# PI (rms)

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.

PI (rms) is useful in the case where the PI is of a known distribution. For example, a stress test may be developed in which sinusoidal PI is intentionally injected. Assuming the injected PI is the only PI present, the system can be calibrated using the PI (rms) measurement.

The PI (rms) 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. The variance of the RN,PI histogram is calculated.
4. PI (rms) is the square root of the RN,PI variance minus the square of the RN value.