[time-nuts] Dual Mixer

Bob Camp lists at rtty.us
Wed May 12 00:34:04 UTC 2010


As long as the idea is comparing the oscillators here in the basement, there are a lot of things you can do. 

As soon as the "bragging rights" bug cuts in, knowing that the numbers are correct becomes an issue. With many noise measurement issues the assumption that "better is correct" can serve pretty well. The biggest exception to that is messing up the math. Math errors can go both ways. 


On May 11, 2010, at 8:23 PM, John Miles wrote:

> My thinking is that anything that provides a close visual match with ADEV
> traces from professional-grade instrumentation is OK for the most part,
> assuming enough different sources and slopes are observed to make that
> "close visual match" judgment meaningful.  At the very least, homebrew DMTD
> and PLL instrumentation is good for the sort of relative comparisons that
> are most often called for, without paying extraordinary attention to
> filtering.  ("Which of these oscillators is better?  Is there room for
> optimization of these disciplining parameters?  Did that last tweak make
> things better or worse?")
> It's interesting to contemplate the transfer-function math needed to product
> an 'exact' ADEV response, but we must remember that we are, after all,
> measuring noise, and ADEV is by no means the last word in characterization
> of all noise types/slopes.  No matter how much effort goes into the
> measurement, we're dealing with input data that is unrepeatable to one
> extent or another, and most professional tools still yield estimates with
> substantial error bars.
> For those who need maximum assurance, Symmetricom and others will cheerfully
> sell you a box that is NIST-traceable, implements known-correct transfer
> functions, was hand-soldered by unmarried women over the age of 50, and
> costs more than your car.
> For the rest of us, though, any instrument that is accurate enough to be
> limited by the inherent repeatability of the measurement being done is
> probably OK, and not worth getting into public (or private) pissing matches
> over.
> I'm spending this week comparing a few different ADEV-measurement
> approaches, including Warren's.  Each of them has different advantages and
> constraints.  Tentative results so far suggest that the PLL error-voltage
> measurement technique can indeed give good results down to at least the
> 1E-13/tau neighborhood, where "good" means "more or less indistinguishable
> from what you get from a TSC 51xxA."
> As Warren suggests, it does appear to be important to oversample; e.g., by
> sampling at 480 Hz you can obtain very good ADEV conformance at t=0.01
> seconds or longer.  Getting AC line noise out of the picture can be a
> challenge at these timescales, obviously.
> I'll contact you (Bert) offline to get one of your boards up here.  I think
> I can help you out with at least one option for software support, and in any
> event it'd be great to run some of the same tests on your project.
> -- john, KE5FX
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>> Behalf Of EWKehren at aol.com
>> Sent: Tuesday, May 11, 2010 4:31 PM
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] Dual Mixer
>> Allow me to apologize to start this again. Bert
>> In a message dated 5/11/2010 7:25:39 P.M. Eastern Daylight Time,
>> warrensjmail-one at yahoo.com writes:
>> (My  apologies to all, this is a game Bruce and I play every time I bring
>> up
>> my  simple tester.)
>> Bruce wrote:
>>> "So you now actually  integrate/average the frequency over the sampling
>>> interval (Tau) after  rejecting the need to do this for months?"
>> Yes, I integrate/average  just the same as I have always done it from day
>> one.
>> Did you finally  understand how the integration works using most any ADC?
>> Hint: it's done  with oversampling the tau zero time.
>> (and a LP filter set to a value above  the tau zero but below the
>> oversamping
>> rate)
>> The VERY SAME thing I have  been trying to tell you from day one,
>> something
>> that you have chosen to  ignore.
>> The very original Block diagram that I posted shows it, if you need  more
