Scope Measurement Hints

Analyze Harmonic Distortion Using FFT

With amplifiers so widely used in electronic devices, harmonic distortion is a common problem faced by design engineers.

To characterize a prototype amplifier, you can input a spectrally pure sinewave and look at the amplifier output on your scope. In this example (Figure 1), the sinewave looks distorted. To get a different point of view, try doing an FFT of the sine wave (Figure 2). Observe the harmonics.

The FFT gives you more quantitative information on how much harmonic distortion there is in the amplifier design. The fundamental frequency is at 50 kHz. The second harmonic at 100 kHz is only 17.81 dB down from the fundamental, indicating serious harmonic distortion. Built-in FFT capability in a scope lets you take a quick look at the frequency domain, in addition to the time domain.