what are the units when we plot the absolute of fft

Hey.

if a use the command plot(abs(fft(Data))) where Data has dimension 1xN, what do i have as units on x-axis? Is it Hz or rad/sample?

Thanks in advance

Answer 1

[Star Strider]

‘Is it Hz or rad/sample?’

Both, depending on the chosen interpretation. The ‘sampling frequency’ (‘Fs’) of the vector is by definition 1, so it could be interpreted as going from 0 Hz to Fs-1, although if it is shifted first with fftshift, it would go from -Fs/2 to +Fs/2 (or -1/2 to +1/2). Transforming that to radians would equate to (-pi to +pi).

This is the reason that it is best to use data with known sampling times, so that it is then possible to calculate a frequency vector, removing all ambiguity.

At least that is how I interpret it.