1 Time Domain Jitter Measurements

There are three types of jitter that are measured in the time domain: period jitter, cycle-to-cycle jitter, and time interval error (TIE) jitter.

The period jitter represents the maximum deviation of an oscillator’s period in a waveform over a specified number of clock cycles. Given a number of clock cycles (typically in the 10,000+ range) each period is measured in order to calculate the mean value, standard deviation, and peak-to-peak value. As this measures the clock period deviations over time, there is no reference to an ideal value. Any difference between the mean and expected value can be looked at as an offset and easily designed around.

The cycle-to-cycle jitter represents how much the oscillator’s period changes between any two adjacent cycles. Much like period jitter, cycle-to-cycle jitter is not measured in reference to an ideal value. Cycle-to-cycle jitter for an oscillator is reported as a peak value that defines the maximum deviation between the rising edges of any two consecutive clocks. For example, if one period is –20ps from the average and another period is +15ps from the average, the maximum cycle-to-cycle jitter is 20ps. There are some measuring devices that calculate the cycle-to-cycle jitter by applying a first order difference operation to the period jitter.

TIE jitter measures how far each edge of the clock varies from its ideal position. Unlike period and cycle-to-cycle jitter, TIE jitter measurements require the tester to know where the ideal edges should be and, for this reason, it is difficult to observe TIE in real time with an oscilloscope. In most cases, TIE jitter measurements require the tester to capture and post-process the data. The TIE may also be obtained by integrating the period jitter, after first subtracting the nominal (ideal) clock period from each measured period. TIE is important because it shows the cumulative effect that even a small amount of period jitter can have over time.