It has been found that any repeating, non-sinusoidal waveform can be equated to a combination of DC voltage, sine waves, and/or cosine waves (sine waves with a 90 degree phase shift) at various amplitudes and frequencies.
Above excerpt from allaboutcircuits.com. I understand that a non-sine wave can be broken down into a combination of harmonic sine waves. What I dont understand is how it states above that it can also be from DC voltage which isnt alternating like a sine wave.
Could someone help clarify? Thanks