Frequency Content within a signal

Hello,

I am trying to get the frequency content of a signal. Basically, the measurements are encoders readings with noise. I want to find out which frequency is due to the encoder output and want to design a filter to filter out the unwanted frequencies. I know fft transforms the data from time domain, but I am having hard time remember how to read the graph/output of fft. My plan is to estimate the unwanted freq. to get the cut-off freq. for the band-pass filter.

Any help is greatly appreciated.

Answer 1

[Image Analyst]

Maybe you should try pwelch() instead (it's in the Signal Processing Toolbox). Basically zero out elements of the spectral array and transform back to see if it does what you want. The elements closest to the origin will be low frequencies while those farther away are higher frequencies. I do have some demos but it's for an image, not a 1D signal. For what it's worth, they're attached.

[Image Analyst]

By the way, have you considered using a lock in amplifier?