My last statement is not quite correct. "If the drift is too big we will over shoot to the next higher integer number and this will bias the average up too high."
In the case where the 100Mhz clock drifts both up and down the number could average to the correct fraction. But now the rate of drift as well as it's magnitude would need to be correct. Accuracy is not important here just the precision, as the number is not important just the rate of change of the number indicates error. I need to think more about this .... Stanley ----- Original Message ---- From: Stanley Reynolds <stanley_reyno...@yahoo.com> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Sun, June 27, 2010 5:23:13 PM Subject: Re: [time-nuts] yet another GPSDO design, or so Jitter may not be the correct word. I also don't know how accurate or repeatable the averaging effect is as described in the article. But I do believe the amount of drift is important for this to work as stated before. Independent is required, if locked then this is the extreme case of no drift and average value will be the same as all the samples are the same integer number. To see the fraction between the integer we need our data set to have some number of samples above and below and hopefully in a ratio to indicate the real measurement. If the drift is too small then we will tend to calculate an average too small, as more samples will fall to the low side the high side is never reached. If the drift is too big we will over shoot to the next higher integer number and this will bias the average up too high. Stanley ----- Original Message ---- From: "ewkeh...@aol.com" <ewkeh...@aol.com> To: time-nuts@febo.com Sent: Sun, June 27, 2010 4:27:39 PM Subject: Re: [time-nuts] yet another GPSDO design, or so Stanley I am not an expert but it is not only the jitter it is the fact that since the two sources are not linked the independent drift of the 100 MHz causes a distribution of the count. Again even with the same resolution the count error will be four X and the subsequent correction will be to scale, with out changing the code. Bert In a message dated 6/27/2010 4:00:59 P.M. Eastern Daylight Time, stanley_reyno...@yahoo.com writes: Without knowing your 100Mhz clock would it not need less jitter to average out +- one count at 100Mhz vs one count at 24Mhz? The GPS error has improved with better hardware/software in the receivers as well as turning off SA. So I'm not so sure the GPS error will wash out the counter's error. Even if it does, as a want to be time nut I need a better counter which is why more is better, a faster count with a recipical counter equals better unless the faster counter no longer averages out the +- count. Stanley ----- Original Message ---- From: "ewkeh...@aol.com" <ewkeh...@aol.com> To: time-nuts@febo.com Sent: Sun, June 27, 2010 1:17:51 PM Subject: Re: [time-nuts] yet another GPSDO design, or so Going from 24 to 100 MHz only gives you smaller steps (resolution) every thing else stays the same. If the he saw 2 to 3 nsec should be more like 8, going to 100 MHz will improve it by a factor of 4. In a redesign of the total system I would have two sample sizes maybe stay with 30 or go to 50/60 and in the Rub. mode 200 maybe 300 sec. Let us not forget what we start out with the GPS signal does not allow us to take advantage of the full resolution. Do not forget I did this to get smaller D/A steps and am not able to rewrite the code, basically fooling the controller that the error should call for a 1.7 E-13 correction when in reality the error is 4.3 E-14 and the resulting step is also 4.3 E-14 per D/A bit. Bert In a message dated 6/27/2010 1:43:40 P.M. Eastern Daylight Time, stanley_reyno...@yahoo.com writes: Yes I see your need for a reduced range with smaller steps. But I was looking for smaller steps to improve the tracking accuracy without a loss of the benefit of averaging. From the QST article: "Interestingly, it is desirable to have the frequency of U7 drift slightly rather than being synchronized with the VCXO. A slight random drift averages out the count ambiguity that is inherent in any pulse-counting device. My measurements indicate that the simple phase-measuring circuit I use is consistently accurate to 2 or 3 ns (for a 30-second measurement), while without drift, the resolution would be limited to 42 ns. The $5 crystal oscillator module drifts adequately" So the drift should just cover the area of uncertainty that is one cycle, too much drift would reduce accuracy, not enough and the average is of no benefit. One extreme no jitter, average doesn't work as it doesn't distribute the samples over the range of uncertainty. The other case too much jitter and the best to expect is an average weighted to one side or the other (+-1 count) with the extreme producing multiple counts of error. Something about this makes me nervous maybe the part about "slight random drift" what is slight at 24 Mhz is it also slight at 100 Mhz ? An average of 30 samples does have a limit to what it will correct. Stanley ----- Original Message ---- From: "ewkeh...@aol.com" <ewkeh...@aol.com> To: time-nuts@febo.com Sent: Sun, June 27, 2010 8:58:55 AM Subject: Re: [time-nuts] yet another GPSDO design, or so Stanley the faster counter also has the jitter, no change, as long as it is not tied to the input frequency. The 24 MHz is not unique, the 100 MHz is same technology just four times faster and thus gives me smaller steps on the D/A and since I use it on Rub. the full range of the 18 bit covers the full tuning range of the Rub. Bert In a message dated 6/27/2010 9:05:12 A.M. Eastern Daylight Time, stanley_reyno...@yahoo.com writes: I have been thinking about a faster counter also but the Shera board was depending on the jitter in the 24 Mhz clock to average out the +- count. The faster clock would reduce the need for this but without the right amount of jitter we lose the benefit of this average. Stanley <snip> _______________________________________________ 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. _______________________________________________ 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.