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 2 added 81 characters in body edited Apr 22 '15 at 20:12 Russell Borogove 61811 gold badge55 silver badges1212 bronze badges An analog signal has only one frequency unless it changes periods. That is absolutely not true, except for the special case of a sine wave signal. See Fourier series. Take a square wave, for instance, of period p. Its fundamental frequency (which is what you're thinking of) is f = 1/p, but it also has frequency components called harmonics of decreasing amplitudes at frequencies 3f, 5f, 7f... The lowpass filter graph shows that the gain (Vout / Vin) decreases as the frequency increases, so if you were to pass a square wave through the filter, the fundamental would remain strong, but the upper harmonics would be reduced in strength; as it turns out this tends to "round off" the corners of a square wave and make it more similar to a sine wave. An analog signal has only one frequency unless it changes periods. That is absolutely not true, except for the special case of a sine wave signal. Take a square wave, for instance, of period p. Its fundamental frequency (which is what you're thinking of) is f = 1/p, but it also has frequency components called harmonics of decreasing amplitudes at frequencies 3f, 5f, 7f... The lowpass filter graph shows that the gain (Vout / Vin) decreases as the frequency increases, so if you were to pass a square wave through the filter, the fundamental would remain strong, but the upper harmonics would be reduced in strength; as it turns out this tends to "round off" the corners of a square wave and make it more similar to a sine wave. An analog signal has only one frequency unless it changes periods. That is absolutely not true, except for the special case of a sine wave signal. See Fourier series. Take a square wave, for instance, of period p. Its fundamental frequency (which is what you're thinking of) is f = 1/p, but it also has frequency components called harmonics of decreasing amplitudes at frequencies 3f, 5f, 7f... The lowpass filter graph shows that the gain (Vout / Vin) decreases as the frequency increases, so if you were to pass a square wave through the filter, the fundamental would remain strong, but the upper harmonics would be reduced in strength; as it turns out this tends to "round off" the corners of a square wave and make it more similar to a sine wave. 1 answered Apr 22 '15 at 19:44 Russell Borogove 61811 gold badge55 silver badges1212 bronze badges An analog signal has only one frequency unless it changes periods. That is absolutely not true, except for the special case of a sine wave signal. Take a square wave, for instance, of period p. Its fundamental frequency (which is what you're thinking of) is f = 1/p, but it also has frequency components called harmonics of decreasing amplitudes at frequencies 3f, 5f, 7f... The lowpass filter graph shows that the gain (Vout / Vin) decreases as the frequency increases, so if you were to pass a square wave through the filter, the fundamental would remain strong, but the upper harmonics would be reduced in strength; as it turns out this tends to "round off" the corners of a square wave and make it more similar to a sine wave.