[SI-LIST] : RE: ADC jitter specs (formerly Diff clocks length matching)

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From: Daniel, Erik S. ([email protected])
Date: Thu Apr 19 2001 - 06:57:52 PDT


I still maintain that phase noise and "jitter" are directly related as long
as "jitter" is properly defined and the integrations are performed properly.
I do not agree phase noise test sets are being obsoleted by digitizing
scopes -- you yourself indicated the "jitter" of digitizing scopes was
limited to ~1 ps, and the ADC and radar community have much tighter
requirements. In addition, it is difficult to obtain a complete spectrum of
phase noise with a digitizing scope. As others have pointed out in an
earlier thread, a spectrum analyzer is a good compromize in some cases.

I agree that we should probably desist and take the discussion off line, but

> -->> Are you SURE that the timing jitter is not limiting you?
> 16 bits minus
> 13 bits = 3 bits which is a factor of 8. 8 times 10 fs = 800
> fs or about 1
> ps

can we at least agree publicly that 8 * 10 fs = 80 fs << 1 ps ? :-) To
answer your question, yes, we are fairly confident we know the sources of
limiting noise and jitter is not a major contributor.

                                        - Erik

Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: [email protected]
Rochester, MN 55905 Web: www.mayo.edu/sppdg/

> -----Original Message-----
> From: Larry Miller [mailto:[email protected]]
> Sent: Thursday, April 19, 2001 8:34 AM
> To: 'Daniel, Erik S.'; Larry Miller
> Cc: [email protected]
> Subject: RE: ADC jitter specs (formerly Diff clocks length matching)
> Dr Daniel,
> Replies below -->>
> -----Original Message-----
> From: Daniel, Erik S. [mailto:[email protected]]
> Sent: Wednesday, April 18, 2001 12:06 PM
> To: [email protected]
> Cc: [email protected]; Daniel, Erik S.
> Subject: Re: ADC jitter specs (formerly Diff clocks length matching)
> Larry-
> Part of my confusion may have been that you were apparently mixing the
> separate 10 bit 10 MHz case and the 7.5 bit 1.5 GHz case that
> Tom mentioned.
> Also, I do not understand why you bring the ADC amplitude
> into it -- as is
> evident by the equations, the LSB-limited jitter spec depends
> only on the
> sampled frequency and the effective number of bits, as the
> LSB is defined
> relative to the ADC full scale. In any event, it sounds like
> we agree on
> the equations.
> -->> I am sure we agree on the equations. I think we probably
> do not agree
> on the attainability of some of the numbers you get by just
> assigning a
> sampling frequency and number of bits. And the ADC full scale
> is important
> in a mixed digital-analog signal system because of the fixed
> sizes of other
> nearby noises in the system. I'm sure that Maxim would be
> happier with a +/-
> 1-volt input range.
> A few years ago in my pre-networking days I worked on CT Scanner data
> acquisition systems (rather the other corner of the playing
> field from what
> we have been talking about, 24-bit dynamic range at low audio
> frequencies)
> and the same equations held.
> Your approach is mathematically correct, but it smacks a bit
> of legislating
> reality-- to me, anyway. Saying "...we require 10 fs of
> jitter...." and
> actually getting it are not the same thing.
> ---------------
> Regarding the followng comment:
> > [Larry Miller] -->> Yes, I get that number. And you are
> talking p-p jitter
> with a
> > noise bandwidth of at least 0.75 GHz.
> --->> Sorry, gotta break this apart for discussion:
> I'm not sure exactly what you mean by "p-p jitter with a
> noise bandwidth of
> at least 0.75 GHz".
> By noise bandwidth I mean the upper range of frequencies
> where the system
> has a (possible) noise sensitivity (gain, if you like) of
> more than 1. This
> is an old thing from George Philbrick and Analog Devices
> op-amp methodology.
> (Anyone else around here remember "The Lightning Empiricist?") It is
> directly related to the bandwidth of what you would use to
> measure such a
> system (like scope bandwidth) and not surprisingly is exactly
> equivalent to
> what you say below.
> ----------------
> Jitter specs seems to be somewhat fleeting. I'm more
> comfortable with jitter defined as an integration of phase
> noise spectral
> density over some frequency range of carrier offsets -- the particular
> choice of offset frequency range is very much application
> specific and is
> somewhat independent of the sampled frequency.
> -->> You are in good company in this. Some oscillator
> manufacturers, such as
> Valpey-Fisher, use this method (with, I think, Aeroflex equipment).
> Unfortunately, the answers we got with this method never
> matched up with
> what we saw in our real systems in our lab (at a major Canadian Telco
> manufacturer). Typically, the results from V-F were 1/10 of
> what we saw with
> the best oscilloscopes we had and also similarly disagreed
> with our Time
> Interval Analyzer (TIA) readings. Since V-F made good stuff
> and we wanted to
> buy it, we basically had to make our own measurements in our
> own (jitterly)
> way to get correlation with other manufacturers who do use jitter
