Eye diagram on an oscilloscope

[roz77]

I'm working on some stuff using an oscilloscope, and I need to figure out how to obtain an eye diagram, or eye pattern on it. Right now I just have a function generator hooked up to it. There's a sync output on the function generator as well as the output for the signal. Do I just hook up the sync output to the external trigger input of the scope? Is there anything else I have to do? Thanks for your help.

 

[berkeman]

I'm working on some stuff using an oscilloscope, and I need to figure out how to obtain an eye diagram, or eye pattern on it. Right now I just have a function generator hooked up to it. There's a sync output on the function generator as well as the output for the signal. Do I just hook up the sync output to the external trigger input of the scope? Is there anything else I have to do? Thanks for your help.

That's pretty much it. You're using a digital o'scope with a "persistence" mode, right?

 

[roz77]

What exactly do you mean by persistence mode? I know I can turn on infinite persistence, but I'm not sure 1) what this means and 2) if it's the same things as persistence mode.

 

[berkeman]

What exactly do you mean by persistence mode? I know I can turn on infinite persistence, but I'm not sure 1) what this means and 2) if it's the same things as persistence mode.

Yes, you would generally use "infinite persistence" mode to form the eye diagram. You turn it on, and the display accumulates all of the trace data. If you have a stable waveform, there will be very little movement on the display, and the "eye" will be nice an open. If there is a lot of jitter in the display from whatever source, that will start to close the eye over time.

http://en.wikipedia.org/wiki/Eye_diagram

.

 

[roz77]

Here's the problem. I have a function generator that's outputting a sine wave, and that shows up fine on the scope. I hook up the sync output of the function generator to the external trigger of the scope, then hit the button for infinite persistence, and nothing seems to happen.

 

[berkeman]

Here's the problem. I have a function generator that's outputting a sine wave, and that shows up fine on the scope. I hook up the sync output of the function generator to the external trigger of the scope, then hit the button for infinite persistence, and nothing seems to happen.

Probably because it is a nice clean sine wave, that always re-traces the same path on the screen.

Try making an RC circuit, and vary the R or C values. Or vary the amplitude output on the sig gen. You should see "ghost" traces that are saved by the persistence.

 

[jsgruszynski]

http://forums.tm.agilent.com/community/viewtopic.php?f=19&t=2935

 

[sophiecentaur]

To get a good eye diagram, you need a long(ish) sequence of pseudo random data to represent a read data signal, with the scope synced to the clock. This is a pretty trivial thing to produce with a short shift register and the appropriate knock-backs.
With a simple, repeated, waveform you can't get an eye pattern.