Re: [time-nuts] Allan variance Vs Plain Old Accuracy
> Something I rather miss is some good old phase or frequency plots - > especially if done at different timescales - which seem to be becoming > rather less common now. > As well as having a plot of ADEV or its relations, seeing what the > reference is doing and when is useful, and most ADEV plots using so > few values of tau does not help. > > Angus. Angus, I agree with you. Each representation has its merits and a combination of several is often necessary to tell the whole story. One feature (problem?) of ADEV is that it's a statistic and so some anomalies, clearly revealed in the phase or frequency domain, are hidden in the statistics. On the other hand, the ability of ADEV to resolve noise types, gives it a greater power than linear strip charts. A recent example of using all three graph types is here: http://www.leapsecond.com/pages/8607-drift/ On your comment about too "few values of tau" -- since adev is calculated by software tools these days, there is less reason to use only a few tau per decade. What I and others sometimes use is "all tau" or "many tau" where the tools calculate as many tau as necessary so that the plot is as close to a continuous, non-interpolated line as possible. For a well-behaved oscillator this is overkill (see above example) since all the points land on the line anyway, but for oscillators with any sort of periodic frequency modulation (e.g., an OCXO with bad tempco), the many tau method turns an adev plot into a sort of spectral plot. A cute example of the power of a many tau adev plot is here: http://www.leapsecond.com/museum/earth/ This is a free adev tool that calculates as many tau as you like: http://www.leapsecond.com/tools/adev1.htm /tvb ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
On Thu, 14 Feb 2008 11:00:32 -, you wrote: >Hi All, > >This comment is bound to get you all going. > >Maybe I'm being stupid, but why does everyone use Allan Variance and not >plan old accuracy? > >I am very familiar with David Allan's full article on Allan Variance. >However Allan Variance isn't the same as accuracy. > >Accuracy is what is important to most people. And that's not RMS but peak >to peak, e.g worse case. And not averaged over 24 hours but averaged over 1 >second or less. Something I rather miss is some good old phase or frequency plots - especially if done at different timescales - which seem to be becoming rather less common now. As well as having a plot of ADEV or its relations, seeing what the reference is doing and when is useful, and most ADEV plots using so few values of tau does not help. Angus. ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
Hello Martyn, >Rubidium's oscillators usually stay within 1E-10 accuracy, about five times >better than any OXCO unit I've measured. yes, your data shows this. This is actually also reflected in your ADEV measurements, your data shows that at 100s and 500s the Rb has about 10x better ADEV performance than the OCXO, and looking at your 3600s plot the OCXO wanders around in about that time frame, so there is some correlation here. So the ADEV numbers are not all that unrelated to frequency accuracy :) Please see Toms' ADEV measurements for four GPSDO's that he posted a couple of days ago, the ADEV numbers for those units are a bit, to significantly better than your ADEV measurement numbers, I think the reason is that the units he tested very likely all had DOCXO's. >From your stability measurements it looks like the OCXO was a single oven unit. A double oven unit's stability spec versus temp is usually 10x to 100x better than a single oven, so should perform much better at a marginal cost increase. Frequency errors in GPSDO's are primarily caused by two sources: 1) The accuracy of the GPS. Any GPS errors especially from 200s and longer will of course affect the frequency accuracy since the OCXO follows the GPS tightly at averaging times longer than 1000s (typically). Rb's are usually locked with much larger averaging times, so the OCXO has to be very good in the interim. 