Slicing up the eye

Any successful serial communication system design includes a receiver that can make a decision at an instant in time as to whether the transmitted serial stream is above or below a particular threshold voltage. From this it decides whether the incoming signal is a data 1 or a data 0. Sensible system designers place this decision point as far as possible from rising edges, falling edges, high level, and low level—in other words, in an unobstructed part of the eye, usually the center, as shown by the “plus sign” in the figure below.

To test designs, a common tool used is the bit-error-rate tester, or BERT. Most BERTs have the ability to move the decision point away from the optimum position shown above in time, voltage, or both. By moving the decision point, it is possible to probe, or “slice up,” other parts of the eye and, by measuring the errors encountered, profile the anatomy of the eye diagram for a more complete understanding of receiver tolerance to excessive noise or jitter on the transmitted stream.

Moving the decision point around is like “slicing up the eye” because it gives the designer a different view of the eye and corresponding BER profile. Here are three ways of doing the slicing:

1. Probing jitter: This is a common test carried out with a BERT and done by probing through the crossing point of the eye, as shown in the figure below. This measurement has been given various names, including BERTScan, bathtub jitter, and jitter peak. It is more fully described in MJSQi. This measurement taken by a BERT has the advantage that the BERT sees every bit, and so is most likely to capture rare jitter events.

2. Probing signal to noise: A second common eye test that arose out of the fiber-optics industry is Q-factor. This is particularly useful in systems whose performance is limited by noise. Here, the decision point is used to probe a vertical slice through the eye halfway through the bit period, as shown in the figure below. Measuring the way the BER rolls off toward the middle of the eye gives an indication of how much noise is present and how it will affect the link.

3. BER contour: This approach to slicing is a superset of the last two approaches, and also the perfect linkage between BER and eye diagrams. Here, the decision point is stepped around the inside of the eye and the BER profile mapped out on a series of angled slices around the eye, as show below. This provides a much more complete and clearer picture of where system parametric problems may be lurking. For BER-contour slicing, some instruments, such as the Tektronix BERTScope, have been optimized to make this a fast and revealing measurement.

Are you doing any slicing, or is the reported BER a satisfactory result for your receiver testing? If you are doing eye slicing, how have you found it to be useful?