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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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!

__________________________________________

Yahoo! DSL Something to write home about.

Just $16.99/mo. or less.

dsl.yahoo.com

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FFT methods almost always have edge effects of various sorts. If padding

the ends of the measurement with real data works for your case, that seems

to me to be about the best thing to do. The farther the data you care about

is from the edges, the better off you will be. If you still have edge

problems, other common methods of dealing with them are to multiply the

data by a weighting function or add tapers to the ends of the data to bring

it down to zero. For example, you could multiply your spare data on the

ends by a quarter cycle of a sin wave that it is 0 at the ends and 1 where

it meets the data you care about.

--

Bill Ossmann

Philips Ultrasound

e-mail: bill.ossmann@philips.com

Vinod <papineni_vinod@yahoo.com> wrote on 12/07/2005 11:16:03 AM:

> 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!

>

>

>

>

> __________________________________________

> Yahoo! DSL â€“ Something to write home about.

> Just $16.99/mo. or less.

> dsl.yahoo.com

>

>

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