"David L. Mills" <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> 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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