Re: [time-nuts] Characterising frequency standards
Ulrich, 2009/4/11 Ulrich Bangert df...@ulrich-bangert.de: So why would my counter show any significant differences between a 1 sec or 2 sec gate time? suppose your source has a 0.5 Hz frequency modulation. Would you see it with 2 s gate time or a integer multiple of it? Would you notice it with 1 s gate time or an odd integer of it? Agreed, if the source is modulated at exactly 1/2 the input frequency, the measurement would be blind to it. So the way to account for this would be to take half the readings, then skip one cycle and take the other half. Examination of the data would then show the modulation. I've just done a Google search for dead time correction scheme and I just turn up results relating to particle physics where it seems measurements are unable to keep up with the flow of data, hence the need to factor in the dead time of system. Google for the STABLE32 manual. THIS literature will bring you a lot further, many well documented source examples in Forth and PL/1, hi. F.e. you may look here: http://www.wriley.com/ Thanks for the pointers. Kind regards, Steve Best regards Ulrich Bangert -Ursprungliche Nachricht- Von: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] Im Auftrag von Steve Rooke Gesendet: Freitag, 10. April 2009 12:55 An: Discussion of precise time and frequency measurement Betreff: [!! SPAM] Re: [time-nuts] Characterising frequency standards Ulrich, 2009/4/10 Ulrich Bangert df...@ulrich-bangert.de: Steve, I think the penny has dropped now, thanks. It's interesting that the ADEV calculation still works even without continuous data as all the reading I have done has led me to belive this was sacrosanct. The penny may be falling but it is not completely dropped: Of course you can feed your ADEV calculation with every second sample removed and setting Tau0 = 2. And of course you receive a result that now is in harmony with your all samples / Tau0 = 1 s computation. Had you done frequency measurements the reason for this appearant harmony is that your counter does not show significant different behaviour whether set to 1 s gate time or alternate 2 second gate time. So why would my counter show any significant differences between a 1 sec or 2 sec gate time? Nevertheless leaving every second sample out is NOT exactly the same as continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a DEAD TIME of 1s. There are dead time correction schemes available in the literature. I've just done a Google search for dead time correction scheme and I just turn up results relating to particle physics where it seems measurements are unable to keep up with the flow of data, hence the need to factor in the dead time of system. This form of application does not appear to correlate with the measurement of plain oscillators. Yes there is dead time, per say, but I fail to see how this can detract significantly from continuous data given a sufficient data set size (as for a total measurement time). I guess what we need is a real data set which would show that this form of ADEV calculation produces incorrect results, IE. the proof of the pudding is in the eating. 73, Steve Best regards Ulrich Bangert -Ursprungliche Nachricht- Von: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] Im Auftrag von Steve Rooke Gesendet: Donnerstag, 9. April 2009 14:00 An: Tom Van Baak; Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] Characterising frequency standards Tom, 2009/4/9 Tom Van Baak t...@leapsecond.com: The first argument to the adev1 program is the sampling interval t0. The program doesn't know how far apart the input file samples are taken so it is your job to specify this. The default is 1 second. If you have data taken one second apart then t0 = 1. If you have data taken two seconds apart then t0 = 2. If you have data taken 60 seconds apart then t0 = 60, etc. If, as in your case, you take raw one second data and remove every other sample (a perfectly valid thing to do), then t0 = 2. Make sense now? It's still continuous data in the sense that all measurements are a fixed interval apart. But in any ADEV calculation you have to specify the raw data interval. I think the penny has dropped now, thanks. It's interesting that the ADEV calculation still works even without continuous data as all the reading I have done has led me to belive this was sacrosanct. What I now believe is that it's possible to measure oscillator performance with less than optimal test gear. This will enable me to see the effects of any experiments I make in the future. If you can't measure it, how can you know that what your doing is good or bad. 73, Steve -- Steve Rooke - ZL3TUV G8KVD JAKDTTNW Omnium finis imminet
Re: [time-nuts] Characterising frequency standards
Tom, 2009/4/11 Tom Van Baak t...@leapsecond.com: Nevertheless leaving every second sample out is NOT exactly the same as continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a DEAD TIME of 1s. There are dead time correction schemes available in the literature. Ulrich, and Steve, Wait, are we talking phase measurements here or frequency measurements? My assumption with this thread is that Steve is simply taking phase (time error) measurements, as in my GPS raw data page, in which case there is no such thing as dead time. Yes, phase measurements as in the original GPS.dat data set on your site. 73, Steve /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. -- Steve Rooke - ZL3TUV G8KVD JAKDTTNW Omnium finis imminet ___ 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] Characterising frequency standards
