Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges
Hi, it's funny, Randy seems to be the only engineer here really familiar with the Motorola innards. Aren't there any Motorola GPS engineers here?? Would love to hear some of the nitty gritty details of how the firmware/hardware works, what you guys think about the iLotus products, etc etc. For example, do the receivers get calibrated during production to minimize the effects of SAW filter variations etc? Could more modern tuners that use digital filters instead of SAW's perform better (see digital TV tuners from Xceive, NXP, etc)? Would it make sense to replace the TCXO with a good OCXO and use that crystal's output instead of a divided-down 1pps? How much does the Antenna LNA's noise figure affect the stability? The NDA's will hopefully expire soon, so please do spill out the secrets!! bye, Said ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of t...
In a message dated 12/21/2006 10:44:29 Pacific Standard Time, [EMAIL PROTECTED] writes: 1 second raw samples: 10.40 ns then removing small linear frequency offset, 1 second samples: 9.36 ns 30 second averages: 9.62 ns 300 second averages: 10.0 ns Did I do something wrong? PHK, what do you think about this? /tvb Hi Tom et. al, Has anyone tried looking at this sawtooth/bridge noise in the frequency domain, then using brick-wall FIR or better IIR low-pass filters below the modulation frequency of the bridges, and sawtooth? There are IIR low-pass filters that can be extremely efficient (120dB stopband attenuation) with only a small number of coefficients. Matlab is great to calculate these based on passband/stopband response etc. With a bit of memory, it is would be possible to do a 512-tap IIR or FIR low-pass filter for example with a 1/100Hz to 1/1000Hz cut-off frequency. The sampling frequency is 1Hz of course. For fun this filter could be scaled to 96KHz sampling frequency, and would then be equivalent to a 96Hz to 960Hz brick-wall low pass. Essentially simple averaging is a really poor-mans FIR low pass filter with all coefficients set to 1.0. We should be able to do so much better than that with IIR filtering, especially since the frequency behavior of the sawtooth, bridges, etc seems to be somewhat well understood and the filter can thus be highly optimized to filter out exactly these periodic noises. Has anyone tried something like this? bye, Said ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Wikipedia Dual-modulus Prescaler item
Dunno if it's still relevant, but the Wikipedia article on dual-mode prescalers is confusing, to say the least. Here's a much better description, written by a master of explaining the complicated to the uninitiated in simple terms - Dean Banerjee :- http://www.national.com/AU/files/PLL_Building_Blocks.pdf regards Grant Hodgson ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging
Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? To explore this I used TAC32 to record the 1 pps sawtooth correction message from a Motorola M12+ receiver for about 1 hour, during which time many bridges occurred (1). Excel's statistical toolbox was then used to examine the data. Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Averaging the sawtooth/bridge correction data for several averaging times produced the following results (2): Avg TimeStandard Deviation Residual Jitter none 8.4 nsec +/- 15 nsec 30 sec1.53+/- 4.3 100 sec 0.79+/- 2.2 300 sec 0.33+/- 0.7 It is evident that jitter is greatly reduced with a bit of time-averaging. In addition, the hanging bridges quickly disappeared into the residual jitter of the smoothed data. It appears to me that a typical GPSDO, which has an integration time in the range of 100's to many 1000's of sec is not likely to be impaired by the sawtooth/bridge noise of a GPS rcvr. A GPS-based clock is a different story since a precise 1pps timing signal without time averaging would be desirable. In summary, it appears that 1pps sawtooth/bridge noise can be ignored for a GPSDO. In some designs it may even be helpful by introducing further deterministic randomness to the phase measurement process. Regards, Brooks (1) the M12+ correction-message resolution is 1 nsec and this seems adequate for a jitter statistics investigation. But as a check, I compared the correction message data with the actual 1 pps jitter measured with a 5370B TIC, a PRS10 and a M12+ . This approach has higher resolution but does not change the conclusions. (2) I choose 30 sec as the shortest averaging time because 30 sec is the summation time of the phase-measuring circuit of my GPSDO design and hence the shortest integration time available. Of course, the PLL filter configuration switches can extend the
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges-theeffect of time averaging
