[time-nuts] Dual Mixer

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue May 11 22:46:43 UTC 2010


So you now actually integrate/average the frequency over the sampling 
interval (Tau) after rejecting the need to do this for months?


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 uisng 
> 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, Doen'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
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