2009/4/11 Magnus Danielson mag...@rubidium.dyndns.org: Tom Van Baak skrev: Nevertheless leaving every second sample out is NOT exactly the same as continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a DEAD TIME of 1s. There are dead time correction schemes available in the literature. Ulrich, and Steve, Wait, are we talking phase measurements here or frequency measurements? My assumption with this thread is that Steve is simply taking phase (time error) measurements, as in my GPS raw data page, in which case there is no such thing as dead time. I agree. I was also considering this earlier but put my mind to rest by assuming phase/time samples. Dead time is when the counter looses track of time in between two consecutive measurements. A zero dead-time counter uses the stop of one measure as the start of the next measure. This becomes very important when the data to be measured has a degree of randomness and it is therefore important to capture all the data without any dead time. In the case of measurements of phase error in an oscillator, it should be possible to miss some data points provided that the frequency of capture is still known (assuming that accuracy of drift measurements is required). If you have a series of time-error values taken each second and then drop every other sample and just recall that the time between the samples is now 2 seconds, then the tau0 has become 2s without causing dead-time. However, if the original data would have been kept, better statistical properties would be given, unless there is a strong repetitive disturbance at 2 s period, in which case it would be filtered out. Indeed, there would be a loss of statistical data but this could be made up by sampling over a period of twice the time. This system is blind to noise at 1/2 f but ways and means could be taken to account for that, IE. taking two data sets with a single cycle space between them or taking another small data set with 2 cycles skipped between each measurement. An example when one does get dead-time, consider a frequency counter which measures frequency with a gate-time of say 2 s. However, before it re-arms and start the next measures is takes 300 ms. The two samples will have 2,3 s between its start and actually spans 4,3 seconds rather than 4 seconds. When doing Allan Deviation calculations on such a measurement series, it will be biased and the bias may be compensated, but these days counters with zero dead-time is readily available or the problem can be avoided by careful consideration. I'm looking at what can be acheieved by a budget strapped amateur who would have trouble purchasing a later counter capable of measuring with zero dead time. I believe Grenhall made some extensive analysis of the biasing of dead-time, so it should be available from NIST FT online library. I'll see what I can find. Before zero dead-time counters was available, a setup of two counters was used so that they where interleaved so the dead-time was the measure time of the other. I could look at doing that perhaps. I can collect some references to dead-time articles if anyone need them. I'd happy to. 73, Steve Cheers, Magnus ___ 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. -- Steve Rooke - ZL3TUV G8KVD JAKDTTNW Omnium finis imminet ___ 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] Characterising frequency standards
Steve Steve Rooke wrote: 2009/4/11 Magnus Danielson mag...@rubidium.dyndns.org: Tom Van Baak skrev: Nevertheless leaving every second sample out is NOT exactly the same as continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a DEAD TIME of 1s. There are dead time correction schemes available in the literature. Ulrich, and Steve, Wait, are we talking phase measurements here or frequency measurements? My assumption with this thread is that Steve is simply taking phase (time error) measurements, as in my GPS raw data page, in which case there is no such thing as dead time. I agree. I was also considering this earlier but put my mind to rest by assuming phase/time samples. Dead time is when the counter looses track of time in between two consecutive measurements. A zero dead-time counter uses the stop of one measure as the start of the next measure. This becomes very important when the data to be measured has a degree of randomness and it is therefore important to capture all the data without any dead time. In the case of measurements of phase error in an oscillator, it should be possible to miss some data points provided that the frequency of capture is still known (assuming that accuracy of drift measurements is required). If you have a series of time-error values taken each second and then drop every other sample and just recall that the time between the samples is now 2 seconds, then the tau0 has become 2s without causing dead-time. However, if the original data would have been kept, better statistical properties would be given, unless there is a strong repetitive disturbance at 2 s period, in which case it would be filtered out. Indeed, there would be a loss of statistical data but this could be made up by sampling over a period of twice the time. This system is blind to noise at 1/2 f but ways and means could be taken to account for that, IE. taking two data sets with a single cycle space between them or taking another