[ntp:questions] NTP phase lock loop inputs and outputs?

David L. Mills mills at udel.edu
Wed May 28 18:08:55 UTC 2008


Bill,

Read it again. Judah takes multiple samples to reduce the phase noise, 
not to improve the frequency estimation.

Dave

Unruh wrote:

> You must have read a different paper than that one. I found it (through our
> library) and it says that if you have n measurements in a time period T,
> the best strategy is to take n/2 measurements at the beginning of the time
> and n/2 at the end to minimize the effect of the white noise phase error on the
> frequency estimate. That is perfectly true, and gives an error which goes
> as sqrt(4/n)delta/T rather than sqrt(12/n)(delta/T) for equally spaced
> measurements (assuming large n) T is the total time interval and delta is the std dev of each phase measurement . But it certainly does NOT say that if you have n
> measurements, just use the first and last one to estimate the slope. 
> 
> If you have n measurements, the best estimate of the slope is to do a least
> squares fit. If they are equally spaced, the center third do not help much
> (nor do they hinder), but a least squares fit is always the best thing to
> do. 
> 
> 
> "David L. Mills" <mills at udel.edu> writes:
> 
> 
>>Bill,
> 
> 
>>NIST doesn't agree with you. Only the first and last are truly 
>>significant. Reference: Levine, J. Time synchronization over the 
>>Internet using an adaptive frequency locked loop. IEEE Trans. UFFC, 
>>46(4), 888-896, 1999.
> 
> 
>>Dave
> 
> 
>>Unruh wrote:
> 
> 
>>>"David L. Mills" <mills at udel.edu> writes:
>>>
>>>
>>>
>>>>Bill,
>>>
>>>
>>>>Ahem. The first point I made was that least-squares doesn't help the 
>>>>frequency estimate. The next point you made is that least-squares 
>>>>improves the phase estimate. The last point you made is that phase noise 
>>>
>>>
>>>No. The point I tried to make was the least squares improved the FREQUENCY 
>>>estimate by sqrt(n/6) for large n, where n is the number of points (assumed
>>>equally spaced) at which the phase is measured. I am sorry that the way I
>>>phrased it could have been misunderstood.
>>>
>>>
>>>The phase is ALSO improved proportional to sqrt(n)
>>>. 
>>>This assumes uncorrelated phase errors dominate the error budget. 
>>>
>>>
>>>
>>>
>>>
>>>>is not important. Our points have been made and further discussion would 
>>>>be boring.
>>>
>>>
>>>Except you misunderstood my point. It may still be boring to you. 
>>>
>>>
>>>
>>>
>>>>Dave
>>>
>>>
>>>>Unruh wrote:
>>>>
>>>>
>>>>>"David L. Mills" <mills at udel.edu> writes:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Bill,
>>>>>
>>>>>
>>>>>>If you need only the frequency, least-squares doesn't help a lot; all 
>>>>>>you need are the first and last points during the measurement interval. 
>>>>>
>>>>>
>>>>>Well, no. If you have random phase noise, a least squares fit will improve
>>>>>the above estimate by roughly sqrt(n/4) where n is the number of points.
>>>>>That can be significant. It is certainly true that the end points have the
>>>>>most weight ( which is why the factor of 1/4). Ie, if you have 64 points,
>>>>>you are better by about a factor of 4 which is not insignificant. 
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>The NIST LOCKCLOCK and nptd FLL disciplines compute the frequency 
>>>>>>directly and exponentially average successive intervals. The NTP 
>>>>>>discipline is in fact a hybrid PLL/FLL where the PLL dominates below the 
>>>>>>Allan intercept and FLL above it and also when started without a 
>>>>>>frequency file. The trick is to separate the phase component from the 
>>>>>>frequency component, which requires some delicate computations. This 
>>>>>>allows the frequency to be accurately computed as above, yet allows a 
>>>>>>phase correction during the measurement interval.
>>>>>
>>>>>
>>>>>He of course is not interested in phase corrections. 
