We talked a bit about characterizing transmitters.
Joel gave a whole talk on oscilloscopes which are the tool that we use these days.
Usually it's eye-diagram based, measured with a scope.
In the old days when I first started with BERTs about, a long time ago, like 17 years ago,
BERT receivers in some cases were better than scopes at characterizing some transmit phenomenon,
but those days are gone, scopes have gotten really good,
so virtually every high-speed channel transmitter is characterized with a scope.
Just relatively quick.
It's intuitive, like Joel was telling you, it's time versus voltage.
You can look at the single-valued waveform or you can overlay those and make an eye diagram.
Often it involves testing against a mask,
so you can understand that if you have nice margins around that mask,
you have a good transmitter.
Receivers, the test is BER-based.
The figure of merit for a receiver is what kind of BER can you get in that channel?
The output waveform of the receiver, so the receiver is going to make bit decisions
and somehow transmit those bit decisions back to a measuring instrument.
We'll talk more about that.
The output waveform of the receiver gives you no idea whatsoever.
You could have this beautiful eye where virtually every bit is in error,
but it'll still look great on the scope.
You'd see this pristine eye coming out of your receiver
after the slicer has made the bit decisions.
You'll have no idea unless you measure the BER.
You have to measure that the correct decisions were made
and that it will not only tell you if the decisions were made
but how well your clock recovery is performing.
As I've learned over the years, it's a lot less intuitive than transmitter testing.
People have a gut feel, and they've gone through EE school
and have been using scopes their whole career.
There aren't that many BERT-based classes in EE school.
We use BER instead of eye measurements.
What we do is we send a known input pattern,
so this could be PRBS or it could be a user pattern
or a pattern that the industry has determined
that's not a PRBS pattern but a memory-based pattern,
but you know what the pattern is.
You put it into the receiver, it makes the decisions,
and it somehow outputs those decisions
and you see that the decisions that are output, there are a couple differences here.
That guy was supposed to be a 1, and that guy was supposed to be a 1.
Anyway, you see that there were some differences between the output pattern and input pattern.
Those we record as bit errors.
What is BER?
You simply ratio the bits in error over the total bits transmitted, and that's BER.
Number of bits received in error over the number of bits transferred.