Frequency stability analysis is used to characterize periodic signals, for example, from oscillators. There is a variety of factors that limit the stability of an oscillator, e.g., electronic jitter or frequency drifts induced by temperature changes or aging of the device. One characterizes an oscillator by comparing it to a reference clock of higher quality. The Time Tagger comes with a software-based external clock that allows using arbitrary reference clocks. Ideally, an atomic clock will be used as a reference to extend the long-time stability of the Time Tagger.

The metrics for frequency stability analysis are described in the IEEE 1139 standard. The Time Tagger's FrequencyStability measurement class provides a collection of metrics, including the Allan Deviation (ADEV), the Time Deviation (TDEV), and the Hadamard Deviation (HDEV). The FrequencyStability class calculates all available metrics in parallel, on-the-fly, and even tracks the current frequency- and phase error.

There are two primary factors that will determine the quality of your measurement. For short time intervals under investigation, the discretization noise of the Time Tagger is the primary factor. For a Time Tagger Ultra with roughly 10 ps temporal resolution, the minimal ADEV you can calculate corresponds to the inverse of this resolution: For a time interval of 1 s, an ADEV of 10^-11 is achievable, at 1 ms you might achieve 10^-8. For long time intervals, the quality of the reference oscillator is the key factor. Without an external reference, you can rely on the internal clock of the Time Tagger Ultra with a stability of 8 ppb. With an atomic clock and our novel software clock, you can extend this stability by several orders of magnitude.

Please note that depending on your required accuracy, our entry model, the Time Tagger 20 may not provide enough stability on the intermediate time scale to allow the use of high precision external software clocks. Please consider a Time Tagger Ultra for your frequency stability analysis measurements.