topic Re: Better Fit in Mathcad

Better Fit

awibroe — Thu, 03 May 2018 13:13:03 GMT

Hi to all,

In the attached (MCP3) file, can anyone give me some suggestions on how to get a better plot of f(x)? I have tried varying splines but the plot is quite erratic toward the end where as I would expect it to be quite uniform.

Thanks,

Andy

Re: Better Fit

DJF — Sat, 21 Apr 2018 14:16:54 GMT

Here's a Schumaker quadratic spline. Bit of a kink at 80, but it's closer than what you had.

I fit the quadratic and then fit that with a b-spline so you can use it easily.

4.0 and pdf attached.

Also, linear interpolation might be good enough for your data, but there's not much fun in that.

Re: Better Fit

Werner_E — Sat, 21 Apr 2018 19:41:06 GMT

I guess a polynomial of 9th degree is too wavy, too?

Its interesting that we can even demand for a higher order polynomial:

Re: Better Fit

LucMeekes — Sun, 22 Apr 2018 11:07:17 GMT

Is this the measured filter characteristic (X in Hz, Y in dB) of an elliptic low-pass filter? https://en.wikipedia.org/wiki/Elliptic_filter

Or a Tshebyshev low-pass filter? https://en.wikipedia.org/wiki/Chebyshev_filter

Then you should be able to find a better fitting with the corresponding filter formulae.

It would also seem then, that you did not measure enough points to accurately catch the passband ripple: considering the very steep roll-off after about 80 Hz I'd expect many more ripples in the passband.

Like this:

Success!
Luc

Re: Better Fit

awibroe — Tue, 24 Apr 2018 17:38:35 GMT

Hi, no this is points on a stability curve that I’ve created in MC for a ship. I extracted the points to a fresh sheet to make this easier. Yes I guess I could have simulated more points...

Cheers

Andy