small data set with 2 cycles skipped between each measurement. An example when one does get dead-time, consider a frequency counter which measures frequency with a gate-time of say 2 s. However, before it re-arms and start the next measures is takes 300 ms. The two samples will have 2,3 s between its start and actually spans 4,3 seconds rather than 4 seconds. When doing Allan Deviation calculations on such a measurement series, it will be biased and the bias may be compensated, but these days counters with zero dead-time is readily available or the problem can be avoided by careful consideration. I'm looking at what can be acheieved by a budget strapped amateur who would have trouble purchasing a later counter capable of measuring with zero dead time. You don't need a full featured counter for this application. One can easily implement a zero deadtime counter or the equivalent thereof in an FPGA. I believe Grenhall made some extensive analysis of the biasing of dead-time, so it should be available from NIST FT online library. I'll see what I can find. You still need to know the phase noise spectrum of the source being characterised. Before zero dead-time counters was available, a setup of two counters was used so that they where interleaved so the dead-time was the measure time of the other. I could look at doing that perhaps. Very easy to do at low cost in an FPGA. I can collect some references to dead-time articles if anyone need them. I'd happy to. 73, Steve Cheers, Magnus ___ 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. Brice ___ 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] Characterising frequency standards
Steve Steve Rooke wrote: If I take two sequential phase readings from an input source and place this into one data set and aniother two readings from the same source but spaced by one cycle and put this in a second data set. From the first data set I can calculate ADEV for tau = 1s and can calculate ADEV for tau = 2 sec from the second data set. If I now pre-process the data in the second set to remove all the effects of drift (given that I have already determined this), I now have two 1 sec samples which show a statistical difference and can be fed to ADEV with a tau0 = 1 sec producing a result for tau = 1 sec. The results from this second calculation should show equal accuracy as that using the first data set (given the limited size of the data set). You need to give far more detail as its unclear exactly what you are doing with what samples. Label all the phase samples and then show which samples belong to which data set. Also need to show clearly what you mean by skipping a cycle. I now collect a large data set but with a single cycle skipped between each sample. I feed this into ADEV using tau0 = 2 sec to produce tau results = 2 sec. I then pre-process the data to remove any drift and feed this to ADEV with a tau0 = 1 sec to produce just the tau = 1 sec result. I now have a complete set of results for tau = 1 sec. Agreed, there is the issue of modulation at 1/2 input f but ignoring this for the moment, this should give a valid result. Again you need to give more detail. Now indulge me while I have a flight of fantasy. As the effects of jitter and phase noise will produce a statistical distribution of measurements, any results from these ADEV calculations will be limited on accuracy by the size of the data set. Only if we sample for a very long time will we see the very limits of the effects of noise. What noise from what source? Noise in such measurements can originate in the measuring instrument or the source. For short measurement times quantisation noise and instrumental noise may mask the noise from the source but they are still present. The samples which deviate the most from the median will occur very infrequently and it is statistically likely that they will not occur adjacent to another highly deviated sample. We could pre-process the data to remove all drift and then sort it into an array of increasing size. This would give the greatest deviations at each end of the array. For 1 sec stability the deviation would be the greatest difference from the median of the first and last samples in the array. For a 2 sec stability, this same calculation could be made taking the first two and last two readings in the array and calculating their difference from 2 x the median. This calculation could be continued until all the data is used for the final calculation. In fact the whole sorted data set could be fed to ADEV to produce a result that would show better worse case measurement of the input source which still has some statistical probability. In theory, if we took an infinite number of samples, there would be a whole string of absolutely maximum deviation measurements in a row which would show the absolute worse case. Is any of this valid or just bad physics, I don't know, but I'm sure it will solicit interesting comment. No, not poor physics but poor statistics. 73, Steve 2009/4/10 Tom Van Baak t...