Tektronix 071-0590-00 Confidence Level in BER Measurement, Stress Testing, Additional Reading

BERT Primer

Confidence Level in BER Measurement

At first glance, one might think that the BER is known after receiving just one bit error. It would be expressed as 1 over the total number of bits received. But how do we know what the “true” BER is? After all, the first errored bit could have been just a freak “glitch”. In reality, from a statistical confidence point of view, we must measure long enough to get many errors. This could take a very long time.

Confidence Requires Collecting Many Errors

If the transmission system had a true BER of 10-12, and we were running at 100 Mb/s, the average time between errors would be 10,000 seconds. At 3600 seconds in an hour, the average time between errors would be nearly 3 hours! We need more than just one “error event” to have any confidence at all in stating an “error rate”. Also, the more errors we accumulate, the greater our confidence level that the BER measurement is truly representative of the device under test.

For a Poisson distribution of errors , BER accuracy is 1 over the square root of the number of errors. An accuracy of 5% requires 400 errors to have been counted. Conversely, for 95% confidence that an error rate is less than some limit, the DUT must be error free for three times the reciprocal of that limit. For example, to assure an error rate less than 10 per hour, the test must run error free for 0.3 hours.

Additional Reading

Dr. Dan Wolaver has written an excellent article on confidence levels in BER measurements titled: “Measure Error Rates Quickly and Accurately".

The article appears later in this section on BERT Technology/ Technical Articles.

Stress Testing

Test times can be dramatically reduced in BER measurement by stressing the device under test to increase errors. This is done by either:

*Adding Attenuation

*Adding Jitter

*Adding Noise

Attenuation degrades S/N (signal-to-noise) ratio, and thus reduces the size of the data “eye” in amplitude. Adding jitter on the clock (modulating the clock period, causing the clock edge to jump around) effectively reduces the data “eye” in width. A third technique is to add noise, which also degrades S/N. Any of these methods will increase the BER in a known way which allows extrapolation of what the error rate would be without stress. Measuring BER in the presence of added stress is an essential technique in testing high quality (very low error rate) systems quickly.

GB1400 User Manual B-9