Hi The simple answer is a filter at the highest sample rate (say 1 second) that impacts the 1 second data. You then decimate from there. If you want 1 second data that “looks right” you start at something higher (say 0.1 second) and filter there. The data set is filtered once (if at all) and decimated from there.
Bob > On Jul 30, 2016, at 12:04 PM, Kevin Rosenberg <ke...@rosenberg.net> wrote: > > Hmm, I might have answered my own question: filter to the fast samples to the > equivalent noise bandwidth (ENBW) of the lower desired sampling rate and then > decimate. > >> On Jul 29, 2016, at 9:44 PM, Kevin Rosenberg <ke...@rosenberg.net> wrote: >> >> Hi Bob, >> >> You have a good point. That leads to the question is what is the “best” >> measurement technique when you are sampling at a more smaller interval than >> the desired tau? >> >> SDRs sample at high rates. The slowest the USRP N2x0 can sample is just >> under 200Ksps. For easy math, let’s assume we sample at 1Msps but we want to >> record only 1sps for a long-term measurement. What’s best way to handle the >> 1e6 to 1 ratio of available samples to desired samples? One method is to >> discard 999,999 samples and just record the phase difference with a true tau >> of 1 sec. The other is to take a window of 1e6 samples and output the >> average phase difference over that 1 second window. Is your point that >> averaging samples that are more frequent than the tau will overestimate >> stability at the tau? If using averaged data, would it be “less lying” to >> multiply the ADEV by the sqrt of the length of the averaging window? >> >> I’d appreciate your thoughts on the subject, >> >> Kevin >> >>> On Jul 29, 2016, at 6:51 PM, Bob Camp <kb...@n1k.org> wrote: >>> >>> HI >>> >>> Keep in mind that if you apply pre-filtering, an ADEV plot is lying to you >>> …. >>> >>> Bob >>> >>>> On Jul 29, 2016, at 6:58 PM, Kevin Rosenberg <ke...@rosenberg.net> wrote: >>>> >>>> Jeff, >>>> >>>> Thanks for your very useful paper Oscillator Metrology with SDRs[1]. I >>>> created a C++ program and checked residuals using a 10 MHz clock split >>>> to the A and B channels of a LFRX and BasicRX boards and sampled at 1 >>>> Mhz. Using boxcar averaging of 1000 samples at 1 kHz, I was impressed >>>> by the low noise floor approaching that of my Timepod which was >>>> several times the cost. I included the Allan Deviation without >>>> averaging showing the sqrt(1000) increase in noise floor without the >>>> averaging[2]. >>>> >>>> I had a question about your experience. You mentioned using a input >>>> signal near the maximum of the USRP’s ADC to get the best SNR. I >>>> reviewed the schematics and application notes. I found a maximum Vpp >>>> mentioned of 3.3V. I was wondering what voltage you were using to >>>> drive the USRPs. When I go above 1.5-2 Vpp, I start getting signal >>>> distortions and not much increase in the amplitude. >>>> >>>> Many thanks for publishing your work in this area. >>>> >>>> Kevin >>>> >>>> [1] >>>> https://arxiv.org/abs/1605.03505 >>>> >>>> [2] >>>> <usrp-pn.png>_______________________________________________ >>>> time-nuts mailing list -- time-nuts@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@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@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@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@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
