Basic question about differential pairs

That should read .25pf cap, the formatting left the point on the previous line.

We scatter ground test points around our fast boards to assist probing. They often come in handy for other things, like kluging.

But these super-low cap sampling and fet probes really work pretty well without a ground, even single-ended. The capacitance of the probe body to the world is enough to keep the body sort of grounded, so waveform fidelity is usually good. Having the probe ungrounded adds some RF noise to the signal, but you can average that back out, at least for periodic signals. We often do useful measurements just holding the probe tip *near* a trace or part pin.

Well, it's displaying the voltage difference betweeen the lines. That's what a differential line receiver will act on.

What's really ugly is if the two transmission lines are loosely coupled and have different prop delays. Then they both look OK as measured single-ended at the receiver, but the actual diff voltage can have plateaus. Nasty for jitter and eye diagrams.

Now the time alignment matters. The differential waveform is no longer a nice trapezoid, so its "risetime" becomes a matter of redefinition. If they both cross their midpoints at about the same time, you can shift your thinking to from risetime slew rate and things become quantifiable. The zen explanation is that the waveform is what it is.

Why not? Just use half of the probe.

John

x-no-archive:

Is that FET probe a DIFFERENTIAL probe or single ended probe. Are you putting the scope probe ground lead on one side of the diff line?

You should be using a DIFFERENTIAL probe or use two single ended probes into two channels of the scope set up to display the differential voltage. i.e V1-V2. In this case it does not really matter what you do with the 2 ground leads of the two probes as long as you connect them to each other with a short connection.

Most so called differential PWB traces are not really operating differentially but rather are operating as two single mode lines to the ground plane that happen to carry opposite singals. Read up on even and odd mode impedance.

Mark

Mark

Yes, it's a diff probe.

Yes, but how do I know what the rise time the scope displays means? The scope is single ended, so the probe is doing a conversion. What does a 50ps rise time mean on a single ended display when you are looking at a diff pair? The signals grow apart in 50ps, but what does it mean for each single ended signal?

I've called them "complementary pairs" before because of the weak coupling but it never caught on.

Thanks.

Thanks but it turns out the probe is more complex than I first thought, there are several heads that can be used and you must use the correctly trimmed lead ins... So before freaking out about rise times I'm gonna see if it was measured properly. Then I can continue freaking out.

Since it is high speed differential the rise / fall times are usually well matched for each driver leg and between legs. It kind of has to be this way. So to the extent you can measure either leg individually they should look the same but with smaller voltage swings. Kind of like the differential outputs on ECL. Also think of current steering pairs.

That each one transitions in 50 ps, but at half the amplitude. And the individual signals may have DC offsets that you can't see.

What matters is that the diff probe shows you what a diff line receiver will see.

John