1pSec Jitter

What do you mean by 1 psec jitter? Do you mean Rj, Dj, Tj? Are you measuring Time Interval error, cycle-to-cycle, or something else?

I would recommend doing some reading. You can start at

Scroll down to "Key Library Information" and download (and read) all of the White Papers and Application notes.

One of the first things you'll find is that it's probably impossible to measure 1psec of jitter. As with any other measurement, there is the concept of the smallest measureable unit. In the jitter world, this is the Jitter Measurement Floor, and typical values are 80 fsec to 2 psec.

I personally can't imagine anything going on in an FPGA that would be affected by 1 psec of jitter. More info would be advisable.

GS

Sorry,

The Osc driving the FPGA calls for 1pSec jitter.

I am just trying to understand the 2 methods and how they relate to FPGA requirements.

JG

The only meaningful way to measure jitter is RMS. And even then, you have to specify the time interval over which it's to be measured. Peak-peak is poorly defined, but figure it's roughly 5 times RMS.

In the telecom biz, any time variances measured within 0.1 second or less is "jitter", and above that it's "wander."

A normal sampling scope measures one or at most a few periods of the input signal, which is "short-term" or "single period" jitter.

1 ps jitter is hard to measure. There are crystal oscillators that can do less than 1 ps. By the time you pass it through an FPGA, expect the result to be 10's of ps, maybe more.

Why do you need 1 ps jitter?

John

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Circa 1970 I built a system for transmitting voice band, which was basically pulse duration modulated. The start was controlled by a separate clock. IIRC signal/noise measurements on the results indicated less that 1psec short term jitter. My memory seems to specify a pulse width in the range 0.25 to 1.25 uSec at about 12 Khz repetition rate, and a s/n ratio of better than 90 db.

We were only interested in the noise level in the telephone audio band, roughly 300 hz to 3600 hz. We traded off repetition rate to simplify (and cheapen) equalization and aliasing filters, and met all signal quality objectives.

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The oscillator calls for? I thought this was a requirement from the FPGA. Do you mean that the FPGA calls for an oscillator jitter not to exceed 1 ps? What is the frequency? Is the FPGA running a PLL based on the oscillator? Basically, what fundamentally is setting the jitter requirement and why?

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Thad

I'd put my money on the "idiot manager" option. Idiot systems engineers also exist - "we've got this circuit which introduces 99psec of jitter, and the error budget is 100psec, so the clock can't introduce more than

1psec of additional jitter".

Then there is idiot sales/marketing person who tells you that he/she can sell hundreds of units if you can just break the second law of thermodynamics.

--
Bill Sloman, Nijmegen

Hello Joe,

Pretty tough requirement. Do you want to build some kind of Doppler?

Jitter is usually looked at via an eye diagram on a blazingly fast scope. The scope manufacturers have app notes about that. But since your xtal mfg told you about two methods why not ask them?

Depends what that FPGA is and what you want to do with it.

Regards, Joerg

Do you mean the application specifies an oscillator with

Tight requirement. What is the specific application?

There are more than two methods for measuring jitter (depending on just what it is you are trying to measure). Frequency domain measurements are commonly used for Dj prediction (although different test equip. mfrs use different techniques). Time domain for cycle to cycle and random jitter. Long term drift (just what long term is depends on the system) may or may not be an issue - that (just like all the other jitter sources) is system dependent.

Note that different mfrs equipment will give you different results - even a different set of probes will vary the measurement, particularly at the speed you seem to need.

What FPGA? What application?

When testing high speed links I designed the physical layer for, (5Gb/s) we used Tektronix equipment. Based on what I saw, they had some of the best equipment. Look here

for some app notes.

In our application, we had to worry more about cycle to cycle and short term peak / rms jitter, but without knowing more I can't say what your app would consider an issue.

Cheers

PeteS

Probably the same guy that was upgrading to a 32-bit CPU and needed a

32-bit ADC to match.

John

bits ~= dB/6

Best regards, Spehro Pefhany

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So 32 bits is 192 dB. Isn't that just about the ratio of 1 atm to the threshold of hearing?

John

This web page says < 1E9, so >180dB.

by their info the threshold of pain is 0.003 of 1 atm.

I guess things would get nonlinear when you start to approach one bar even if it didn't rupture your eardrums.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
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Have you ever seen that stock footage of a nuke? There's a very visible shock wave of some kind, that's obviously traveling faster than Mach 1.

Cheers! Rich

Yep, 20 uPa RMS to 1 ATM or 101 kPa RMS is 0 to 194 dB SPL; from the lowest threshold of hearing for sensitive youngsters to sound levels on the launchpad during a large rocket launch, way into the nonlinear region. Now if we could only find a microphone and preamp to cover the entire range ...

***Warning***Off-topic***Warning

If you are talking about the "stock footage of a nuke" that I think you are talking about, I believe that spherical front is a visualization of the Cherenkov Effect.

See:

Roberto Waltman

[ Please reply to the group, ] [ return address is invalid. ]

Supersonic shock waves. What's next?!

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  Keith

Clips the negative swings, for sure.

John

Technici should be open minded: "nothing is impossible untill it's proven" at my work we've a 128 channel 32 (or 34) bit AD converter !! Stef

I was getting ~27 bits out of a ADS1252 (after averaging). That is only a $6 part.

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John Devereux