XCORR of two harmonic signals has a triangular shape. Why?

Hello! I have two signals (H and Z), which are almost harmonic. Both vectors contain 4018 elements each (I attach the picture). I want to calculate cross correlation between them, and what I get is shown on the second picture. My code is simple:

C=xcorr(H,Z,'coeff')

What can be the reason of such triangular shape? I tried to calculate xcorr of sin and cos, and even added trend to one of them, but it's not triangular, it must be also harmonic... Why can this happen?

Answer 1

[Christoph F.]

> What can be the reason of such triangular shape?

The reason is that both signals have a significant DC component/offset. Try removing it. For example

C=xcorr(H-mean(H),Z-mean(Z),'coeff')

or a more sophisticated method like highpass filtering or forward-backward highpass filtering with filtfilt.