 Cycles-per-time spectral density

In cycles per observation, frequency lies in $[-1/2,1/2]$ and $\gamma (h)=\int e^{2\pi ih\omega }{f}_c(\omega )d\omega$. Compared with angular-frequency density $f_a$, one has $f_c(\omega )=2\pi f_a(2\pi \omega )$. In particular, white noise of variance ${\sigma }^2$ has $f_c={\sigma }^2$.