Tektronix 071-0590-00 user manual Amplitude UI peak-to-peak Frequency, Jitter Tolerance

BERT Technical Articles

Jitter Tolerance

In general, the noise and imperfect equalization of the transmission system itself introduce much more jitter than the clock source does. If the jitter, qi, of the received data exceeds the receiver’s jitter tolerance, the receiver will begin making errors. When the BERT is recovering its own clock, the instrument receiver’s jitter tolerance must be greater than the system receiver’s for all waveforms. Otherwise, the BERT will report an incorrectly high error rate.

It is impossible to check all waveforms. Fortunately, however, if the BERT receiver has a higher jitter tolerance for sinusoidal jitter at all jitter frequencies, fm, then it has a higher jitter tolerance for all jitter waveforms.

The maximum data phase jitter,θe max, between the data and the clock that the receiver can tolerate. The first failure may be the setup time when the decision circuit samples the data with the clock.

Or the clock-recovery circuit may fail to maintain lock if θe is too great. In any case, θe max is generally between 0.3 and 1.0 UI peak-to-peak. If the system’s θe max is unknown, it can be measured.

The transfer function from θi to θe is:

⎜θe/θi⎜=fm/(f2 m=f2B)1/2 (Equation 2)

where fm is the frequency of the sinusoidal jitter.

Then the maximum data jitter a receiver can tolerate is θi max=θe max/⎜θe/θi⎜

Below fB, θI max descends at one decade per decade. Above that frequency (where the clock recovery can’t follow the data jitter), θi max=θe max.

For proper operation, the BERT receiver’s jitter tolerance curve must lie above that of the system receiver.

A BERT designed to supply jitter has a sinusoidal generator that modulates the clock phase. Both the frequency, fm, and the amplitude, A, of the phase are selectable over certain ranges. The range of fm is from f0 to f4, and the range of the amplitude depends on fm. At low frequencies, the bit-error-rate tester must generate higher amplitudes than are needed at high frequencies (see following figure).

Amplitude

UI peak-to-peak

A1
A2
f1	f2	f3	f4
	Frequency

Figure 5. The range of jitter frequencies and amplitudes that a BERT should generate.

B-30 GB1400 User Manual