Spectrum Decreasing Detection

Hi,

How can I build a model in simulink that detects decreasing of spectral components ?

For example, if I have a signal with a spectrum between 0 and 100Hz and at particular instant of time it turns between 0 and 50Hz gradually, how can I detect when it decreases to 10% of initial spectrum ?

I hope I'm clear in the explanation.

[Wayne King]

When you say in Simulink, do you have the DSP System Toolbox?

Wayne

[cyberdyne]

Yes.