Negative frequency deviation in crystal oscillator

Question

What is the significance of a negative frequency deviation in the freq deviation vs temperature profile of a typical OXCO (e.g. the diagram seen on this page)? My understanding is that a positive deviation on these sorts of diagrams is ± some window around the specified frequency (and so would include situations where the deviation is negative relative to spec), but I haven't been able to find much about what a negative deviation means in this context.

Answer 1

You’re overthinking it. In terms of the 1st plot, a positive deviation simply means that the crystal is oscillating too fast. Negative, too slow.

Generally though you’re correct. The Allan Deviation plot at the bottom doesn’t care about the 1st order rate (i.e. the frequency offset). It is a statistic used to characterize the frequency stability, or performance, of the clock across averaging intervals (τ).This statistic allows you to inspect the short/long term behavior of a clock, as well as the oscillator’s constituent noise types.