measuring jitter with a Rigol scope

We got some pretty big orders and we figured that we'd need about 1000 hours of production test time to get the stuff out. So I am designing a couple of new test sets and the kids will grind out a pile of Python, to replace classic manual test procedures with more automated stuff.

A couple of the product tests need measurement of time jitter. We had been using old Tek 11801C sampling scopes to measure jitter, but they are gigantic and slow. So we got to wondering if a cheap Rigol could measure jitter.

The DS1204B is a 4-channel 200 MHz, 2Gs/s scope for $1395. It doesn't do any jitter measurements. It will measure the time between rising edges of two channels, so we'll work with that.

It's always hard to figure out what scopes are doing internally; that's not often discussed in manuals. Experimentally, the DELAY A->B measurement seems to work well in both free-run and single-shot mode. In ss mode, one can measure a delay, say 5 ns from rising A to rising B, and it's right, and one can change that to 6 ns, fire one more acquisition, and that will be right too.

So we wrote some Python to do a bunch of DELAY A->B measurements and compute the RMS jitter. We used one of our DDGs as the source, which has about 6 ps RMS jitter between outputs. The Rigol reported different jitters depending on whether we were measuring continuous or single-shot. Single-shot takes longer to run, about 100 ms to fire each shot and read the delta-T. Continuous mode lets us snoop the time delta in about 10 ms (this is Ethernet) but reports more jitter, so I'm not sure what it really means.

Taking measurements in single-shot mode and doing the math seems to have a jitter measurement floor a bit below 30 ps RMS, which is amazing for a $1600 scope with a 2 GHz ADC clock, a 500 ps sampling interval. Shannon was one smart dude.

We generated some test signals with known jitter. Once the input jitter gets above 500 ps RMS or so, it doesn't seem to matter whether we use continuous or s/s mode for the time-interval measurement.

For lower jitter measurements, 100 acquisitions in s/s mode might be enough, and that will take about 10 seconds, which is tolerable.

Scopes that internally measure jitter are fabulously expensive, which is silly because it's just a little math.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin
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Apparently the manufacturers know that they can ask for a scope with certain capabilities.

More and more there is standard hardware used in both cheap simple and more elaborate devices, and there is a number of expensive license keys to unlock the software capabilties.

Reply to
Rob

And they are competing with people who can suck waveforms out and do their own math.

Someone could start a web site that posts free libraries of Python or whatever, to do the math that the scope makers charge big bux for. Or someone could write a program that does cool scope math from extracted waveforms of popular scopes. Maybe somebody has already done that.

We have a LeCroy scope that will do a software PLL on an acquired NRZ data waveform and make eye diagrams with no trigger. That's pretty cool.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

The picoscope 5000 costs 1100 USD and have 3ps RMS jitter and amazing 13000 waveforms per second

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Cheers

Klaus

Reply to
Klaus Kragelund

That's kind of suspicious.

Even more suspicious. Are you seeing some sort of weird.... aliasing product being damped down by single shot mode?

Customers that want jitter measurements are closer to being cash cows, would be my guess.

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Les Cargill
Reply to
Les Cargill

And they have to watch out for people who reverse-engineer the thing and publish license key generators...

In the low-end range there are already interfaces that just sample the input data and do all further processing in a PC or tablet, and of course open-source variants will appear or already have appeared.

There are scopes that can demodulate often used serial protocols and display the decoded frames in human-readable form.

And of course tricks like doing FFT on the sampled signal and display the result in frequency domain, a simple spectrum analyzer.

Reply to
Rob

I presume you're aware of the PLL problems in some Rigol scopes.

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Reply to
Tom Gardner

Stupid idea: Assume both signals are step functions (or equivalent for measurement purposes). Feed to a fast ECL XOR gate (or equivalent..more later). You get a pulse with width equal to the time difference of the inputs ie, the jitter. Non-ideal characteristics of the XOR will easily make the width larger, maybe (hopefully) by a fixed amount, and if not, can be experimentally determined. What is nice,is a VOM (or equivalent) could then make the measurement directly.

Now,the ECL XOR gate (or equivalent) must be as symmetrical as possible, meaning the actual circuit AND layout must be as symmetrical as possible. And a number of ECL chips are NOT symmetrical. In that case, you could try two (selected) same-type connected A,B on one, and B,A on the other to an OR scheme...

You may be forced to DIY with discretes using a front-end very much like an op-amp..

Reply to
Robert Baer

Wow, I didn't expect a scope manufacturer not having control of their clock

It's a nice read, he really spend some time digging into that PLL problem

Cheers

Klaus

Reply to
klaus.kragelund

There are a number of semi-analog techniques for making picosecond time-interval measurements, similar to what you describe. One is a pulse-width stretcher: charge a cap at current I during the time interval to be measured, and then discharge it at I/100, and measure the discharge time with a counter.

Another way is to start a linear ramp during the time to measure, and then stop it and measure the voltage.

We make a few time-interval counters that "timestamp" a number of edges, to maybe 48 bit resolution, and subtract the timestamps to get the time differences. The LSBs of the stamp are generated by a technique similar to what the Rigol is doing: bandlimit the edges, digitize (gotta have multiple samples on the rising edge) and do the math to compute the time-of-rise to sub-clock resolution.

People are doing picosecond time interval measurements in custom ICs and even inside FPGAs.

I'm guessing that a $350 Rigol scope can do time interval and jitter measurement well below 100 ps accuracy.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

That could be hacked into a pretty good time interval counter, with statistics. Looks like single-shot resolution would be about 130 fs, but the jitter will dominate.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Can you not use the Math function of the Rigol in A-B mode, and then use the pulse width measurement on that ? I dont know whether the Math function works in the hardware front-end, or in the captured signal, but it might be worth a look.

Reply to
Adrian Jansen

Computing jitter takes a small program, with setup, a FOR loop, and a final computation. Inside the FOR loop one might want to fire a single-shot acquisition. I don't think scope math functions can do that, but maybe some can.

One of the guys has written some Python libraries to talk to various scopes, and can do stuff like jitter on top of that. It's just a tad slow; the Rigol needs about 100 ms per shot to report the delta-T.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Do you actually need to look at multiple single-shot triggers, or could you capture a single record with several cycles of the DUT signal and look at the zero crossing times? That's the usual difference between a scope's "fr ee" firmware statistics calculations and the optional jitter-analysis packa ges on the market. (That, and anything involving clock recovery.)

The TDSJIT1 software that came with my TDS694C does this sort of thing ( ht tp://

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) but I've never needed to mess with i t since I'm usually more interested in the frequency domain. It might be f un to write a scope-agnostic version of that.

-- john, KE5FX

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John Miles, KE5FX

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