AUBRAUX

Jitter

Jitter is displacement of signal edges from their ideal positions in time.

What is Jitter?

Jitter is displacement of signal edges from their ideal positions in time. It is also called timing distortion, phase jitter and timing jitter.

Jitter is made of two components: Deterministic (DJ) and Random (RJ). Deterministic jitter is further made up of Periodic Jitter (PJ) and Data Dependent Jitter (DDJ). DDJ is again made up of Duty Cycle Distortion (DCD) and Inter Symbol Interference (ISI).

What are the sources of Jitter?

Jitter is introduced by all electrical and optical components used to generate, transmit and receive signals. There are many sources of jitter:

Inter-symbol interference. Sources of intersymbol interference are mismatched impedances, reflection in signal path and frequency dependent attenuation over the signal bandwidth in the signal path.
Phase noise of the oscillator or time base generator.
Cross talk from adjacent signal traces.
Power supply noise. Noise on power carrying layers or ground layers in a PCB.
EMI radiation from RF sources and AC power lines.
SSO (Simultaneously switching outputs) noise.
Poorly designed PLLs and DLLs.
Thermal noise, shot noise and flicker noise.

Why is Jitter so important?

Importance of jitter in high frequency/ high speed systems:

It can introduce errors in digital signals.
It can result in loss of synchronization in PLLs (phase locked loops) and DLLs (delay locked loops).
It can cause loss of synchronization in frame or packet based systems.

How do I measure Jitter?

You can measure Jitter with JitLab. JitLab is a stand alone software which you can install on a regular PC or on the oscilloscope itself. It uses a digital oscilloscope to measure jitter. Supported digital oscilloscopes are from Tektronix and Agilent having LAN connectivity. Any digital oscilloscope with a GPIB port can also be connected using a GPIB card with VISA support.

Links & References:

Understanding and Performing Precise Jitter Analysis - By Tektronix
Analyzing Jitter Using a Spectrum Approach - By Tektronix