Tektronix 071-0590-00 user manual + Peak, Examining Jitter

BERT Technical Articles

To stress the noise margin, the PRBS spectrum must have components below the coupling circuit’s cutoff frequency, fL. For example, a 23-stage PRBS generator with a bit rate, fc, of 44.7 Mbits/s has a pattern length of 223-1=8,388,607 bits.

The fundamental frequency, fF, is fc/8,388,607=5.33 Hz. If fL=32 kHz, thousands of the pattern’s spectral components are removed. The fraction of power removed is given by πfL/fc, and the square root of that figure is the rms error as a fraction of the signal level:

rms error = πfL/fc = 0.047

This error appears as Gaussian noise with an rms value that is 4.7% of the noise margin. The more spectral components below the cutoff frequency, the more Gaussian the noise is. However, if fundamental would be the PRBS pattern length was 27-1, the 350 kHz, which is greater than fL. The noise margin wouldn’t be stressed, and the formula for rms error wouldn’t hold.

Similarly, a PRBS pattern will stress the clock-recovery circuit if the pattern’s fundamental is within the B. If it is, the pattern introduces random fitter in the recovered clock. The jitter’s

magnitude depends on fB and on offsets within the circuit. The rms jitter is not a function of the PRBS pattern length, but the peak jitter increases with pattern length.

Examining Jitter

An important factor in a data transmission system is jitter. Ideally, all clock and data signals in the systems have a constant frequency with no phase modulation. In practice, a clock source has some phase modulation, or jitter, and noise and imperfect equalization introduce additional jitter. The following discussion examines clock-source jitter alone, assuming that noise and distortion contribute no jitter.

If the received data waveform, F, is viewed on an oscilloscope synchronized data, the 1s and 0s overlap to produce an “eye” pattern (see figure below).

Ideal pulse position

- Peak

Superimposed pulses with jitter modulation

+ Peak

Jitter modulation

Figure 4. Viewed on an oscilloscope synchronized to the received data, phase error, or jitter, θe shows up as a widening of the recovered clock's waveform.

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