@leapsecond.com: I think the penny has dropped now, thanks. It's interesting that the ADEV calculation still works even without continuous data as all the reading I have done has led me to belive this was sacrosanct. We need to be careful about what you mean by continuous. Let me probe a bit further to make sure you or others understand. The data that you first mentioned, some GPS and OCXO data at: http://www.leapsecond.com/pages/gpsdo-sim was recorded once per second, for 400,000 samples without any interruption; that's over 4 days of continuous data. As you see it is very possible to extract every other, or every 10th, every 60th, or every Nth point from this large data set to create a smaller data set. Is it as if you had several counters all connected to the same DUT. Perhaps one makes a new phase measurement each second, another makes a measurement every 10 seconds; maybe a third counter just measures once a minute. The key here is not how often they make measurements, but that they all keep running at their particular rate. The data sets you get from these counters all represent 4 days of measurement; what changes is the measurement interval, the tau0, or whatever your ADEV tool calls it. Now the ADEV plots you get from these counters will all match perfectly with the only exception being that the every-60 second counter cannot give you any ADEV points for tau less than 60; the every-10 second counter cannot give you points for tau less than 10 seconds; and for that matter; the every
Re: [time-nuts] Characterising frequency standards
Bruce Griffiths wrote: ... Brice An impostor? An alias? :-) ___ 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] Characterising frequency standards
Rex wrote: Bruce Griffiths wrote: ... Brice An impostor? An alias? :-) And I thought I was alluding to aliasing of the phase noise spectrum not the characters of the alphabet. Bruce ___ 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] Characterising frequency standards
Steve Rooke skrev: 2009/4/11 Magnus Danielson mag...@rubidium.dyndns.org: Tom Van Baak skrev: Nevertheless leaving every second sample out is NOT exactly the same as continous data with Tau0 = 2 s. Instead it is data with Tau0 = 1 s and a DEAD TIME of 1s. There are dead time correction schemes available in the literature. Ulrich, and Steve, Wait, are we talking phase measurements here or frequency measurements? My assumption with this thread is that Steve is simply taking phase (time error) measurements, as in my GPS raw data page, in which case there is no such thing as dead time. I agree. I was also considering this earlier but put my mind to rest by assuming phase/time samples. Dead time is when the counter looses track of time in between two consecutive measurements. A zero dead-time counter uses the stop of one measure as the start of the next measure. This becomes very important when the data to be measured has a degree of randomness and it is therefore important to capture all the data without any dead time. In the case of measurements of phase error in an oscillator, it should be possible to miss some data points provided that the frequency of capture is still known (assuming that accuracy of drift measurements is required). Depending on the dominant noise type, the ADEV measure will be biased. If you have a series of time-error values taken each second and then drop every other sample and just recall that the time between the samples is now 2 seconds, then the tau0 has become 2s without causing dead-time. However, if the original data would have been kept, better statistical properties would be given, unless there is a strong repetitive disturbance at 2 s period, in which case it would be filtered out. Indeed, there would be a loss of statistical data but this could be made up by sampling over a period of twice the time. This system is blind to noise at 1/2 f but ways and means could be taken to account for that, IE. taking two data sets with a single cycle space between them or taking another small data set with 2 cycles skipped between each measurement. Actually, you can take any number of 2 cycle measures and be unable to detect the 1/2 f oscillation without detecting it. In order to be able to detect it you will need to take 2 measures and be able to make an odd number of cycles trigger difference between them to have a chance. The trouble is that the modulation is at the Nyquist frequency of the 1 cycle data, so it will fold down to DC on sampling it at half-rate. Canceling it from other DC offset errors could be challenging. Sampling it at 1/3 rate would discover it thought. An example when one does get dead-time, consider a frequency counter which measures frequency with a gate-time of say 2 s. However, before it re-arms and start the next measures is takes 300 ms. The two samples will have 2,3 s between its start and actually spans 4,3 seconds rather than 4 seconds. When doing Allan Deviation calculations on such a measurement series, it will be biased and the bias may be compensated, but these days counters with zero dead-time is readily available or the problem can be avoided by careful consideration. I'm looking at what can be acheieved by a budget strapped amateur who would have trouble purchasing a later counter capable of measuring with zero dead time. Beleive me, that's where I am too. Patience and saving money for things I really want and allowing accumulation over time has allowed me some pretty fancy tools in my private lab. Infact I have to lend some of my gear to commercial labs as I outperform them... I believe Grenhall made some extensive analysis of the biasing of dead-time, so it should be available from NIST FT online library. I'll see what I can find. I recalled wrong. You should look for Barnes Tables of Bias Functions, B1 and B2, for Variance Based on Finite Samples of Processes with Power Law Spectral Densities, NBS