Quick Answer: It depends.
Jitter is short term variation (generally we take short term to mean changes with frequencies greater than 10Hz, below that we are talking about Wander) in the position of a clock signals rising and falling edges. From this simple concept though the word ‘Jitter’ can take on multiple meanings depending on how it is measured and characterized. Future posts will explore these different approaches to Jitter and clarify what ‘Low Jitter’ means.
In the meantime it can be helpful to look at a traditional Figure Of Merit in the clock industry, the integrated phase jitter measurement using no additional filtering, to get a sense of the performance range that is covered by modern clock oscillators. On the Vectron website we have summarized data on a selection of devices at standard frequencies. You can find the data summarized here.
One thing that you might notice is that ‘low jitter’ devices cover a wide range that goes from below 1ps all the way down to 23fs and that these numbers are strongly dependent on the details of the measurement, even within the narrow definition of phase jitter we have given for the data in the table. Further variation in phase jitter would be seen if we superimposed additional filtering on the data beyond that already assumed by the ‘brick wall’ filter used during measurement.
The message here is that benchmarks like phase jitter are useful, but even then details about the measurement can lead to a range of results. When comparing clocks that claim ‘low jitter’ performance always keep in mind the differences between what ‘Jitter’ actually means (more on this to follow) and the particulars of how the numbers you are looking at were calculated.