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BERT Primer
B-10 GB1400 User Manual
Other BERT Features
In addition to measuring Bit Error Rate, BERTs may have additional features
which enhance ease of use or expand test capabilities. Some of these might be:
* Eye Width Measurement
* Auto Synch Feature
* Pattern Locking Methods
* Pattern Loading Software
* Error Insertion
* Internal Jitter Generation
Eye Width Measurement
Since the BERT Receiver must have the ability to delay the received clock, it
may have the ability to run the clock edge through a range sufficient to cover the
received data “eye” . The “eye” width is the time during which the received data
exhibits clean logic “1” or “0” levels. Ideally the “eye width” is one over the bit
rate. At the edge of the “eye”, the signal is transitioning from one level to the
other. The “Eye Width” for any Error Rate is a measure of a system’s timing
and jitter tolerance. A designer might have to meet a spec such as “eye width at
500Mb/s and 10
-9
BER must be 1.0ns or greater”. The manufacturer is then
confident that there is sufficient margin in his product to live up to his customers’
expectations. Vertical compression of the eye (from ISI, echoes, regenerator
output variations, decision-level instability, etc.) from ideal can be used to
extrapolate error rate, and derive minimum S/N ratios to assure specified
performance.
The user should run the test equipment back-to-back (i.e., Transmitter connected
directly to Receiver with no device in the test path) first, to establish maximum
eye width. BERTs inherently have their own set-up and hold times, so back-to-
back results will always be a little less than perfect.
Auto-Synch
Auto-Synch is the ability of the BERT Receiver to search in time and in
amplitude level for the optimum place to put the Clock edge for the cleanest logic
level information. This positioning must be done, whether automatically or
manually, before any meaningful BER measurement can be made. All of our
BERT products have this Auto-Synch feature.