Tektronix 071-0590-00 user manual Bert Affects Accuracy

BERT Technical Articles

A BERT designed to measure jitter has a phase demodulator connected to the covered clock. The range of fm is from f1=10 Hz to f4; the amplitude depends on fm.

The BERT’s jitter generation and jitter measurements specifications should be somewhat higher than the maximum jitter that a system can typically tolerate. As a result, a BERT with jitter generation can find a receiver’s θe max by setting fm=f4 and increasing A until the receiver begins to make errors. In addition, the clock-recovery bandwidth, fB , can be estimated by lowering fm until the jitter tolerance begins to exceed

θe max.

BERT Affects Accuracy

Several BERT characteristics can affect the accuracy of system measurements. For instance, a BERT transmitter may have a very jittery clock source, such as an open-loop voltage-controlled oscillator. If so, the system will have a higher error rate or less margin than when the system’s clock source is used. Measuring the clock-source spectral density and calculation θe rms will uncover this problem.

Another snag may be that a BERT receiver has a very narrowband clock-recovery circuit, such as crystal controlled phase locked loop. The small fB will probably make the BERT less tolerant of jitter than the system its testing. In high jitter situations, therefore, the BERT will report a higher bit error rate than the actual rate. A BERT transmitter with jitter generation can measure its own receiver’s jitter tolerance and that of the system to see which is greater.

The question of whether a BERT data pattern fairly represents live traffic is more difficult to answer. Experience indicates that long PRBS patterns (N greater than 14 ) stress the system more than most live traffic does. But it is rare traffic with unusually stressing patterns that is most interesting. Therefore, the best users can do is to anticipate these patterns and generate them with fixed patterns from the BERT transmitter.

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GB1400 User Manual B-31