What is Jitter in a Crystal Oscillator?

Jitter is a method of describing the stability of an oscillator in the Time Domain. It combines all the noise sources together and shows their effect with respect to time.

Let us consider a simple pulsed signal chain. Ideally, the duration of a perfectly pulsed signal at a frequency of 1 MHz, would be exactly every 1 microseconds, with an alternating edge every 500 ns (See Fig. 1 below).

But in the real world, this does not happen. In reality, the position and amplitude of the alternating edge varies, which causes uncertainty about when the next edge of the signal will occur. This uncertainty is know as Jitter (time domain) and phase noise (frequency domain).

Jitter measures the variations of signal period in the time domain, describing how far the signal period has wandered from its ideal value. Typically, deviations below 10 MHz are not classified as jitter, but as wander or drift. 

There are two main types of jitter: deterministic and random. 

Deterministic Jitter is created by identifiable interference signals. It is always bounded in amplitude, has specific (not random) causes, and cannot be analyzed statistically.

There are four main sources of deterministic jitter:

• Crosstalk between adjacent signal traces 
• EMI radiation on a sensitive signal path
• Noise from power layers of a multi-layer substrate
• Simultaneous switching of multiple gates to the same logic state

Random Jitter describes timing variations caused by less predictable influences like temperature which can affect the mobility of semiconductor crystal material, or semiconductor process variations etc.

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