Tom, lacking a Cesium or a Maser i can only perform such measurements INSIDE the closed loop of my GPSDO. I do not remember whether an Rb or my FTS1200 was used as the LO when this data was achieved, but as i tried to make clear that should be of no real concern with a loop constant of abt. 1200 s. Perhaps this is a good example that a lot of interesting measurements can be done even without owning the real expensive equipment. You should be able to generate you exactly the same plots when you take the SAW corrected data and the uncorrected data from the file that i have sent you. Best regards and a Merry Christmas for everyone in the group Ulrich Bangert, DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Tom Van Baak Gesendet: Freitag, 22. Dezember 2006 13:03 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges-theeffect of time averaging Have a look to http://www.ulrich- bangert.de/html/photo_gallery_44.html Ulrich, Thanks a nice one. What 1PPS reference did you use when that M12 data was collected? /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi- bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] LUCENT RFTG-m-RB
I would of replied, but I already turned my computer off for the night. To answer Stork's question, the GPS is not built into the RFTG RB module, it's built into the RTFG XO module. But the RB is disciplied off the XO/GPS, so there is still hope. Note - neither RFG model has/uses a GPS that I have seen. They must just phase lock off another signal? I did get a reply from a person who has both units and the chassis. The chassis is just an open frame with a few connections for splitting signals and whatnot. His XO unit got hit by lightning though so it is out of action right now. Anyhow, here's what he said about the pinouts on the RB: - On mine there's an interface cable that might be important to you. Its a dB9 male to male that connects J5 on the Rb to J5 on the XO. I'm guessing that it brings the GPS receiver info to the Rb module from the XO. Pinout looks weird. Crisscrossed row by row. 1 - 5 2- 4 3- 3 4- 2 5- 1 6- 9 7- 8 8- 7 9- 6 part number on the cable is 105589-001 The J6 RS422/1pps cable is also dB9 male. It connects up to the divider on the chassis. It brings a similar cable from the XO unit together in the divider/combiner and then outputs it as three dB15s. Part number of that cable is 105590-003. Jason It's interesting the way this group handles questions. Say you ask about a manual for this Lucent do-dad, and you get no answers. Here are some possibilities for no answer: Others consider you hopelessly incompetent because you did not find the manual in some obscure corner of the net by yourself. Others are focused on something else and have no time for you. Others do not find Lucent Rb cards interesting. The cards are crap, but nobody wants to break the news to you. There is no manual. Welcome to the joys of e-mail. Bill Hawkins A bunch of new LUCENT RFTG-m-RB listed on ebay. What's the bottom line? Are they or are they not GPS disiplined? Stork ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Wikipedia Dual-modulus Prescaler item
Thanks to all those that have replied to my query - it seems a simple concept, and the Wikipedia article just confused me. And thanks for that PDF link Grant - the four-modulus counter looks frightening though - I'll have to sit down with a cold towel round my head to think about that one :-) Peter ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging
Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? To explore this I used TAC32 to record the 1 pps sawtooth correction message from a Motorola M12+ receiver for about 1 hour, during which time many bridges occurred (1). Excel's statistical toolbox was then used to examine the data. Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Averaging the sawtooth/bridge correction data for several averaging times produced the following results (2): Avg TimeStandard Deviation Residual Jitter none 8.4 nsec +/- 15 nsec 30 sec1.53+/- 4.3 100 sec 0.79+/- 2.2 300 sec 0.33+/- 0.7 It is evident that jitter is greatly reduced with a bit of time-averaging. In addition, the hanging bridges quickly disappeared into the residual jitter of the smoothed data. It appears to me that a typical GPSDO, which has an integration time in the range of 100's to many 1000's of sec is not likely to be impaired by the sawtooth/bridge noise of a GPS rcvr. A GPS-based clock is a different story since a precise 1pps timing signal without time averaging would be desirable. In summary, it appears that 1pps sawtooth/bridge noise can be ignored for a GPSDO. In some designs it may even be helpful by introducing further deterministic randomness to the phase measurement process. Regards, Brooks (1) the M12+ correction-message resolution is 1 nsec and this seems adequate for a jitter statistics investigation. But as a check, I compared the correction message data with the actual 1 pps jitter measured with a 5370B TIC, a PRS10 and a M12+ . This
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging
Brooke Clarke wrote: Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? To explore this I used TAC32 to record the 1 pps sawtooth correction message from a Motorola M12+ receiver for about 1 hour, during which time many bridges occurred (1). Excel's statistical toolbox was then used to examine the data. Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Averaging the sawtooth/bridge correction data for several averaging times produced the following results (2): Avg TimeStandard Deviation Residual Jitter none 8.4 nsec +/- 15 nsec 30 sec1.53+/- 4.3 100 sec 0.79+/- 2.2 300 sec 0.33+/- 0.7 It is evident that jitter is greatly reduced with a bit of time-averaging. In addition, the hanging bridges quickly disappeared into the residual jitter of the smoothed data. It appears to me that a typical GPSDO, which has an integration time in the range of 100's to many 1000's of sec is not likely to be impaired by the sawtooth/bridge noise of a GPS rcvr. A GPS-based clock is a different story since a precise 1pps timing signal without time averaging would be desirable. In summary, it appears that 1pps sawtooth/bridge noise can be ignored for a GPSDO. In some designs it may even be helpful by introducing further deterministic randomness to the phase measurement process. Regards, Brooks (1) the M12+ correction-message resolution is 1 nsec and this seems adequate for a
