How to demodulate Amplitude Modulation in decay curves?

Hi I´m dealing with decay curves for my final paper of BSc.

Now I have the problem, that these decay curves have an amplitude modulation. You see a high-frequent-oscillation and something like a superposition with a low-frequent-oscillation.

The problem is, I want only the high-frequent oscillation. I´ve tried to filter the signal with butterworth, but it don´t work. Also I have tried mtalbs function amdemod, but it also doesn´t work. Maybe you have another idea how i can handle the Amplitude Modulation. Sorry for my bad english.

Thanks in advance.