# [time-nuts] Allan variance by sine-wave fitting

d.schuecker at avm.de d.schuecker at avm.de
Thu Nov 23 11:09:08 EST 2017

```The harmonics limit the perfomance if you want to find the frequency of a
given, sampled sinewave with linear methods. Thats at least my finding when
I built a device to measure grid frequency fast and with high accuracy. I
had to use high-Q digital filters for the fundamental. Their slow transient
response limited the speed of new frequency measurements.

Cheers
Detlef
DD4WV

"time-nuts" <time-nuts-bounces at febo.com> schrieb am 23.11.2017 16:34:39:

> Von: Tim Shoppa <tshoppa at gmail.com>
> An: Discussion of precise time and frequency measurement
<time-nuts at febo.com>
> Datum: 23.11.2017 16:35
> Betreff: Re: [time-nuts] Allan variance by sine-wave fitting
> Gesendet von: "time-nuts" <time-nuts-bounces at febo.com>
>
> I wonder how much a fitting approach is affected by distortion
(especially
> harmonic content) in the waveform.
>
> Of course we can always filter the waveform to make it more sinusoidal
but
> then we are adding L's and C's and their tempcos to the measurement for
> sure destroying any femtosecond claims.
>
> Tim N3QE
>
> On Wed, Nov 22, 2017 at 5:57 PM, Ralph Devoe <rgdevoe at gmail.com> wrote:
>
> > Hi,
> >        The fitting routine only takes up 40 uS of the 1 sec interval
> > between measurements, as shown in Fig. 1 of the paper. This is less
than
> > 10(-4) of the measurement interval. It just determines the phase
difference
> > at the start of every second. I don't think the filtering effect is
very
> > large in this case.
> >         The interesting thing is that good results are achievable with
such
> > a short fitting interval. One way to think of it is to treat the
fitting
> > routine as a statistically optimized averaging process. Fitting 40 uS,
that
> > is 4096 points at 10 ns/point,  should reduce the noise by a factor of
64
> > (roughly). The single shot timing resolution of the ADC is about 10 pS
(see
> > Fig. 4), so dividing this by 64 brings you down into the 100's of fs
range,
> > which is what you see.
> >
> > Ralph
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```