Hi, thanks to everyone who helped me with my last question, lots of responses and fast too :) I have a simulink model running where I am performing FM and measuring adjacent channel power. I can't really quantitise this inside simulink so I send the output of my FM modulator to my workspace where I perform the FFT. During the simulation i can watch the output of the modulator with a FFT block and things look good. When I modulate with a square wave there is alot of power in the adjacent channel (around -50dB) and if I remove the modulating signal so I just have a carrier, the adjacent channel is around -120dB. So I think all of this works fine. Back in standard matlab I wrote a small function to look at 50% overlapping frames (pretty much what the simulink fft block does) and calculates the average adjacent channel power ratio...which always ended up being around 35dB no matter what kind of modulating signal was being used...so something is wrong here. On closer inspection it seems that the PSD plot I get from the fft doesn't go any lower than about -50dB (in my frequency range of interest) compared to my simulink model which shows it around the - 80 to -100dB region. Has anyone experienced this before? or knows a work around? It could be caused by something that I'm doing wrong...but I've checked my working many times and it's really not that complicated:/ I'm just stumped for directions to go at this point. I'm happy to send plots/files etc to better explain the problem Thanks Aaron