Quantitative or qualitative serial testing?
Eye diagrams are an intuitive way of viewing parametric performance. When done correctly, an eye shows every possible pattern combination overlaid, one on top of the other. With all combinations in one place, as shown below, the oscilloscope provides a rich set of tools that makes it easy to see when rise times are too slow, when overshoot is present, or when the eye is closed due to jitter. This is a qualitative way of really seeing the signal. So, why not use eye diagrams as the main measurement of link performance?
In order to construct an eye, an oscilloscope measures voltage samples, not actual bits. These voltage snapshots differ from measured BER points in important ways:
- Scopes grab a small part of any signal at a time, and that small part is taken only at a rate of a few hundred thousand per second. This compares with real data bits flying past at rates of 10 billion per second (for a 10 Gb/s link). This is a sparse sampling of what is really going on, and makes it unlikely that an oscilloscope will catch the one mistake in 1,000,000,000 bits that is being sought.
- Oscilloscopes and BERTs have very different methods of acquiring data–the former has very wide bandwidth but sparse sampling; the latter measures every bit but has a limited input frequency response, more in line with the network equipment that it is emulating. These differences can result in significant differences between each class of instrument.
In the real world, of course, the trade-offs aren’t always so black and white. Oscilloscopes are becoming faster and more capable of evaluating a high-speed link’s BER performance. At the same time, BERTs are becoming better at isolating the exact source of problems in a link and deliver a very accurate BER on long patterns.
With these trade-offs in mind, what are you using to determine your serial link’s performance? A BERT for a final test score only? An oscilloscope for a qualitative view using eye diagrams and estimated BER? Or both?
Chris Loberg is a senior technical marketing manager at Tektronix Inc.