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Raiko17-Peridot
17-Peridot
January 24, 2011
Solved

Finding roots of a non-rational function

  • January 24, 2011
  • 2 replies
  • 8524 views

Hello folks,

I have a weird problem: for some measurement data I'm trying to find the roots. This data appears to be best approximated by a sine function. As MathCad's roots/polyroots function works with rational functions only (or am I wrong on this one?) I tried to use the MATCH function together with the control parameter "near".

Weird enough, it finds only a few of the roots (see graph in the attached MC 11 file). Why doesn't it work and how could I improve this?

Thanks in advance

Raiko

Best answer by MikeArmstrong

Slightly different approach and I have used your match function.

Mike

2 replies

M
MikeArmstrong1-Visitor
1-Visitor
January 24, 2011

Your fitting functions doesn't look right, have you tried using the smoothing functions with Mathcad?

Clipboard01.jpg

Mike

R
Raiko17-PeridotAuthor
17-Peridot
January 24, 2011

Hello Mike,

yes this fitting function is an approximation by utilising a sine-function. I tried smoothing, but I'm more confident with the sine function.

Nonetheless, this doesn't solve my problem: finding the roots. Do you believe that MATCH might work better with a smoothed data set?

Thanks

Raiko

R
RichardJ19-Tanzanite
19-Tanzanite
January 24, 2011

You don't have the correct x-axis for the zero points.

The data doesn't look very sinusoidal to me. If you look at the spectrum there's at least two main frequency components. I think even two frequency components could not model that data very well though.

I have included a different approach to find the zeros using a function that finds the maxima or minima in data (I've posted this function before).

R
Raiko17-PeridotAuthor
17-Peridot
January 24, 2011

Hello Richard,

you're correct in guessing that this isn't a pure sinusoidal function. It is a oscillator who's hampered by a second oscillator ; coupled therefore.

However, with this fit it is easier to determine the frequency of the oscillation. I still don't get it why the match function fails! Any idea?

Anyhow, thanks a lot for your time. Your approach yields better results.

Raiko

M
MikeArmstrong1-Visitor
1-Visitor
February 3, 2011
You'll find the "near" in Match at the Zeros function. Match(=,fir,"near") is assigned to Q.

I take it the "near" is used to instruct the Match function to find scalars close to zero?

Where did you find the use for this? Can't see it in any of the help folders?

Mike

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