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How to test a USB2 cable with a BERT (bit error rate tester)?

How to test a USB2 cable with a BERT (bit error rate tester)?

How to test a USB2 cable with a BERT (bit error rate tester)?


We have what can be best described as a usb 2 cable of questionable quality.

We want to use a bit error rate tester to decide whether or not this cable is up to the job of supporting high-speed USB 2 data (480 Mbit).

I am aware there are other ways of checking the performance of the cable, but these other methods are not the subject of this post.

Our bit error rate tester is an Agilent ParBERT system. It has a clock module, that can run up to 675MHz, and it is possible to generate data using this clock pass it though the DUT and analyse what comes out the other side...

We are currently using PRBS 2^15-1 data.

Everything is working and we are not getting any errors. My understanding is that the BERT tester plays around until it finds the optimum sampling point (in the centre of the eye), and then synchronises to it... Assuming the DUT isn't too terrible, the expectation would be that there would be no errors...

Ok, so here's the thing I don't really understand. If the BERT synchonises to the middle of the eye and there are no errors, then is this test really representative of a real system? It would kind of suggest that as long as there is a little bit of an eye opening, there will be no errors - I am pretty certain this statement is rubbish...

So my question is how to do set up the BERT to test the system is a mannor that lets me know whether my DUT will work?

I hope this has made some sense. Thank you for taking the time to read this and for any responses.

Kind regards,


RE: How to test a USB2 cable with a BERT (bit error rate tester)?

Just wanted to add that I am thinking that the BER bath-tub might be the key, but I only partially understand that at the moment...

RE: How to test a USB2 cable with a BERT (bit error rate tester)?

The eye opening is only a qualitative way of measuring the noise. The BER is a very sensitive measure of the noise. Now if you have a huge signal to noise ratio you won't have errors.

If the cable is long and/or has high loss, the signal will be greatly reduced and the noise may then cause errors.

The eye opening will also show finite bandwidth and could be used to compare cable types.

RE: How to test a USB2 cable with a BERT (bit error rate tester)?

A single cable or a cable design?

If it's USB, how are you getting to the analog data?  How are you ensuring that the meter is providing the same termination, etc., that the USB receiver is providing?


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