nanosecond.
The image fringe will grow approximately by one pixel for every 1.6 meters of cable mismatch.

The skew will become very important in all analogue VGA transmitters over CAT cable. Here a special 'skew-free' cable is the best choice.
All CAT cable have a certain length difference between the four twisted pairs. Each pair has a slightly different number of turnarounds per length unit to minimize crosstalk between the pairs. A different number of turns results in a different length of the individual pair, thus resulting in a length difference between the four pairs. This length difference is the cause for a different times needed for the signals to travel on the wire pairs. The resulting travel time difference over 100m is measured as skew. The skew for a good CAT6 / 7 cable is in the range of some nanoseconds.