Technical Note 375, Janurary 1969 as well as Barnes and Allan Variance Based on Data with Dead Time Between the Mesurements NIST Technical Note 1318, 1990. A ahort into to the subject is found in NIST Special Publication 1065 by W.J. Riley as found on http://www.wriley.com along other excelent material. The good thing about that material is that he gives good references, as one should. Before zero dead-time counters was available, a setup of two counters was used so that they where interleaved so the dead-time was the measure time of the other. I could look at doing that perhaps. You should have two counters of equivalent performance, preferably same model. It's a rather expensive approach IMHO. Have a look at the possibility of picking up a HP 5371A or 5372A. You can usually snag one for about 600 USD or 1000 USD respectively on Ebay. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go
Re: [time-nuts] Characterising frequency standards
Bruce Griffiths skrev: Rex wrote: Bruce Griffiths wrote: ... Brice An impostor? An alias? :-) And I thought I was alluding to aliasing of the phase noise spectrum not the characters of the alphabet. So it is not a case of shot noise of Bruce fingers? :) I know mine has some, and besides that there are several bugs in the language unit... Cheers, Magnus ___ 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] Characterising frequency standards
Hej Magnus Magnus Danielson wrote: Bruce Griffiths skrev: Rex wrote: Bruce Griffiths wrote: ... Brice An impostor? An alias? :-) And I thought I was alluding to aliasing of the phase noise spectrum not the characters of the alphabet. So it is not a case of shot noise of Bruce fingers? :) I know mine has some, and besides that there are several bugs in the language unit... Cheers, Magnus ___ 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. More a case of digital jitter. Perhaps the control system phase noise was too high. Bruce ___ 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] Characterising frequency standards
Hello Steve, Try this... take Tom's sample data set, run the numbers. Then, using a good random number generator, make another data set by randomly throwing out half (or more) of the samples (to simulate a non ZDT counter). Run the numbers again. See how they change. This should give you a good idea of how using a standard counter would affect your adev numbers. _ Rediscover Hotmail®: Get e-mail storage that grows with you. http://windowslive.com/RediscoverHotmail?ocid=TXT_TAGLM_WL_HM_Rediscover_Storage1_042009 ___ 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] HP 1938 revisited
Fluke.l (China) was selling a number of 1938's on Ebay. I snagged one just to have a piece of HP history. It works just fine, but I've noticed something a little strange. Comparing the 1938 to both my cesium and GPS standards, there's a distinct periodic 1ns phase shift every second. Seems to smoothly advance 1ns for 500 ms, then retard back to the original phase point over the next 500ms. Question: is this to be expected? I'm assuming this is from the AFC loop, but I would have expected it to be better damped. I've searched both this group and the HP group; there really doesn't seem to be a great deal of info about these, other than schematics and some nice variance plots on leapsecond. Just what is the serial port and the PIC data lines useful for, if anything? -- Bill Ezell -- They said 'Windows or better' so I used Linux. ___ 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] HP 1938 revisited
wje wrote: Fluke.l (China) was selling a number of 1938's on Ebay. I snagged one just to have a piece of HP history. It works just fine, but I've noticed something a little strange. Comparing the 1938 to both my cesium and GPS standards, there's a distinct periodic 1ns phase shift every second. Seems to smoothly advance 1ns for 500 ms, then retard back to the original phase point over the next 500ms. Question: is this to be expected? I'm assuming this is from the AFC loop, but I would have expected it to be better damped. The AFC loop (as opposed to EFC) is a purely analog loop in which there is no mechanism for a 1 Hz oscillation. Now if you are talking about the EFC, that is another story. Depending on how you are driving the EFC, you can pickup noise from any number of sources. I vaguely remember there was an LED that flashed at 1 Hz if everything was working OK. You might see if that is the source of what you are seeing. I've searched both this group and the HP group; there really doesn't seem to be a great deal of info about these, other than schematics and some nice variance plots on leapsecond. Just what is the serial port and the PIC data lines useful for, if anything? The serial port can be used to change the oven temperature set point and IIRC monitor various oven parameters. Probably nothing you want to play with if you want a working oscillator. If you want to experiment, I have released the software to talk to the serial port. It should be archived somewhere. In production, we ramped the temperature up and down and found the exact turnover temperature for each individual oscillator and set the oven to the turnover. This required that we make crystals that actually had a turnover. Many 10811 crystals did not have turnovers, only an inflection point. Rick Karlquist N6RK Designer of E1938A ___ 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] Characterising frequency standards
