SFPL und SNR of Algorithms

Hello,

I would like to evaluate the Dynamic Range and Signal to Noise Ratio of different FM and AM demodulation algorithms, but I stumbled into a problem. When I use the functions Matlab provides [sfdr() and snr()] and measure against the noise floor, the value decreases (i.e. the noise floor increases) when I lengthen the simulation time. On the other hand, when I calculate the FFT with the function fft() the noise floor decreases with increasing simulation time.

Can somebody explain to me what is happening here? Or point me in the right direction to evaluate the SFDR und SNR of algorithms in Matlab?

Thank you so much!

On the lefthand side a simulation wir 2^15 points and on the righthand side with 2^25 points. The simulation results are similar with SFDR. Otherwise there are no differences between the simulations.