FRF Plotting

I want to plot FRF vs Frequency graph.I have a matrix 'a' of acceleration & a force matrix 'f' both of 30000X1 size, collected in 3secs with a time step of 0.0001 sec.I am using the following steps to plot FRF. ffa=fft(a); fff = fft(f); frf, H=ffa./fff; aH =abs(H); x=0:0.1:30000; semilogy(aH) After plotting,I am not getting the correct first natural frequency at the first peak compared to the one I obtained from ANSYS (=14.5Hz).For plotting on Y-axis wether we have to take FRF in dB by using 20*log10(aH)?Where am I wrong in the procedure? Does the time steps have any role while finding fft?.

[Rick Rosson]

Please format your code.

Answer 1

[bym]

Your x-axis scaling is incorrect. When you do

semilogy(aH)

it plots the amplitude versus the matrix index. You need to scale the x axis. See

doc fft

for example and detailed explaination