Agilent Technologies J-BERT N4903, User Manual

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Periodic Jitter and Bit Error Information
When the incoming signal is sampled at the transition point, periodic
jitter manifests itself in the bit error record. An example may be helpful
to understand the phenomenon.
Let us assume we expect and correctly receive a simple 0, 1, 0, 1, 0, ...
pulse signal. If we would sample this signal one clock period earlier (-1
UI), we would see a bit error rate (BER) of 1.0. This is the maximum
BER for this pattern.
Let us also assume, this signal is overlaid by a periodic jitter source with
sinusoidal characteristics.
Now we sample the incoming signal at the transition point (-0.5 UI from
the optimum sampling point).
The result is illustrated in the figure below.
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.
5
Advanced Analysis
282 Agilent J-BERT N4903 High-Performance Serial BERT