Electronic Design

Jitter Is The Designer's Nemesis—Until You Need It To Check Out Your System

Jitter problems plague many high-speed designs today, from semiconductor applications to telecommunications systems and just about everything in between. Ironically, however, when a designer wants to create jitter—perhaps to evaluate a system's tolerance to this pernicious effect—it is difficult to generate and control.

Fortunately, some AWGs are ideally suited to simulating jitter. This is particularly true of the AWG 610 from Tektronix. It allows for extremely fine jitter because of its high-speed sample rate of 2.6 Gsamples/s, and its Quick Edit or region-shift capability, which simulates jitter that is a fraction of the sample clock.

The Quick Edit feature can create jitter with picosecond resolution. It works on the simple premise that when a data point is moved vertically, it also will shift the signal horizontally. The waveform increments in the AWG 610 are less than a sample interval, creating a new data point that is interpolated between the sample intervals to derive the shifted value. The minimum shift value is defined by the minimum sample rate and the vertical resolution of the AWG. In the case of the AWG 610, its 2.6-Gsample/s sample rate, combined with its 8-bit resolution, produces a 400-fs minimum shift.

So what does this all have to do with jitter? By creating several copies of an original waveform, modifying each copy with a different shift value, and then outputting them sequentially, a jitter signal with several jitter transitions can be produced. The number of waveforms needed is determined by the number of jitter signals needed to simulate the real world. Waveforms can be shifted in both the horizontal and vertical direction.

In the horizontal direction, the Quick Edit horizontal control feature can be used to make precise increments. In the vertical direction, the waveform can be shifted at an increment defined by the number of vertical bits—in this case, 8 vertical bits. An unmodified waveform and the effects of performing waveform shifting, expansion, and compression can be seen in the figure.

Quick Edit can only create a single waveform. To simulate a jitter test signal, all the test waveforms need to be output sequentially. The AWG 610, though, supports waveform sequencing. Once the jitter waveforms have been created, the designer can string them together into a waveform sequence, precisely simulating a signal with jitter.

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