2) The stability of the OCXO. The more inherently stable the OCXO, the less error correction control the loop has to do, the better the frequency accuracy (and ADEV), and the further-out the time constant for the GPS locking can be pushed. Many OCXO vendors claim their DOCXO units to be Rb replacements. I think with the aid of a good GPS receiver, this can actually be accomplished for most applications, or at least GPSDO's can get close to the same performance. But it does require very good OCXO's and GPS performance, and you are right Rb's are on average more accurate due to this requirement. GPSDO's still have some advantages: * a good Rb with a built-in GPS is still more (to a lot more) expensive than a very good DOCXO-based GPSDO, but the accuracy differences may actually be small * the Rb requires much higher power consumption * The Rb generates more RF noise (due to all of the RF related frequencies and switching regulators etc inside the unit) * an Rb has a lamp-design-related limited lifetime, and their MTBF is lower than typical GPSDO's * some commercial GPSDO's can operate up to +85C which most commercial Rb's cannot do (SRS PRS-10 is limited to 65C on it's baseplate for example, meaning the ambient has to be much <<65C) * GPSDO's are typically smaller and lighter than GPS-disciplined RB's I think the choice is still not that clearly differentiated. bye, Said **The year's hottest artists on the red carpet at the Grammy Awards. Go to AOL Music. (http://music.aol.com/grammys?NCID=aolcmp0030002565) ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
At 10:56 AM 2/14/2008, Richard (Rick) Karlquist wrote... >Modern counters have "interpolators" (now called "time to digital >converters") that can measure fractions of a cycle. It appears he used an SRS620, which despite being called a "counter," really measures a time interval, and then computes the frequency. "Counter" is a misnomer. I was just "tweaking" him a bit for giving conclusions based on measurements from an unstated instrument with an unstated timebase. ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
Martyn, I think you're comparing "apples to oranges" by mixing short term stability of a 10MHz source with long term stability of a time reference, since different processes are responsible for the observed variances. Your data suggests that the OXCO device you've characterized is orders of magnitude worse than the most recent devices Tom Van Baak posted data for. Am I missing something here? Pete Rawson ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
Modern counters have "interpolators" (now called "time to digital converters") that can measure fractions of a cycle. Even the old Agilent 53132, designed 15 years ago, measures any frequency to 12 significant figures in one second. For example, it will display 10 MHz to .1 Hz using a 1 second gate time. Rick Karlquist N6RK Mike S wrote: > At 06:00 AM 2/14/2008, Martyn Smith wrote... >> I have an article on my web site > > You might want to proof read that again. "very gone Allan variance," > and there's more. > >> where I compare a OXCO based unit versus my rubidium's unit. > > Please explain how a counter resolves to .0003 cycles in a one second > gate. A counter, well, counts. Counting involves natural numbers. Also, > please tell us what time base was used on this unspecified "counter" > for these measurements. > > > ___ > 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
At 06:00 AM 2/14/2008, Martyn Smith wrote... >I have an article on my web site You might want to proof read that again. "very gone Allan variance," and there's more. >where I compare a OXCO based unit versus my rubidium's unit. Please explain how a counter resolves to .0003 cycles in a one second gate. A counter, well, counts. Counting involves natural numbers. Also, please tell us what time base was used on this unspecified "counter" for these measurements. ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
ailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] > Sent: Wednesday, February 13, 2008 4:11 PM > To: [EMAIL PROTECTED]; time-nuts@febo.com > Subject: Re: [time-nuts] GPS Locked and Unlocked Performance > Comparison > > Hello Tom, > > Bruce mentioned there is a validity bit that can be checked for > holdover. I wonder if a small micro can be used to hold the EFC > voltage steady without much effort. Or maybe using Super-Caps in the > loop filter? > > Or maybe use one of those new 24 bit Sigma-Delta ADC/DAC chips to > capture a > 24 bit word (ADC) and feed that to the 24 bit DAC during holdover. > Kind of a > 24 bit high-precision sample-and-hold circuit. > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.20.4/1277 - Release Date: 2/13/2008 8:00 PM -- Message: 7 Date: Thu, 14 Feb 2008 11:00:32 - From: "Martyn Smith" <[EMAIL PROTECTED]> Subject: [time-nuts] Allan variance Vs Plain Old Accuracy To: Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Hi All, This comment is bound to get you all going. Maybe I'm being stupid, but why does everyone use Allan Variance and not plan old accuracy? I am very familiar with David Allan's full article on Allan Variance. However Allan Variance isn't the same as accuracy. Accuracy is what is important to most people. And that's not RMS but peak to peak, e.g worse case. And not