Bruce, 2009/4/12 Bruce Griffiths bruce.griffi...@xtra.co.nz: Steve Steve Rooke wrote: If I take two sequential phase readings from an input source and place this into one data set and aniother two readings from the same source but spaced by one cycle and put this in a second data set. From the first data set I can calculate ADEV for tau = 1s and can calculate ADEV for tau = 2 sec from the second data set. If I now pre-process the data in the second set to remove all the effects of drift (given that I have already determined this), I now have two 1 sec samples which show a statistical difference and can be fed to ADEV with a tau0 = 1 sec producing a result for tau = 1 sec. The results from this second calculation should show equal accuracy as that using the first data set (given the limited size of the data set). You need to give far more detail as its unclear exactly what you are doing with what samples. Label all the phase samples and then show which samples belong to which data set. Also need to show clearly what you mean by skipping a cycle. Say I have a 1Hz input source and my counter measures the period of the first cycle and assigns this to A1. At the end of the first cycle the counter is able to be rest and re-triggered to capture the second cycle and assign this to A2. So far 2 sec have passed and I have two readings in data set A. I now repeat the experiment and assign the measurement of the first period to B1. The counter I am using this time is unable to stop at the end of the first measurement and retrigger immediately so I'm unable to measure the second cycle but is left in the armed position. When the third cycle starts, the counter triggers and completes the measurement of the third cycle which is now assigned to B2. For the purposes of my original text, the first data set refers to A1 A2. Similarly the second data set refers to B1 B2. Reference to pre-processing of the second data set refers to mathematically removing the effects of drift from B1 B2 to produce a third data set which is used as the data input for an ADEV calculation where tau0 = 1 sec with output of tau = 1 sec. I now collect a large data set but with a single cycle skipped between each sample. I feed this into ADEV using tau0 = 2 sec to produce tau results = 2 sec. I then pre-process the data to remove any drift and feed this to ADEV with a tau0 = 1 sec to produce just the tau = 1 sec result. I now have a complete set of results for tau = 1 sec. Agreed, there is the issue of modulation at 1/2 input f but ignoring this for the moment, this should give a valid result. Again you need to give more detail. In this case the data set is constructed from the measurement of the cycle periods of a 1Hz input source where even cycles are skipped, hence each data point is a measurement of the period of each odd (1, 3, 5, 7...) cycle of the incoming waveform. In this case the time between each measurement is 2 sec so ADEV is calculated with tau = 2 sec for tau = 2 sec. This data set is then mathematically processed to remove the effects of drift, bearing in mind the 2 sec spacing of each data point, and ADEV is then calculated with tau0 = 1 sec for tau = 1 sec. Now indulge me while I have a flight of fantasy. As the effects of jitter and phase noise will produce a statistical distribution of measurements, any results from these ADEV calculations will be limited on accuracy by the size of the data set. Only if we sample for a very long time will we see the very limits of the effects of noise. What noise from what source? PN - White noise phase WPM, Flicker noise phase FPM, White noise frequency WFM, Flicker noise frequency FFM and Random walk frequency RWFM. Noise in such measurements can originate in the measuring instrument or the source. Indeed, and this is an important aspect to consider as we have been discussing the effects of induced jitter/PN to a frequency standard when it is buffered and divided down. Ideally measurements of ADEV would be made on the raw frequency standard source (eg. 10MHz) rather than, say, a divided 1Hz signal. For short measurement times quantisation noise and instrumental noise may mask the noise from the source but they are still present. Well, these form the noise floor of our measurement system. The samples which deviate the most from the median will occur very infrequently and it is statistically likely that they will not occur adjacent to another highly deviated sample. We could pre-process the data to remove all drift and then sort it into an array of increasing size. This would give the greatest deviations at each end of the array. For 1 sec stability the deviation would be the greatest difference from the median of the first and last samples in the array. For a 2 sec stability, this same calculation could be made taking the first two and last two readings in the array and calculating their difference from 2 x the median. This
Re: [time-nuts] Characterising frequency standards
