[time-nuts] Phase, One edge or two? (was Digital mixing with a D Flip Flop)

Bob Camp kb8tq at n1k.org
Wed Oct 22 20:42:11 EDT 2014


Hi

The more you “curve fit” or “average” the more you are filtering the data. Filtering does indeed impact the ADEV both at short tau’s and longer tau’s. You need to be very careful if you filter or you will mess up the data.

Bob

> On Oct 22, 2014, at 7:42 PM, Didier Juges <shalimr9 at gmail.com> wrote:
> 
> Even more effective would be to sample the entire 10MHz waveform instead of just the zero crossing. By doing a best fit of the entire waveform, you should be able to estimate the zero crossing with much greater precision because now the noise is averaged over the entire waveform instead of a single point at the zero crossing.
> 
> I wish my signal processing were better than they are and that I had some time to evaluate that.
> 
> Didier KO4BB
> 
> 
> On October 22, 2014 1:09:11 PM CDT, WarrenS via time-nuts <time-nuts at febo.com> wrote:
>> 
>> 
>> The recent  discussions about the simple digital mixer got me thinking
>> about
>> the performance vs. complexity trade offs when measuring accurate, high
>> resolution, phase drift differences between two oscillators.
>> It would seem to me, that using both the positive and negative slope
>> edges
>> of the high freq sinewave signal is a better way to go.
>> Is using just one edge, acceptable for a 'state of the art' Phase drift
>> measurements?
>> 
>> I am not suggesting  the KISS approach is the wrong solution for Simon.
>> I am questioning if the paper posted, is the best way for CERN to make
>> a
>> state of the art femtosecond DDMDT?
>> 
>> Here is an extreme example of throwing away useful data for the sake of
>> simplicity:
>> When measuring phase drift of a 10 MHz osc using just a 1PPS signal,
>> 19,999,999 other possible data points are being discarded.
>> Using all possible data points could decrease the noise floor
>> considerably.
>> (by ~5,000 to 1)
>> 
>> ws
>> 
>>>> 
>> -------------------------------------------------------------------------------
>>>> Tom Posted
>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip
>>>> Hi Simon,
>>>> 
>>>> Some additional info. I first heard about the D-FF method of
>> frequency 
>>>> comparison in the late 90's (from Rick Hambly, I think) on the old
>> gps 
>>>> mailing list. It sounded really interesting. Since then, the subject
>> has 
>>>> turned up every few years on this list. But each time, the topic
>> seems to 
>>>> go away quietly with little or no data, plots or explanation. In 
>>>> addition, none of the commercial products I've taken apart appear to
>> use 
>>>> this approach. Hmm. So that begs the question -- what's really going
>> on, 
>>>> and why.
>>>> 
>>>> I'm enjoying this thread because you've shown both technical
>> competence 
>>>> and optimistic persistence. Perhaps once and for all, with your
>> efforts, 
>>>> we can settle this matter. You will either find a working
>> combination 
>>>> with excellent performance, or you will uncover enough uncontrolled 
>>>> variables that you never want to try it again. Either way, we all
>> learn a 
>>>> lot. Keep the photos, data, and plots coming.
>>>> 
>>>> Thanks,
>>>> /tvb
>>>> ------------------------------------------------------------------
>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip
>> Flop
>>> 
>>>> Bruce posted 
>>>> 
>> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf
>>>> 
>>>> among other things illustrates a modified approach to the offset 
>>>> generator by replacing the intermediate phase locked VCXO with a
>> bandpass 
>>>> filter.
>>>> 
>>>> ------------------------------------------------------------------
>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip
>> Flop
>>> Simon posted   www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ...
>>> The idea is based on the following article which describes creating a
>>> digital DMTD with an FPGA for clocks @ 125mhz:  > 
>> 
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> 
> -- 
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