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging
Hi Bruce: OK so the plot at will level off at about 5E-14. Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Dr Bruce Griffiths wrote: Brooke Clarke wrote: Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? To explore this I used TAC32 to record the 1 pps sawtooth correction message from a Motorola M12+ receiver for about 1 hour, during which time many bridges occurred (1). Excel's statistical toolbox was then used to examine the data. Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Averaging the sawtooth/bridge correction data for several averaging times produced the following results (2): Avg TimeStandard Deviation Residual Jitter none 8.4 nsec +/- 15 nsec 30 sec1.53+/- 4.3 100 sec 0.79+/- 2.2 300 sec 0.33+/- 0.7 It is evident that jitter is greatly reduced with a bit of time-averaging. In addition, the hanging bridges quickly disappeared into the residual jitter of the smoothed data. It appears to me that a typical GPSDO, which has
Re: [time-nuts] LUCENT RFTG-m-RB
Did anyone get the pinout for the p1 24v on the rfg-rb?? Mine has an LPRO. Norm n3ykf ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges- theeffect of time averaging
Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? You should continue to use 5000 second averagesare fine. When you use various ADEV programs they will be able to plot at 5000 s and any multiples of 5000 s, including 1e6. Have Fun, Brooke Clarke - Original Message - From: Brooke Clarke [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Friday, December 22, 2006 14:27 Subject: Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges- theeffect of time averaging Hi Bruce: OK so the plot at will level off at about 5E-14. Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Dr Bruce Griffiths wrote: Brooke Clarke wrote: Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? To explore this I used TAC32 to record the 1 pps sawtooth correction message from a Motorola M12+ receiver for about 1 hour, during which time many bridges occurred (1). Excel's statistical toolbox was then used to examine the data. Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges- theeffect of time averaging
Hi Tom: The goal is to get the C field set. If by using only 5,000 seconds I'm not getting the full precision of the GPS system, then a longer averaging time would allow more accurate setting, nes pa? Have Fun, Brooke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Tom Van Baak wrote: Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? You should continue to use 5000 second averagesare fine. When you use various ADEV programs they will be able to plot at 5000 s and any multiples of 5000 s, including 1e6. Have Fun, Brooke Clarke - Original Message - From: Brooke Clarke [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Friday, December 22, 2006 14:27 Subject: Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges- theeffect of time averaging Hi Bruce: OK so the plot at will level off at about 5E-14. Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Dr Bruce Griffiths wrote: Brooke Clarke wrote: Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: Brooks, Excel computed that the unaveraged correction data had a standard deviation of 8.4 nsec, which is consistent with the actual measured 9.5 nsec rms jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark and Hambly, p. 15). Even if this scientifical improvement has not found its way into Excel: A certain Mr. Allan has shown that the standard deviaton is NOT the appropiate measure for noise processes in oscillators. Therefore he had to find a new statitistics on its own. If you don't own a software to calculate ADEV and other relevant statistical measures with you may download one for free from my homepage: http://www.ulrich-bangert.de/plotter.zip But the question remains what time averaging is needed to reduce the sawtooth/bridge jitter from a typical +/-15 nsec to something negligible, perhaps +/-1 nsec? Have a look to http://www.ulrich-bangert.de/html/photo_gallery_44.html If you can read it it will immediatly give you the answer to your questions: in order to get to a certain precision draw a horizontal line at this precisision on the vertical axis and at the two crossing points read the necessary time for SAW corrected and uncorrected data on the horizontal axis. Nevertheless, pardon to contradict you: One simply has NO choice to average this long or to average that long. You have to set the regulation loop time constant up to exactly where the OCXO's tau-sigma-diagram meets the receiver's tau-sigma. Every loop time constant different from that is a faulty design and nothing else. The regulation loop dynamics may be improved a bit by pre-averaging the phase data before they are fed into the loop but not by computing the arithmetic mean over a time but by a gliding exponential average as is explained in detail in the PRS-10's handbook. Due to stability reasons even this time constant of this pre-filter is more or less fixed to abt. 1/3 the main loop's time constant. Regards Ulrich Bangert,DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera Gesendet: Donnerstag, 21. Dezember 2006 18:50 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] GPS orthodontics: sawteeth hanging bridges - theeffect of time averaging Recently there has been some mention of the influence of 1pps sawtooth and hanging bridges jitter on the performance of a GPSDO. It would seem to me that the jitter must average to zero in the long run, for if it did not the 1pps signal would drift away from its relation to UTC. But the question remains what time averaging is needed
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges -theeffect of time averaging
Knew there was something I liked about Ulrich! Laphroaig! I have a 30 and 40 in my collection... waiting for an excuse to open my 40. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] GPS orthodontics: time averaging theory
- Original Message - From: Dr Bruce Griffiths [EMAIL PROTECTED] To: Brooks Shera [EMAIL PROTECTED]; Discussion of precise time and frequency measurement time-nuts@febo.com Brooks Stop fooling yourself try reading: /Time Interval Averaging: Theory, Problems, and Solutions/, David Chu, HP Journal June 1974 pp12-15. Bruce Bruce Thanks for pointing out the interesting article by Dave Chu. It presents a fine statistical analysis of the uncertainties associated with time averaging, especially as it applies to his HP5345A counter design. His analysis seems to support my controller design as well. The pitfalls Dave mentions are: PARTIAL PULSE BIAS: very narrow gated clock pulses are not counted, thereby introducing a bias as computed in his eq(1). Note that all the parameters on the right side of eq(1) are constant, thus the bias is constant. A constant bias is important for a frequency counter or a TIC since all measurements will be slightly off, but for phase locking it makes no difference, it just moves the phase setpoint a tiny bit. Forget the synchronizer. COHERENCE: if time intervals are repeated at a rate coherent with the clock frequency, statistical averaging is impaired. Dave's 5375A design uses random clock phase modulation via a Zener diode noise source to avoid coherence. I used a cheap 24 MHz xtal drifting clock which is surely incoherent with GPS. When I designed my phase detector I was, of course, aware of the coherence issue and I considered various clock randomizing approaches: a VCXO driven by a random number generator (but randomness is hard for an VAX, early Bell Labs UNIX, clearly too much for a PIC), a Geiger counter modulated VCXO (requires a high voltage PS), etc. The cheap xtal won out. Forget the coherence issue. COMPUTE THE QUANTIZATION ERROR: for me, this is the most interesting part of Dave's paper . His results are summarized in eq(5) in the box on p.15. For the worst case, when the time interval being measured falls midway between 2 clock pulses, the rms uncertainty is T/(2 x sqrt(N)), where T is the clock period, and N is the number of measurements. For the 30 sec counter readout interval in my design this gives a worst-case rms uncertainty of 3.8 nsec. For a more realistic integration time of 1000 sec the rms worst-case quantization uncertainty is expected to be 0.66 nsec. Dave's eq(5) predicts a much smaller uncertainty when the average time interval duration is near an integral number of clock pulses. In fact, the uncertainty is zero when the averaged time interval is exactly an integral number of clock pulses. As it turns out, that is exactly the situation the phase lock loop is trying to achieve, an average phase equal to the phase setpoint - an integer! Dave didn't mention this interesting fact (he was building a counter, not a PLL). Of course, the PLL does not achieve a phase distribution whose average sits exactly on a integer, but it's probably fairly close and the quantization uncertainty should be significantly smaller than the worse-case values. In most cases you can... Forget the quantization error. Brooks ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridgestheeffect of time averaging
[ sorry for the earlier truncated posting ] Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? Or even a little better; perhaps under 1E-13. So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Brooke, You may continue using 5000 second averages. When you use various ADEV programs, they will be able to compute and plot at 5000 seconds and any multiples of 5000 s, including out to 1e6, and beyond. It's usually much better to collect many phase points at smaller tau, rather than wait for (and risk) for a few points at huge tau. By running longer you will see where your FTS 4060 hits its noise floor. It may occur well before 11 days, but you won't know until you take a month or two of data. A plain M12 is fine for this; you don't need a GPSDO. Here's a 12-day run that you'll like: http://www.leapsecond.com/pages/58503-cns2/ /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridgestheeffect of time averaging
[ sorry for the earlier truncated posting ] Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? Or even a little better; perhaps under 1E-13. So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Brooke, You may continue using 5000 second averages. When you use various ADEV programs, they will be able to compute and plot at 5000 seconds and any multiples of 5000 s, including out to 1e6, and beyond. It's usually much better to collect many phase points at smaller tau, rather than wait for (and risk) for a few points at huge tau. By running longer you will see where your FTS 4060 hits its noise floor. It may occur well before 11 days, but you won't know until you take a month or two of data. A plain M12 is fine for this; you don't need a GPSDO. Here's a 12-day run that you'll like: http://www.leapsecond.com/pages/58503-cns2/ /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridgestheeffect of time averaging
[ sorry for the earlier truncated posting ] Suppose that I'm now using the SR620 to make averages of 5,000 seconds and plotting those where the inputs are from an M12+T and a FTS4060 Cesium standard. At 5,000 seconds Ulrich's plot shows about 4E-12. Does that mean with a perfect standard I would expect to see noise of about 4E-12? Or even a little better; perhaps under 1E-13. So I should set the averaging to about 1E6 seconds (11 days)? to get the best possible result? Brooke, You may continue using 5000 second averages. When you use various ADEV programs, they will be able to compute and plot at 5000 seconds and any multiples of 5000 s, including out to 1e6, and beyond. It's usually much better to collect many phase points at smaller tau, rather than wait for (and risk) for a few points at huge tau. By running longer you will see where your FTS 4060 hits its noise floor. It may occur well before 11 days, but you won't know until you take a month or two of data. A plain M12 is fine for this; you don't need a GPSDO. Here's a 12-day run that you'll like: http://www.leapsecond.com/pages/58503-cns2/ /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridgestheeffect of time averaging
I have attached a ADEV plot I generated from some of my work. The sawtooth correction of the M12+ did not converge together on this plot - but you will see its coming together - but on some work I have done on a longer dataset, it did converge. The horizontal increment is binary 1,2, 4, 8, 16,etc in seconds. Noise is the HP53131A time interval counter noise floor M12 is the Motorola M12+ receiver M12SC is the Motorola M12+ receiver saw tooth corrected FRK-L is a rubidium FRS-C is a rubidium Z3801A is one of my two GPDSO FRS-GPS is the FRS rubidium disciplined by the Shera Controller rbgpsdo.pdf Description: Adobe PDF document ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS orthodontics: sawteeth hanging bridges-theeffect of time averaging
Brooke I'm not convinced that one can actually directly derive the GPS timing receiver stability from measurements taken within a feedback loop where the OCXO is locked to the GPS timing receiver output. Surely one has to Yes, I'm nervous about this approach too. correct for the loop transfer function. As tau approaches the loop time constant the accuracy of the stability measure as calculated from the uncorrected phase errors is degraded. Correcting for the effect of the loop transfer function will improve the accuracy a bit. The only way to measure the receiver timing stability is surely to measure it against a very accurate and stable standard was done when testing the M12+ receiver at USNO. When the outliers are rejected these tests indicate that the sawtooth one day stability is around 5E-14 or so, ie somewhat better than Ulrichs plots show. Bruce 5E-14 is exactly the right number. Have a look: A Comparison Between a 58503B and a CNS-II http://www.leapsecond.com/pages/58503-cns2/ /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] [Fwd: Re: GPS orthodontics: time averaging theory]
---BeginMessage--- Brooks Shera wrote: - Original Message - From: Dr Bruce Griffiths [EMAIL PROTECTED] To: Brooks Shera [EMAIL PROTECTED]; Discussion of precise time and frequency measurement time-nuts@febo.com Brooks Stop fooling yourself try reading: /Time Interval Averaging: Theory, Problems, and Solutions/, David Chu, HP Journal June 1974 pp12-15. Bruce Bruce Thanks for pointing out the interesting article by Dave Chu. It presents a nice statistical analysis of the uncertainties associated with time averaging, especially as it applies to his HP5345A counter design. His analysis seems to support my controller design as well. The pitfalls Dave mentions are: PARTIAL PULSE BIAS: very narrow gated clock pulses are not counted, thereby introducing a bias as computed in his eq(1). Note that all the parameters on the right side of eq(1) are constant, thus the bias is constant. A constant bias is important for a frequency counter or a TIC since all measurements will be slightly off, but for phase locking it makes no difference, it just moves the phase setpoint a tiny bit. Forget the synchronizer. This analysis neglects the problem of metastable states. Whilst these cannot be eliminated a simple shift register synchroniser can be employed to reduce the metastable state rate to less than once in the age of the universe or less if required. If one also wishes to produce output pulses in lock step with GPS time, these relatively large temperature dependent constant: offsets are not negligible. This constant offset should be particularly large with the relatively slow 4000 series CMOS counters employed. COHERENCE: if time intervals are repeated at a rate coherent with the clock frequency, statistical averaging is impaired. Dave's 5375A design uses random clock phase modulation via a Zener diode noise source to avoid coherence. I used a cheap 24 MHz xtal drifting clock which is surely incoherent with GPS. When I designed my phase detector I was, of course, aware of the coherence issue and I considered various clock randomizing approaches: a VCXO driven by a random number generator (but randomness is hard for an VAX, early Bell Labs UNIX, clearly too much for a PIC), a Geiger counter modulated VCXO (requires a high voltage PS), etc. The cheap xtal won out. Forget the coherence issue. This is wishful thinking, the same reasoning could be applied to the equally inexpensive crystal used in the Motorola M12+ GPS timing receivers, however the hanging bridges are a manifestation of near coherence. If this occurs with the M12+ timing receiver, why should your phase detector be magically immune to this? COMPUTE THE QUANTIZATION ERROR: for me, this is the most interesting part of Dave's paper . His results are summarized in eq(5) in the box on p.15. For the worst case, when the time interval being measured falls midway between 2 clock pulses, the rms uncertainty is T/(2 x sqrt(N)), where T is the clock period, and N is the number of measurements. For the 30 sec counter readout interval in my design this gives a worst-case rms uncertainty of 3.8 nsec. For a more realistic integration time of 1000 sec the rms worst-case quantization uncertainty is expected to be 0.66 nsec. Dave's eq(5) predicts a much smaller uncertainty when the average time interval duration is near an integral number of clock pulses. In fact, the uncertainty is zero when the averaged time interval is exactly an integral number of clock pulses. As it turns out, that is exactly the situation the phase lock loop is trying to achieve, an average phase equal to the phase setpoint - an integer! Dave didn't mention this interesting fact (he was building a counter, not a PLL). Of course, the PLL does not achieve a phase distribution whose average sits exactly on a integer, but it's probably fairly close and the quantization uncertainty should be significantly smaller than the worse-case values. You've gone a little astray here, the clock pulses of interest are the 24MHz pulses counted by your phase detector, whilst the phase lock loop attempts to lock the phase of the divided down OCXO output to the GPS derived PPS pulse it doesn't lock the 24MHz phase detector clock to anything. In most cases you can... Forget the quantization error. Brooks Bruce ---End Message--- ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] Picket fence technique
Using a GPS timing receiver to quantify the long term stability of an oscillator whose frequency is not a harmonic of 1Hz, then the technique of dividing the oscillator frequency down to 1Hz and logging the time delay between the GPS derived PPS pulse and the leading edge of the divided down reference frequency will incur several phase wraps during the monitoring period. Worse than this if one is using a TIC with a finite dead time (eg HP5370A/B) between measurements some the TIC will not measure some of these intervals. These difficulties can be circumvented by using a picket fence technique as devised by Greenhall to measuring beat frequencies in the paper: A Method for Using a Time Interval Counter to Measure Frequency Stability C. A. Greenhall Communications Systems Research Section This article shows how a commercial time interval counter can be used to measure the relative stability of two signals that are offset in frequency and mixed down to a beat note of about i Hz. To avoid the dead-time problem, the counter is set up to read the time interval between each beat note upcrossing and the next pulse of a 10 Hz reference pulse train. The actual upcrossing times are recovered by a simple algorithm whose outputs can be used for computing residuals and Allan variance. A noise-floor test yielded a df/f Allan deviation of 1.3 X 10 -9/r relative to the beat frequency. When quantifying the long term stability of an oscillator using this method one connects the PPS signal to the TIC START input and the picket fence frequency source (produced by dividing down the reference) to the STOP input. If the GPS pulse sawtooth correction and pulse epoch are recorded the sequence of time intervals measured by the TIC can be unwrapped using the algorithm outlined in the above paper. With a relatively low drift oscillator one need only divide its output down to 1MHz or so. Thus a simple inexpensive single chip divider such as 74HC4017 Johnson counter may be used for oscillator frequencies of up to 20MHz or so. The Question is has anyone considered using this technique? Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