2009/4/13 Magnus Danielson mag...@rubidium.dyndns.org: Dead time is when the counter looses track of time in between two consecutive measurements. A zero dead-time counter uses the stop of one measure as the start of the next measure. This becomes very important when the data to be measured has a degree of randomness and it is therefore important to capture all the data without any dead time. In the case of measurements of phase error in an oscillator, it should be possible to miss some data points provided that the frequency of capture is still known (assuming that accuracy of drift measurements is required). Depending on the dominant noise type, the ADEV measure will be biased. If the noise has a component related to the measurement frequency, agreed, but I have already commented on that before. Indeed, there would be a loss of statistical data but this could be made up by sampling over a period of twice the time. This system is blind to noise at 1/2 f but ways and means could be taken to account for that, IE. taking two data sets with a single cycle space between them or taking another small data set with 2 cycles skipped between each measurement. Actually, you can take any number of 2 cycle measures and be unable to detect the 1/2 f oscillation without detecting it. In order to be able to detect it you will need to take 2 measures and be able to make an odd number of cycles trigger difference between them to have a chance. Agreed. The trouble is that the modulation is at the Nyquist frequency of the 1 cycle data, so it will fold down to DC on sampling it at half-rate. Canceling it from other DC offset errors could be challenging. Comparing the frequency calculated from the data would show a 2Hz offset with the fundamental frequency of the source. Sampling it at 1/3 rate would discover it thought. Agreed. I'm looking at what can be acheieved by a budget strapped amateur who would have trouble purchasing a later counter capable of measuring with zero dead time. Beleive me, that's where I am too. Patience and saving money for things I really want and allowing accumulation over time has allowed me some pretty fancy tools in my private lab. Infact I have to lend some of my gear to commercial labs as I outperform them... Well, that's a goal for me but I'm looking at what is achievable in the short term instead of sitting on my hands. I recalled wrong. You should look for Barnes Tables of Bias Functions, B1 and B2, for Variance Based on Finite Samples of Processes with Power Law Spectral Densities, NBS Technical Note 375, Janurary 1969 as well as Barnes and Allan Variance Based on Data with Dead Time Between the Mesurements NIST Technical Note 1318, 1990. A ahort into to the subject is found in NIST Special Publication 1065 by W.J. Riley as found on http://www.wriley.com along other excelent material. The good thing about that material is that he gives good references, as one should. Thanks for the pointer. I could look at doing that perhaps. You should have two counters of equivalent performance, preferably same model. It's a rather expensive approach IMHO. It may still be cheaper than the purchase of a counter capable of continuous collection, especially if you already have a counter that is capable at 1/2 f. Have a look at the possibility of picking up a HP 5371A or 5372A. You can usually snag one for about 600 USD or 1000 USD respectively on Ebay. I'd have to be a really good boy for Santa to bring me something of that ilk. Perhaps the lotto will come up one day :-) 73, Steve Cheers, Magnus ___ 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. -- Steve Rooke - ZL3TUV G8KVD JAKDTTNW Omnium finis imminet ___ 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] HP 1938 revisited
I, too, snagged one of these since it has the reputation of being the ultimate achievement of crystal oscillator technology with the goal of building a GPS controlled reference using a Brooks Shera controller card and a GPS receiver. Toward that end, since it takes a few minutes for the 1938 to 'lock', is there a signal that can be used to 'turn on' disciplining? It doesn't seem logical to try to discipline the oscillator until it has stabilized and, therefore, if the device is to be used from time to time, not staying on continuously, it will need a way to optimize turn on performance. Thanks in advance and I would appreciate any 'links' to any programs that would allow communication with the oscillator. Joe -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Richard (Rick) Karlquist Sent: Sunday, April 12, 2009 6:33 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] HP 1938 revisited wje wrote: Fluke.l (China) was selling a number of 1938's on Ebay. I snagged one just to have a piece of HP history. It works just fine, but I've noticed something a little strange. Comparing the 1938 to both my cesium and GPS standards, there's a distinct periodic 1ns phase shift every second. Seems to smoothly advance 1ns for 500 ms, then retard back to the original phase point over the next 500ms. Question: is this to be expected? I'm assuming this is from the AFC loop, but I would have expected it to be better damped. The AFC loop (as opposed to EFC) is a purely analog loop in which there is no mechanism for a 1 Hz oscillation. Now if you are talking about the EFC, that is another story. Depending on how you are driving the EFC, you can pickup noise from any number of sources. I vaguely remember there was an LED that flashed at 1 Hz if everything was working OK. You might see if that is the source of what you are seeing. I've searched both this group and the HP group; there really doesn't seem to be a great deal of info about these, other than schematics and some nice variance plots on leapsecond. Just what is the serial port and the PIC data lines useful for, if anything? The serial port can be used to change the oven temperature set point and IIRC monitor various oven parameters. Probably nothing you want to play with if you want a working oscillator. If you want to experiment, I have released the software to talk to the serial port. It should be archived somewhere. In production, we ramped the temperature up and down and found the exact turnover temperature for each individual oscillator and set the oven to the turnover. This required that we make crystals that actually had a turnover. Many 10811 crystals did not have turnovers, only an inflection point. Rick Karlquist N6RK Designer of E1938A ___ 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] HP 1938 revisited
