From: "Poul-Henning Kamp" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Code for phase noise and allan variance Date: Tue, 28 Jun 2005 00:46:44 +0200 Message-ID: <[EMAIL PROTECTED]>
> In message <[EMAIL PROTECTED]>, Magnus Danielson write > s: > >> Does anyone have code which will take an amplitude vs. time input > >> stream and output phase noise and/or allan variance? > > > >I guess you could do it that way, but really, it would be alot of data at a > >high sampling rate to make any usefullness. > > You're wrong Magnus :-) > > You overlook that the A/D converter gives you much more information > about the timing of a zero-crossing than just the sample number: > You can interpolate the zero-crossing to subsample accuracy and > that way get far *better* resolution than the HP5370 Actually, I did think about it, but all I saw was problems, such as dependence of waveshape (we see that in normal counters too) which makes steeper slopes have you loose accuracy. I agree that the A/D performs a form of analog interpolator. But, I also said that it will be alot of unnecessary data and unless you skip data between the last sample to the next sample. > which otherwise > holds the single-shot crown. Actually, cutting edge equipment is at picosecond or even sub-picosecond in single-shot resolution. Also, A/D convert based interpolators is used there but on a much more controled signal than the raw waveform. > I have played a lot with this with a 12bit 20MHz ADC card I have, and > the USRP does 64MHz sampling which is a lot better. > > If your signal is relatively noise-free and of good amplitude I > would not at all be surprised to see single-shot timings well into > the pico-second regime, (compared to the 15nsec a digital use of > 64MHz would give). I'd like to see the error analys of such a system. The signal-dependent error profile would be an interesting exercise. By propper pre-processing or a close to perfect signal, the system performance reaches the quick-and-dirty estimate. So, maybe I am a bit picky, but it can be a bit harder, but I agree that you can acheive sub-sample resolution, but much care must be taken. However, I do like the idea of taking something and use it for something completely different! ;O) Just be a bit careful in verifying the actual performance. > And as I suggested in an email a couple of days ago: It would be > almost perfect for phase-noise measurements as well, thanks to > the dual inputs. Actually, for phase noise you really *can* make use of the sampled waveform if you make it long enought. Toss the waveform into FFT, scrape of the overtones, sum around the base frequency to get the single-sided spectrum and correct for 6 dB slope and you are done. This quick-and-dirty method will be pretty accurate for lower modulations. Cheers, Magnus _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts