# PI (δ-δ)

Periodic Interference (PI) is a measure of the interference that is uncorrelated to the pattern, yet is periodic. Interference caused by a switching power supply is an example of periodic interference.

Dual-Dirac PI (δ-δ) is useful when the distribution is unknown. The dual-Dirac number allows various PI measurements to be compared with one another, even when the distributions are dissimilar. It is a measure of how much the PI affects the low-probability statistics of the interference.

The PIδδ measurement algorithm is:

1. Random Noise (RN) is measured.
2. An RN,PI histogram of the data used to measure the RN is constructed.
3. Using the measured value of RN, a dual-Dirac model is fit to the RN,PI histogram to obtain the PI measurement. The model is fit at a probability of 10E-3.
4. The above steps are repeated, resulting in an increasingly populated RN,PI histogram.