How can the autocorrelation function of an oscillating time series always be positive? I have an oscillating time series which has a Lorentzian shaped power spectrum, centered about a dominant frequency.
After taking the autocorrelation of this time series, I see that it's nearly always positive.
Here's the normalized autocorrelation of my time series:

(a) is a zoomed out autocorrelation, and (b) is zooming into that short time region.
It's an oscillating time series, so the autocorrelation is oscillating as expected.
How can the autocorrelation almost always be positive though?
I'm confused because the autocorrelation of a sinusoid is another sinusoid, which oscillates between both positive and negative. My autocorrelation, however, oscillates only in the positive regime... What would cause this?

If you make your harmonic signal to have a zero mean you'd see the auto correlation have more negative values.
Since your signal is almost always positive and the correlation is a sum of the values it makes sense it will be almost always positive.