>> information.
>> ws
>> *******************
>> Warren
>> So  you now actually integrate/average the frequency over the sampling
>> interval  (Tau) after rejecting the need to do this for  months?
>> Bruce
>> *****************
>> WarrenS wrote:
>>> Bruce
>>> Before we go around again and discuses what my simple  tester can and
>>> can not do and why,
>>> It would be helpful if you  would take the time to better understand
>>> how it works and why it works  the way I have done it.
>>> You really should try one yourself if you  can't see why it works.
>>> You are going to be surprised and embarrassed  at how good it works.
>>> Why you're at it, try the "swing test" with  anything you have. Let me
>>> know how that goes.
>>> I'm not  saying that may tester will match someone's Latest ever
>>> changing NEW  idea of what the "correct AVAR" should be,
>>> After all it just Logs  correct, integrated, Freq difference data of
>>> ANY noise type
>>> and does it without adding any dead time or aliasing all by using
>>> pretty much using ANY ADC capability of over sampling at the tau Zero
>>> rate.
>>> If one then uses the data log with something like the classic  Stable
>>> 32 S/W or Ulrich's Plotter,
>>> it gives is the exact same  results as other methods costing much much
>>> more, over the whole tau  range.
>>> This is limited only be its reference oscillator (Same way that  all
>>> others are limited of course, Doesn't get much better than  that).
>>> If that is not good enough for you, them you need to discuss  the
>>> results with Symmetricon and others that give the same answer as  mine,
>>> not me.
>>> If for some reason you want to set one  up wrong so that it matches the
>>> results of some other special  instrument, I'd be glad to tell you how
>>> to have it add back in the  dead time or aliasing artifact problems or
>>> whatever else you would  like it to do wrong, that it presently does
>>> correctly.
>>> ws
>>> ******************
>>> Bruce wrote
>>> As  long as one is aware that your method (as implemented by
>> you) doesn't
>>> actually measure Allan variance, it may be useful for comparing the
>>> relative stability some sources for small Tau (unfortunately the range
>>> of Tau for which the method may produce useful results depends on the
>>> phase noise characteristics of the sources being compared).
>>> To measure  AVAR the technique has to have the same response to
>> all phase
>>> noise  spectral components as does AVAR.
>>> Since you do not integrate/average  the frequency measures the phase
>>> noise response of the method is not  identical to that used in
>>> calculating AVAR.
>>> This technique  probably works best when white phase noise dominates the
>>> phase noise  spectral region of interest (usually for small Tau).
>>> For those  who can follow the theory, the following paper shows how the
>>> above  method is affected by aliasing etc:
>>> http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
>>> The paper also shows how the required integration (needed to actually
>>> measure AVAR) can be approximated from the discrete sample sequence.
>>> Alternatively one could avoid the numerical integration by
>> replacing  the
>>> ADC with a zero deadtime (ie not a dual slope converter. A  multislope
>>> algorithm like that used in the 34401A (but not the 3458A)  should work
>>> as the signal is integrated continuously) integrating ADC.  One
>>> possibility is to use a VFC as NIST did when they used this  technique
>>> some decades ago.
>>> Of course, the classical  DMTD setup undersamples the phase noise
>>> spectrum and thus may suffer  from aliasing artifacts.
>>> Such aliasing artifacts have no significant  effect when the phase noise
>>> spectrum is flat.
>>> Bruce
>>> *********************
>>> WarrenS wrote:
>>>> For the Really cheap time nuts,
>>>> It sounds like Bert  Kehren has done a great Job building a Dual Mixer
>>>> tester.
>>>> There are other simpler, less standard ways to get good  data for Allan
>>>> Variance and small frequency  differences.
>>>> My VERY simple $10.00 analog tight PLL Tester BB  (Previously posted)
