Oscilloscope Eye Diagram Testing
The images below show the eye diagram for a HDMI cable running at 3.4 Gbps. The picture on top is a measure of a poor quality passive cable; it shows a “closed eye”. On the bottom is the eye from an active cable that boosts the high-speed signals, thus resulting in an “open eye” which confirms the signal integrity of the cable. The degree to which the eye is open is a direct measure of the quality of the cable.
Figure 5: Oscilloscope Eye Diagram Snapshot of Passive (top) and Active Cables (bottom)
While the oscilloscope and data generator is clearly very effective. this kind of test equipment may cost up to $200k. Along with this the equipment is difficult to use and even a single cable measurement can take five minutes. With these drawbacks this system is really only used in pre production qualification of cables and random sampling of production lots. It is not a solution for 100% production testing.
A final method used to guarantee high-speed cable performance is to use a Bit Error Rate checker (BER). This tester generates and transmits high-speed data through the cable under test to a receiver. The receiver will have a prior knowledge of the transmitted data sequence and thus any errors received at the cable output are easily counted. This error count or error rate, produced by this system, is then used to give a coarse pass/fail indication for the cable. The goal is to use a receiver that is representative of what might be attached to the cable in the field. To find this is in fact one of the weaknesses of the scheme. Clearly the use of an atypically high performing receiver in this setup would lead to optimism in the pass/fail assessment.
A further note on this approach is that there is not a one to one correspondence between the bit error rate and the cable eye opening. This is clear for the fact that the eye diagram has two dimensions of information (time and signal swing) and the BER test produces a single number to characterize the cable and as such does not provide a quantitative statement of quality for the cable nor does it guarantee compliance for such standards as HDMI.
NEXT: Environment and Features for a Cable Test System
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