# 1pSec Jitter

Discussion in 'Embedded' started by Joe G \(Home\), Jan 14, 2006.

1. ### Joe G \(Home\)Guest

Hi All,

I have a FPGA system which requres better than 1pSec jitter.

When I ask the Xtal MFG they advise there are 2 methods of measuring Jitter
Peak to Peak and an averaging method

Measuring the same Xtal the result can be significatly differment values
between the 2 methods.

Does any one have any information on the 2 methods (or more) how to measure
jitter.

What would FPGA input expect?

Regards
JG

Joe G \(Home\), Jan 14, 2006

2. ### Gob StopperGuest

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
http://www.agilent.com/find/jitter

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

GS

Gob Stopper, Jan 14, 2006

3. ### Joe G \(Home\)Guest

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

Joe G \(Home\), Jan 14, 2006
4. ### John LarkinGuest

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

John Larkin, Jan 14, 2006
5. ### Chuck F.Guest

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.

--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the

Chuck F., Jan 14, 2006

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?

7. ### bill.slomanGuest

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

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, Jan 15, 2006
8. ### JoergGuest

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

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

Regards, Joerg

http://www.analogconsultants.com

Joerg, Jan 15, 2006
9. ### PeteSGuest

Do you mean the application specifies an oscillator with <1pSec jitter?
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
http://www2.tek.com/cmswpt/tifinder.lotr?cn=oscilloscopes&lc=EN
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

PeteS, Jan 15, 2006
10. ### John LarkinGuest

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

John

John Larkin, Jan 16, 2006
11. ### Spehro PefhanyGuest

bits ~= dB/6

Best regards,
Spehro Pefhany

Spehro Pefhany, Jan 16, 2006
12. ### John LarkinGuest

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

John

John Larkin, Jan 16, 2006
13. ### Spehro PefhanyGuest

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

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/intens.html

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

Spehro Pefhany, Jan 16, 2006
14. ### Rich GriseGuest

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

Rich Grise, Jan 16, 2006
15. ### Glen WalpertGuest

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 ...

Glen Walpert, Jan 16, 2006
16. ### Roberto WaltmanGuest

***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:
http://almaz.com/nobel/physics/cherenkov.html
http://www.gae.ucm.es/~emma/tesina/node4.html

Roberto Waltman

[ return address is invalid. ]

Roberto Waltman, Jan 16, 2006
17. ### KeithGuest

Supersonic shock waves. What's next?!

Keith, Jan 17, 2006
18. ### John LarkinGuest

Clips the negative swings, for sure.

John

John Larkin, Jan 17, 2006
19. ### Stef MientkiGuest

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

Stef Mientki, Jan 17, 2006
20. ### John DevereuxGuest

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

John Devereux, Jan 17, 2006