>>>> pretty much accomplishes the same goals as  his,
>>>> and it can do 1e-13 in a second, and 1e-11 in 10ms   (limited of course
>>>> by the single reference Oscillator  used)
>>>> A simple test that most can do at home, and  still challenges the best
>>>> high end testers out there is Tom's the  swinging Oscillator test.
>>>> http://www.leapsecond.com/pages/10811-g/
>>>> (The results from my PLL  tester is attached)
>>>> ws
>>>> ******************
>>>> ----- Original Message ----- From: <EWKehren  at aol.com>
>>>> To: <time-nuts at febo.com>
>>>> Sent:  Tuesday, May 11, 2010 7:02 AM
>>>> Subject: [time-nuts] Dual  Mixer
>>>>> The Dual Mixer project is  nearing completion.
>>>>> Let me refresh every ones memory as to my  goals.
>>>>> a)  Total cost less than $ 200
>>>>> b)  1 E-13 with a one second offset
>>>>> c)  use parts  attainable by every one
>>>>> d)  easy to assemble only a few  surface mount parts
>>>>> e)  a five channel counter that  yields 1 E 15 resolution and
>>>>> interfaces
>>>>> directly to a PC via  RS232 or USB
>>>>> f)   A  counter that also gives you instant frequency  difference at
>> the
>>>>> sample rate, not only Allan Variance
>>>>> g)  Modular so one can use only the Dual Mixer
>>>>> h)   Modular so one can use multiple units to do simultaneous
>>>>> comparison of
>>>>> more than two oscillators.
>>>>> i)   Isolation between D/M and counter so that the counter can  be
>>>>> powered
>>>>> by the PC USB  port
>>>>> I am happy to report that all goals have  been accomplished, attached
>>>>> is a
>>>>> picture of  the D/M, limitation of the file size does not allow me to
>>>>> attach
>>>>> an  actual board picture, but if you contact me  direct I will send
>>>>> you one,
>>>>> the  final  board is actually nicer since the first layout had to
>>>>> accommodate
>>>>> several   variances.
>>>>> The D/M part leans heavy on the  original NIST unit with a few
>>>>> substitutions
>>>>> and recommendations from Bob Camp. Also beside Opto Couplers
>>>>> SN65LVDS1's
>>>>> have  been included for those that want to  use other
>> counting methods.
>>>>> Selection of  filter  capacitors allow the use at other offset
>>>>> frequencies such  as
>>>>> 10 and 100  Hz. The D/M fits in a standard 74 X 111 X  20 mm Euro case
>>>>> and
>>>>> the counter can be   stacked below or next to it using the Opto
>>>>> Isolators  as
>>>>> the inter connect. The  SYPD-1's fit right on the  board but
>>>>> connections are
>>>>> included to use the  HP 10514  A. As a matter of fact removing the HP
>>>>> mixer
>>>>> board from its housing   fits it nicely on the  board and every thing
>>>>> is still
>>>>> inside the  housing.
>>>>> The counter will handle 1 an 10 Hz offset with a 1 E  14 resolution
>>>>> at 10
>>>>> Hz. Thanks to Richard Mc  Corkle we have great drawings and code,
>>>>> available  to
>>>>> every one.
>>>>> Code, drawings, list of material  and PC board layouts and its  file,
>>>>> will
>>>>> be available to every one once the project is completed.
>>>>> I  need help in the following areas
>>>>> a)  help me create a  nice set of drawings that are computer generated
>>>>> something I  am not able to do
>>>>> b) create the computer program that takes  the output of the counter
>>>>> board
>>>>> and allows  Allan Variance plots, frequency difference and dual
>>>>> temperature
>>>>> readings and plots using RS232 and  USB.
>>>>> c) an independent test by a third party.
>>>>> As I said previously, I am not getting in the business of  supplying
>>>>> parts
>>>>> but will work with people that  will help achieve the three points
>>>>> listed
>>>>> above.  Presently I have boards on order and will have  two
>>>>> uncommitted board
>>>>> sets and  probably  also component kits.
>>>>> Please contact me  directly.
>>>>> Again thank you Corby Dawson, Richard Mc Corkle and  Bob Camp.
>>>>> Bert Kehren    Miami
>> _______________________________________________
>> time-nuts  mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the  instructions there.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

More information about the time-nuts mailing list