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GB1400 User Manual B-15
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BER
SNR (dB)
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Figure 1. Bit-Error-Rate (BER) is plotted here as a function of the signal-to-noise ratio (SNR),
where SNR is S/N
rms
in decibels (electrical), S is magnitude of difference between the signal (1 or
0) and the decision threshold and N
rms
is the rms value of the Gaussian noise riding on the signal
There are four major sources of error in any digital system: noise, jitter, baseline wander, and
intersymbol interference (ISI). A fair number of errors will come from noise; there will always
be background errors. In general, noise manifests itself as random errors that appear infrequently
and sporadically, unless there is a strong source of noise close by.
However, if a great many errors occur in a short period of time, you may be able to correlate the
burst of errors with other events, either natural (lightning around a microwave radio tower) or
man-made (running your test at precisely the same time as the once-a-month test of the back-up
diesel generator).
Jitter, undesired variations in the timing of edge transitions in clock or data waveforms, can
cause frame or clock slips. A certain amount of jitter (residual or incidental jitter) is inevitable
due to phase noise in oscillators, voltage hysterisis in switching transistors, etc.
We can deliberately add jitter into the system by supplying a jittered clock for the BERT
transmitter. This allows us to see how susceptible to jitter clock recovery circuits are. We can
also see whether a transmission circuit element will add jitter. As with adding attenuation,
stressing the transmission system with a certain level of jitter can be a good predictor of error
performance with no added jitter stress.