Interpreting an Error Free Interval Histogram

The Error Free Interval histogram looks at the bit distances between errors. For example, an error that occurs every time a pattern is transmitted will result in a histogram with all error free intervals equal to the pattern length. If the error only occurs on occasional repetitions, then multiple spikes will appear at regular, multiple spacings of the pattern length.

1. One bit between errors is most likely; two bits is next most likely; etc. If this pattern appears in conjunction with a Burst Length histogram length of one, then errors are truly random—which is not as common as you would think. In a fiber optic link, this means the errors are SNR-limited only.

2. This graph shows classical interference; systematic repetitive error at a frequency that corresponds to "n" bits. This could be a digital logic problem within the system under test.

3. A fiber optic link, SNR-limited but with some mild interference occurring as a background, repetitive error, would produce a graph of this type.

4. This pattern is not random. A single peak at the pattern-repeat interval shows pattern sensitivity. Several peaks indicate that sometimes the interval between errors is twice or three times as long, meaning that the error does not occur with every repetition. See also Example 2.

5. This pattern indicates interference external to the system and not bit-aligned. The problem interferes every time—for example, breakthrough from spark plugs, or vibration on a shake table. Interference frequency in this example is (bit rate)/n. See also Example 6.

6. This graph is essentially the same as Example 5, but the interference does not happen on every repetition.

7. A single, high peak at a fixed position reflects a system that always gets errors this number of bits apart. Usually, this is a hardware issue. For example, in magnetic recording you might expect a write error rather than a read error; in fiber optics, a one is seen as a zero every time—perhaps GaAs lazy shoulder after many zeros in a row.

8. On an eight-bit MUX/DeMUX system, this would indicate a differential connective wire is broken or a dry solder joint on one leg of an IC. A Burst Length profile will show one-bit errors, and Error Free Intervals are shown with eight-bit separations. Other common multiples might be 16, 32, 64, or 20 bits for video.