Bruce, how does the three cornered hat scheme work? If I had two LPRO Rubidium oscillators and a TBOLT GPSDO, and I divided each of them down to 100KHz, then I could compare pairs of them with D latches and record 3 different analogue signals of phase difference. One of the LPRO oscillators is in an oven to remove ambient temperature influence. If I ran them for several weeks and logged the signals every 10 minutes, what could I expect to recover from the data and how would I apply the 3 cornered hat scheme? I ask this question because this is about where my building program is taking me. cheers, Neville Michie
On 10/11/2008, at 1:38 PM, Bruce Griffiths wrote: > Joe > J. L. Trantham wrote: >> I have been enjoying this discussion. >> >> Since the original question was the desire to 'compare' the >> frequency of an >> LPRO to a Z3801, it seems that you could consider that from two >> (at least) >> perspectives. >> >> Before I begin, I confess that I am a novice in this arena and please >> correct me in any area that needs it. >> >> The first perspective is the issue of frequency. That seems to me >> to be the >> issue of the average frequency of the LPRO versus the average >> frequency of >> the Z3801. Assuming that there is no gross difference of the 10 MHz >> signals, a lissajous figure (X-Y display) on a scope with the >> appropriate >> bandwidth amplifiers would be a reasonable initial approach. >> >> > The limitation with using Lissajous figures is that eventually the > noise > in the relatively wide oscilloscope bandwidth (due to amplifier noise > and signal phase noise) limits the figure rotation rate that can > reliably discerned. Also the lack of a record of changes limits the > ability to see long term trends/drifts. > > However it can be a useful/instructive starting point particularly > when > the frequency difference is relatively large but still small enough > that > the figure rotation can be seen. >> Assuming that they are both near 10 MHz and you do not know which >> is the >> most accurate (although the Z3801 would seem to be the default >> standard), if >> it takes 10 minutes for a single cycle of the lissajous figure to >> complete, >> then it is 1 cycle per 600 seconds difference between the two and >> therefore >> the two are within 1/600 Hz or 1.67 mHz of each other. If we >> assume that >> they are both close to 10 MHz, then that is 1.67 parts in 10E-10 >> difference >> between the two. Is my logic faulty? >> >> The other perspective is the issue of 'purity'. That is to say, >> what is the >> 'frequency modulation' of the source? This, I think, is the issue >> of phase >> noise. Correct? >> >> That is something that I have not yet had a chance to contemplate >> as far as >> how to measure. It would appear to require a particularly stable >> (pure) >> source as a reference though. Various multiplying or dividing >> protocols >> would seem to introduce a host of other variables that would seem >> to be >> difficult to account for though they might accentuate an impurity >> in the >> signal in question. I have read Bruce's comments and I still do not >> understand the basics of time stamping or how a sound card might >> provide >> this. >> >> > One technique is to use a three cornered hat technique. > If the phase fluctuations of 3 oscillators are statistically > independent, then it is possible to determine the statistical > fluctuations in each of them. >> I would appreciate any direction for further reading regarding >> this and I >> would appreciate any direction/correction/etc. in the thoughts above. >> >> Joe >> >> > Bruce > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