> specifications (which is most of them, actually).
> The telecommunications industry seems to have settled on
> jitter and eye
> openings as a way of evaluating high speed systems.
> Phase noise methods and equipment were popular in evaluating
> radar local
> oscillators (especially ones like YIG oscillators manufactured by
> Watkins-Johnson, for example, and used in frequency-agile microwave
> receivers for radar direction finders like those in the P-3 recently
> acquired by the Chinese government).
> I do consider phase noise testing an out-of-date measurement
> technique for
> digital data transmission systems. I myself am out-of-date on analog
> converters.
> ----------------
> Typically, the high end of
> the integration would be the clock frequency (relating to
> deviations in
> timing from one clock cycle to the very next one), and the
> low end would be
> dictated by the longest coherent time cycle required by the
> system (relating
> to deviations in timing from one clock cycle to another one
> very much later
> -- perhaps 1 millisecond => 1 kHz offset frequency for our
> applications of
> interest). If this integration over the desired frequency
> range yields a
> "jitter" that is larger than ~ 2^(-N)/(2 Pi f), then this
> will appear as
> broadband noise which will limit an ADC's effective number of
> bits to less
> than N for sampled frequencies f for that application.
> -->> "Noise bandwidth of 0.75 GHz"....
> -->> "high end of the integration would be the clock frequency"...
> -->> The clock frequency is 0.75 GHz
> -->> Works for me, just two ways to get the (hopefully) same
> answer, just
> depends on how you hold your mouth when you say it.
> Regarding fs jitter clock sources and our results to date on the 16
> effective bit, 350 MHz center frequency ADC system, I can't
> tell you too
> much as many of the details are proprietary to one of our
> collaborators, but
> I can tell you they have developed a custom clock source that
> has low enough
> jitter for the target system (again, on the order of 10 fs),
> and system
> demonstrations to date have shown about 13 effective bits at a 350 MHz
> center frequency, believed not to be limited by timing jitter.
> Commercially, the best low phase noise sources seem to be
> made by Poseiden
> (www.psi.com.au). The specs on their web page are given in
> terms of phase
> noise spectral density instead of jitter, as the definition
> of "jitter" is
> somewhat loose as I mentioned above, but integrated jitters
> in the 10s of fs
> are possible, again depending on your required integration band.
> -->> Are you SURE that the timing jitter is not limiting you?
> 16 bits minus
> 13 bits = 3 bits which is a factor of 8. 8 times 10 fs = 800
> fs or about 1
> ps which is about as good a clock as I ever saw (and a very
> expensive one at
> that). You know I had to say it...(blush)
> Regarding measurements of "jitter" of this order, I agree one
> would never
> try to measure it with a scope. In my opinion, the best way
> to measure
> phase noise is with a (aptly named) phase noise test set,
> which measures
> phase noise spectral density versus frequency offset from the
> carrier for a
> clock source input.
> -->> My problem with measuring this way is this:
> It is not directly related to what you are interested in for a digital
> system. Integration always gives you a 1/f roll-off and this
> makes it very
> hard to see its effects.
> The clocked data input to a digital device (flip-flop, memory,
> what-have-you) only cares about peak timing excursions. If a
> clock violates
> the input setup requirement (is late) or hold requirement (is
> early) you
> either get an outright error or, what is worse from a problem solving
> viewpoint, metastability. With digital system speeds getting
> so high, the
> margin for error is very small, so you cannot rely on rms
> style measurements
> unless you have a very fault-tolerant system. An oscilloscope
> displays what
> you (and the circuit, to lapse into a teleological vein) are directly
> interested in.
> I have one thing to say in defense of this: it works.
> Multi-GHz systems
> perform with vanishingly small error rates, and this is
> necessary. Gigabit
> and higher data systems with performance worse than 10^-13 to
> 10^-15 error
> rates produce errors often enough to be annoying to humans.
> Jitter numbers can vary a lot, depending upon your sampling
> interval. You
> can get just about any answer you want out of a TIA by
> tweaking the data
> acquisition parameters. However, there are standard jitter testing
> methodologies and test setups that have evolved (like the
> ones in the ANSI
> and IEEE standards) and these give repeatable answers that
> agree very well
> with what you see on high quality test instruments, and, more
> importantly,
> agree with measured performance (error rates) of the
> resulting systems.
> --------------
> Anyway, good luck with your exotic A/D. I think we have tried
> the patience
> of the members of this reflector enough; I see posts
> appearing about how to
> implement "spam filters" on the reflector. I can take a hint....
> Larry Miller

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