The sine waveSometimes, alternating current has a sine-wave, or sinusoidal, nature. This meansthat the direction of the current reverses at regular intervals, and that thecurrent-versus time curve is shaped like the trigonometric sine function. The waveformin Fig. 9-1 is a sine wave.Any ac wave that consists of a single frequency will have a perfect sine waveshape.And any perfect sine-wave current contains only one component frequency. In practice,a wave might be so close to a sine wave that it looks exactly like the sine function on anoscilloscope, when in reality there are traces of other frequencies present. Imperfec-tions are often too small to see. But pure, single-frequency ac not only looks perfect, butactually is a perfect replication of the trigonometric sine function.The current at the wall outlets in your house has an almost perfect sine waveshape,with a frequency of 60 Hz.The square waveEarlier in this chapter, it was said that there can be an alternating current whose mag-nitude never changes. You might at first think this is impossible. How can polarity re-verse 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. 9-2A). The oscillo-scope 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 onthe oscilloscope, because they’re instantaneous. But perfection is rare. Usually, thetransitions can be seen as vertical lines (Fig. 9-2B).A square wave might have equal negative and positive peaks. Then the absolutemagnitude of the wave is constant, at a certain voltage, current, or power level. Half ofthe time it’s +x, and the other half it’s -x volts, amperes, or watts. Some square wavesare lopsided, with the positive and negative magnitudes differing.Sawtooth wavesSome ac waves rise and fall in straight lines as seen on an oscilloscope screen. The slopeof the line indicates how fast the magnitude is changing. Such waves are called saw-tooth waves because of their appearance.Sawtooth waves are generated by certain electronic test devices. These waves pro-vide ideal signals for control purposes. Integrated circuits can be wired so that they pro-duce sawtooth waves having an exact desired shape.Fast-rise, slow-decayIn Fig. 9-3, one form of sawtooth wave is shown. The positive-going slope (rise) is ex-tremely steep, as with a square wave, but the negative-going slope (fall or decay) isgradual. The period of the wave is the time between points at identical positions on twosuccessive pulses.Sawtooth waves167