averaged over 24 hours but averaged over 1 second or less. Although I sell GPSDO using OXCO's, I don't find their accuracy as good as my rubidium disciplined oscillators. I have an article on my web site where I compare a OXCO based unit versus my rubidium's unit. I won't say what OXCO unit it is, but its one of the well known ones on the market that is talked about in this forum. www.ptsyst.com/AppNote2.pdf Also a friend of mine measured one of the current leading OXCO based units (from a very big manufacturer). He found its accuracy was +3.93E-10 and -4023E-10 over a 10.5 hour period. Over the same time the unit displayed a worse case error of 1.4E-12. It tended to drift in one direction for a long time and then drop. So its Allan Var was still very good, but its accuracy was poor. That's another one of my gripes. The frequency accuracy displayed by many units has little bearing on actual frequency accuracy. Rubidium's oscillators usually stay within 1E-10 accuracy, about five times better than any OXCO unit I've measured. And now that I can sell you a Rubidium with GPS locking built in for under $600.00 why buy an OXCO! Ok, I do need an order for > 5000 pieces. Best Regards Martyn -- Message: 8 Date: Thu, 14 Feb 2008 12:03:55 + From: Luis Cupido <[EMAIL PROTECTED]> Subject: Re: [time-nuts] GPS Locked and Unlocked Performance Comparison To: Discussion of precise time and frequency measurement Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Hi, I got also a 10KHz version fit on a CPLD as per request of W7QX in early 2004 (for 100MHz etc) and later for N1JEZ and G6GXK (this time having also 10MHz). All is on my web pages since then but only if you look in detail in the configuration list files you find it... I must rearrange my web layout as some stuff is not so visible, sorry. on the other hand I suffer from extra queries about things I have there... ;-) Anyway, is is the the same old straightforward style of design (as the others) and has both XOR and FF outputs available. Unfortunately I do not have a Jupiter RX so I did not any real performance tests on it only some basic functional tests :-( ... I'm stuck to 1pps ;-) Luis Cupido. ct1dmk http://w3ref.cfn.ist.utl.pt/cupido/ Dave Brown wrote: > There's a published design very similar to James Millers from Andy > Talbot- > > http://www.frars.org.uk/cgi-bin/render.pl?pageid=1285 > > I think they were both published about the same time originally. > He uses a 4046 for the phase detector and suggests a decent OCXO but > otherwise very much the same. > > DaveB, NZ > > ___ > 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 https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts End of time-nuts Digest, Vol 43, Issue 33 * ___ 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.
Re: [time-nuts] Allan variance Vs Plain Old Accuracy
Steady boys and girls... This discussion would be fun however, I get the feeling it would only supply a marketing department with more copy. ___ 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] Allan variance Vs Plain Old Accuracy
Hi All, This comment is bound to get you all going. Maybe I'm being stupid, but why does everyone use Allan Variance and not plan old accuracy? I am very familiar with David Allan's full article on Allan Variance. However Allan Variance isn't the same as accuracy. Accuracy is what is important to most people. And that's not RMS but peak to peak, e.g worse case. And not averaged over 24 hours but averaged over 1 second or less. Although I sell GPSDO using OXCO's, I don't find their accuracy as good as my rubidium disciplined oscillators. I have an article on my web site where I compare a OXCO based unit versus my rubidium's unit. I won't say what OXCO unit it is, but its one of the well known ones on the market that is talked about in this forum. www.ptsyst.com/AppNote2.pdf Also a friend of mine measured one of the current leading OXCO based units (from a very big manufacturer). He found its accuracy was +3.93E-10 and -4023E-10 over a 10.5 hour period. Over the same time the unit displayed a worse case error of 1.4E-12. It tended to drift in one direction for a long time and then drop. So its Allan Var was still very good, but its accuracy was poor. That's another one of my gripes. The frequency accuracy displayed by many units has little bearing on actual frequency accuracy. Rubidium's oscillators usually stay within 1E-10 accuracy, about five times better than any OXCO unit I've measured. And now that I can sell you a Rubidium with GPS locking built in for under $600.00 why buy an OXCO! Ok, I do need an order for > 5000 pieces. Best Regards Martyn ___ 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.
