# ISI (p-p)

ISI (p-p), Inter-Symbol Interference, is a measure of the pattern-dependent interference effects within a signal. It is a measure of how much the amplitude of a bit depends on its location within the pattern.

ISI is measured using the following steps:

1. The instrument acquires 16 samples from each of the first 256 bits in the pattern. If the pattern has fewer than 256 bits, 16 samples are acquired from every bit in the pattern. These samples (up to 4096) are taken in random order at the measurement location, which is normally the center of the bit.
2. The average level of each bit is obtained. These averaged levels are combined into an ISI histogram for the ones and a similar histogram for the zeros. The ISI for each level is calculated as the peak-to-peak width of each histogram.
3. The above steps are continually repeated, displaying a running average level for each bit.