J. L. Trantham wrote: I, too, snagged one of these since it has the reputation of being the ultimate achievement of crystal oscillator technology with the goal of Thanks, we thought it was pretty good :-) Toward that end, since it takes a few minutes for the 1938 to 'lock', is there a signal that can be used to 'turn on' disciplining? It doesn't seem logical to try to discipline the oscillator until it has stabilized and, therefore, if the device is to be used from time to time, not staying on continuously, it will need a way to optimize turn on performance. There is nothing in the E1938A that locks. The closest to lock is the oven current cutting back. After that, the oven will settle within a few minutes to its set point. However, the crystal will exhibit high aging rates for hours. It will not get back to the aging it had when last turned off for something like a day. During this recovery time, you can discipline the oscillator if you like, but it won't give very good holdover performance if you lose GPS. No crystal oscillator will give the kind of performance you need for this application unless it is continuously ovenized. The SC cut is a big improvement over the AT cut in terms of a cold start, but is still not adequate for GPS work right after a cold start. Hope that helps. Rick Karlquist N6RK Designer of E1938A ___ 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] time-nuts Rockwell Jupiter 12-channel GPS receiver OEM module
If nobody has already mentioned it, Fluke.1 has the Rockwell Tu00-D205 high performance 12-channel GPS receiver OEM modules for $9.99 ea with free shipping worldwide, item number: 290306684157 These appear to have the 10KHz output described at http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm tvb measured the performance at http://www.leapsecond.com/pages/gpsdo/ More documentation is at http://www.gpskit.nl/gps-readme.html Mike ___ 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] HP 1938 revisited
Thanks for the info. My plan is to develop a stable GPS disciplined reference suitable for use as a reference for Microwave work in the 10 GHz range that can be used in portable locations with relatively quick start up. Perhaps the 1938 would be better in the shop where it could be left on for weeks/months at a time and an LPRO 101 or a 10811 could be used for the portable application. Reasonably high drift rates could be accommodated if the GPS signal is reliable and the time constants (disciplining rate, drift rate, etc.) are appropriate. Thanks, Joe -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Richard (Rick) Karlquist Sent: Sunday, April 12, 2009 7:57 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] HP 1938 revisited J. L. Trantham wrote: I, too, snagged one of these since it has the reputation of being the ultimate achievement of crystal oscillator technology with the goal of Thanks, we thought it was pretty good :-) Toward that end, since it takes a few minutes for the 1938 to 'lock', is there a signal that can be used to 'turn on' disciplining? It doesn't seem logical to try to discipline the oscillator until it has stabilized and, therefore, if the device is to be used from time to time, not staying on continuously, it will need a way to optimize turn on performance. There is nothing in the E1938A that locks. The closest to lock is the oven current cutting back. After that, the oven will settle within a few minutes to its set point. However, the crystal will exhibit high aging rates for hours. It will not get back to the aging it had when last turned off for something like a day. During this recovery time, you can discipline the oscillator if you like, but it won't give very good holdover performance if you lose GPS. No crystal oscillator will give the kind of performance you need for this application unless it is continuously ovenized. The SC cut is a big improvement over the AT cut in terms of a cold start, but is still not adequate for GPS work right after a cold start. Hope that helps. Rick Karlquist N6RK Designer of E1938A ___ 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] femtosecond jitter anyone?
Chris The biggest problem with the OCXO is probably that it has a square wave output. With careful design it is possible to achieve a jitter of a few tens of femtosec for a logic level output from a limiter, but the OCXO designers are unlikely to have used such a limiter. [...] Bruce Bruce This would be an excellent subject for a tutorial on precision system design. Do you have any links to support your claim of a tens of femtosec for a logic level from a limiter? I am not aware of any logic family that can support that jitter performance. When you post items that stretch the state of the art, it would be nice if you would show us all how to do the same. Mike ___ 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.
