Agilent Technologies J-BERT N4903B, User Manual, Page: 347 / 572

1
0
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1
0
1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1
increasing right shift decreasing right shift increasing left shift decreasing left shift
Signal without jitter
Expected and correctly
sampled data
Periodical,
sinusoid jitter
Signal with jitter
Captured data
Error data 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
Expected data
The jitter source moves the received signal to the right and to the left, but the
sampling point is fixed (dotted lines show the undisturbed signal). Because we are
sampling at the transition point, we get errors when the signal is shifted to the
right and no errors when the signal is shifted to the left.
The error signal shows a very characteristic and reproducible pattern. The resulting
BER for this kind of signal is 0.5. Other jitter frequencies yield different but also
characteristic error patterns and also a BER which is half the maximum BER.
The maximum BER and hence the actual BER are pattern-dependent.
Considering the BER bathtub curve, the sampling point is in the middle of the
descending line.
If random data is used, the average bit error rate at the left-hand side of the jitter
region is 0.5.
The bit error rate at the right-hand side of the jitter region is 0. We therefore expect
a bit error rate around 0.25.
Advanced Analysis
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Agilent J-BERT N4903B High-Performance Serial BERT 347