Two Sources of Signal Attenuation
The first source is the variation in insulation thickness for an individual wire, and the second is the changing distance that an individual wire will have to its partner in its differential pair and its proximity to other wires/shields. The materials and control procedures in the cable construction as well as the particular cable style will directly affect the resulting spread in the attenuation levels in the cable.
Another performance limiting feature of high speed interconnect is intrapair skew. Differential skew is a measure of the difference between the time taken for a signal to propagate along the positive and negative wires in a differential pair. This skew at low levels degrades the performance of the cable but it can be catastrophic at higher levels. Targets for HDMI are a total of 111ps at a 3.4Gbps data rate. One reason for this skew is a variation in the wire insulation thickness and proximity of other wires. These cause a change in the electrical length seen in the two wires in a differential pair.
Another equally common cause of skew is a physical length difference that can exist between the two wires as a result of a non-optimal cable winding process. As with attenuation, with precision in the cable construction and the use of higher performance materials skew can be controlled to a reasonable level.
Cable Test Technology
With an understanding of the significant number of defects that can arise in cables, coupled with the fact that there is an ever-increasing requirement for faster and thinner cables, there is clearly a need for in-production testing for cables. There are a number of technologies in use today to test the quality and functionality of cables.
The first approach uses a cable continuity tester. This is a piece of equipment that checks the resistance between the endpoints of a cable. For a passive cable this tester builds a resistance profile of the cable and from this it can point to some of the catastrophic opens short failures that occur. The limitation with the continuity test however is that the information gathered is all based on DC resistance measurements and the device has no ability to measure frequency dependant components of the cable, such as capacitance and inductance. Without this high frequency response information, no assessment of attenuation or high frequency performance can be made. From this it is clear that a continuity tester is not sufficient to guarantee the signal integrity of the cable.
To measure the high frequency performance of a cable, more advanced equipment is required. For example, a high-speed data generator and an oscilloscope are equipment commonly used to do this. Typically prescribed high frequency data patterns are passed through the cable and an oscilloscope quantifies the damage done by the cable using an eye diagram. An eye diagram measurement gives a complete picture of signal integrity at the end of the cable — all DC or high frequency performance limitations will be captured by the diagram. The diagram is also easy to read in that results are also instantly obvious. It also gives easily interpreted feedback on the quality of the cable (see next page).
NEXT: Oscilloscope Eye Diagram Testing
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