Beating the jitter bug – how to apply multiple measurement strategies to identify noise source

November 03, 2016 // By Andrea Dodini, Keysight Technologies
Digitising an analogue signal can improve the chances that the information it represents arrives at its destination uncorrupted by electrical noise. However, digital signals can be corrupted if noise signals alter their timing enough to push the transitions in the bitstream out of sync with the sampling point. This ‘jitter’ means the bitstream is misinterpreted, and can be a particular problem at very high data rates.

The first step to beating the jitter bug is to know your enemy.

 

What causes jitter?

There are several causes of jitter. Some of them are as follows:

  • System phenomena, such as crosstalk from radiated or conducted signals, dispersion effects, and impedance mismatches.
  • Data-dependent phenomena, due to the pattern of data in the bitstream data. This can result in inter-symbol interference, duty-cycle distortion, and pseudorandom periodicity in the bitstream.
  • Random-noise phenomena, caused by thermal effects, noise associated with electron flow in conductors, shot noise caused by electron and hole noise in semiconductors, or pink noise spectrally related to the inverse of the clock frequency.

 

Random versus deterministic

Jitter sources are often categorised as ‘bounded’ or ‘unbounded’.

Bounded jitter sources reach maximum and minimum phase deviations within an identifiable time interval. Bounded jitter is due to systematic and data-dependent phenomena. Unbounded jitter sources are random, and can (in theory) have infinite amplitude.

The total jitter on a signal, defined as its phase error, is the sum of the deterministic and random jitter affecting it.

The deterministic jitter component is defined by adding the maximum phase advance and phase delay it produces, while the random jitter is the sum of all the random noise sources affecting the signal.

Design category: