[time-nuts] GPS orthodontics: sawteeth & hanging bridges- theeffect of time averaging
Brooke Clarke
brooke at pacific.net
Sat Dec 23 02:23:10 UTC 2006
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" <brooke at pacific.net>
>To: "Discussion of precise time and frequency measurement"
><time-nuts at 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: time-nuts-bounces at febo.com
>>>>>[mailto:time-nuts-bounces at febo.com] 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 Time Standard Deviation Residual Jitter
>>>>>none 8.4 nsec +/- 15 nsec
>>>>>30 sec 1.53 +/- 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 integration to many
>>>>>hours if desired.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>_______________________________________________
>>>>>time-nuts mailing list
>>>>>time-nuts at febo.com
>>>>>https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>
>>>
>>>
>>>
>>>
>>Brooke
>>
>>The GPS system has a frequency noise floor with an Allan deviation of
>>about 5E-14 for averaging periods of a day or more.
>>If you need more accuracy than this one has to use techniques like
>>Common view, All in view and related time transfer techniques to compare
>>one's frequency standards with more accurate standards located elsewhere.
>>
>>Bruce
>>
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>>
>>
>>
>>
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