# vrf Compare Waveform to Ideal

Question asked by papineni_vinod on Dec 7, 2005
Bill,

Thanks for your suggestions. The fft method seems to work better for me,
however there was one problem - the start and end of the data points were
causing larger than expected errors.

In the acquired data, I have addtional points (100 in front/back) which I am
not using in the comparison. So I used this data to "pad" for performing the
fft, and that gave me good results.

I would like to know your opinion if this is cosidered an acceptable practice
and/or what are the implications of doing that.

Best regards,
Vinod

--- Bill Ossmann <bill.ossmann@philips.com> wrote:

> Vinod,
>
> I've been a little hesitant to jump in here because this sort of thing is
> fraught with the possibility of noise-induced errors or just simply
> misunderstanding of what you are trying to do. That said, IF you are just
> dealing with a pure sine wave, and you reliably know the expected amplitude
> and phase of the measurement (There will be gain error.), then you can use
> an arcsin or arccos function, asin(x) or acos(x) to find the phase of any
> point. These functions, of course, only give a phase range of pi radians.
> It will take some finagling to get the correct overall phase, especially
> near +/-1 in the presence of noise. If the Phase_diff.vee method works for
> you that's great. Phase and time are related by the frequency of the sine
> wave:
>
> phase = 2*pi*f*t
>
> so time error would be just the phase error divided by 2*pi*f:
>
> delta-t = delta-phase/(2*pi*f)
>
> --
> Bill Ossmann
> Philips Ultrasound
> e-mail:  bill.ossmann@philips.com
>
> Vinod <papineni_vinod@yahoo.com> wrote on 12/05/2005 03:20:24 AM:
>
> > Hello ALL!
> >
> > This was my original query...
> >
> > > How do I find the phase of a particular point in Sine wavefrom > data
> > (from an Oscilloscope), knowing start time, duration of
> > > waveform , and frequency of input (sine) signal.
> >
> > > How would I then determine the time error of each point, in
> > > relation to an ideal sinewave of the same frequency and
> > > amplitude.
> >
> > While I can find the pahse of the start/particular point in the
> > wavefrom data, I still have difficulty finding the "time error". The
> > requirement is to find the sampling error of the acquisition.
> >
> > The attched vee file has a sample waveform data, which was acquired
> > from an oscilloscope. I had a look at Bill Ossmann's "Phase_Diff.vee"
> > which seems close to what I want, but how to covert the phase error
> > into time error.
> > < http://www.oswegosw.com/SearchVRF.php?mi=9169 >
> >
> > Is there a different approach to this?
> >
> > Best regards,
> > Vinod
> >
> > P.S. Peter I have seen your suggestions. Thanks for the info!

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