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Dave
>>>>>
>>>>>
>>>>>>Unruh wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>Unruh wrote:
>>>>>>>
>>>>>>>
>>>>>>>>>I do not understand this. You seem to be measuring the offsets, not the
>>>>>>>>>frequencies. The offset is irrelevant. What you want to do is to measure
>>>>>>>
>>>>>>>
>>>>>>>>Measuring phase error to control frequency is pretty much THE standard 
>>>>>>>>way of doing it in modern electronics.  It's called a phase locked loop 
>>>>>>>
>>>>>>>
>>>>>>>Sure. In the case of ntp you want to have zero phase error. ntp reduces the
>>>>>>>phase error slowly by changing the frequency. This has the advantage that
>>>>>>>the frequency error also gets reduced (slowly). He wants to reduce the
>>>>>>>frequency error only. He does not give a damn about the phase error
>>>>>>>apparently. Thus you do NOT want to reduce the frequecy error by attacking
>>>>>>>the phase error. That is a slow way of doing it. You want to estimate the
>>>>>>>frequency error directly. Now in his case he is doing so by measuring the
>>>>>>>phase, so you need at least two phase measurements to estimate the
>>>>>>>frequency error. But you do NOT want to reduce the frequency error by
>>>>>>>reducing the phase error-- far too slow. 
>>>>>>>
>>>>>>>One way of reducing the frequency error is to use the ntp procedure but
>>>>>>>applied to the frequency. But you must feed in an estimate of the frequecy
>>>>>>>error. Anothr way is the chrony technique. -- collect phase points, do a
>>>>>>>least squares fit to find the frequency, and then use that information to
>>>>>>>drive the frequecy to zero. To reuse past data, also correct the prior
>>>>>>>phase measurements by the change in frequency.
>>>>>>>(t_{i-j}-=(t_{i}-t_{i-j}) df
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>(PLL) and it is getting difficult to find any piece of electrnics that 
>>>>>>>>doesn't include one these days.  E.g. the typical digitally tuned radio 
>>>>>>>
>>>>>>>
>>>>>>>A PLL is a dirt simply thing to impliment electronically. A few resistors
>>>>>>>and capacitors. It however is a very simply Markovian process. There is far
>>>>>>>more information in the data than that, and digititally it is easy to
>>>>>>>impliment far more complex feedback loops than that.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>or TV has a crystal oscillator, which is divided down to the channel 
>>>>>>>>spacing or a sub-multiple, and a configurable divider on the local 
>>>>>>>>oscillator divides that down to the same frequency.  The resulting two 
>>>>>>>>signals are then phase locked, by measuring the phase error on each 
>>>>>>>>cycle, low pass filtering it, and using it to control the local 
>>>>>>>>oscillator frequency, resulting in their matching in frequency, and 
>>>>>>>>having some constant phase error.
>>>>>>>
>>>>>>>
>>>>>>>>>the offset twice, and ask if the difference is constant or not. Ie, th
>>>>>>>>>eoffset does not correspond to being off by 5Hz. 
>>>>>>>
>>>>>>>
>>>>>>>>ntpd only uses this method on a cold start, to get the initial coarse 
>>>>>>>>calibration.  Typical electronic implementations don't use it at all, 
>>>>>>>>but either do a frequency sweep or simply open up the low pass filter, 
>>>>>>>>to get initial lock.
>>>>>>>
>>>>>>>
>>>>>>>And? You are claiming that that is efficient or easy? I would claim the
>>>>>>>latter. And his requirements are NOT ntp's requirements. He does not care
>>>>>>>about the phase errors. He is onlyconcerned about the frequency errors.
>>>>>>>driving the frequency errors to zero by driving the phase errors to zero is
>>>>>>>not a very efficient technique-- unless of course you want the phase errors
>>>>>>>to be zero( as ntp does, and he does not). 
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>




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