I have a clearly 4Hz periodic signal (see image below). However when I apply a fourier transform, there is no peak at 4Hz. Also, when I do a power spectrum density (using Matlab's pwelch algorithm), there is no peak at 4Hz.
My original signal is sampled at 5ms, and is 141 bins (705ms) long.
Why is this & which alogithm will detect the 4Hz peak?