The square wave

Earlier in this chapter, it was said that there can be an alternating current whose magnitude never changes. You might at first think this is impossible. How can polarity reverse without some change in the level? The square wave is an example of this.
On an oscilloscope, a perfect square wave looks like a pair of parallel, dotted lines, one with positive polarity and the other with negative polarity (Fig. 1A). The oscilloscope shows a graph of voltage on the vertical scale, versus time on the horizontal scale. The transitions between negative and positive for a true square wave don’t show up on the oscilloscope, because they’re instantaneous. But perfection is rare. Usually, the transitions can be seen as vertical lines (Fig. 1B).
A square wave might have equal negative and positive peaks. Then the absolute magnitude of the wave is constant, at a certain voltage, current, or power level. Half of the time it’s +x, and the other half it’s -x volts, amperes, or watts. Some square waves are lopsided, with the positive and negative magnitudes differing.

Fig. 1: At A, a perfect square wave. At B, the more common rendition of a square wave.