Finding a function to best fit a curve

[kelly0303]

Hello! I want to fit a function to the curve I attached (the first image shows the full curve, while the second one is a zoom-in in the final region). Please ignore the vertical lines, what I care about is the main, central curve. It basically goes down slowly and then it has a fast rise. What is the best function that I can try to use to fit this kind of curve? Thank you!

 

[BvU]

What is it ?

Looks like an upside down resonance curve like

(from here)

Also: googling 'low-pass filter resonance curve' yields this nice video.

Perhaps you can try a log scale for the ordinate ?