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.
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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.
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.