Square waves are included because they are useful in tests in which a sinu­soidal input is not feasible and it is desirable to get the best results at a single frequency in the shortest time using binary input signals. The power spectrum of a square wave is given by:

Pk = 0.8M2//c2 for к odd

= 0 for к even

Since this spectrum is identical with the maximum (see Section 2.12), the square wave gives the greatest possible energy at harmonic frequencies. The first harmonic contains 81 % of the total signal energy, and the next nonzero harmonic (к = 3) contains only 9 %.