Harmonics are present in distorted sinusoidal voltage/current which can be resolved into series of sinusoidal wave having multiple frequency of distorted wave's frequency. Pure sinusoidal voltage having same frequency as distorted wave is called fundamental harmonics. Odd multiple frequencies are most liable for severe distortion of fundamental wave. Here 1kHz source is fundamental wave, 3kHz is 3rd harmonics, 5kHz and 7kHz are 5th and 7th harmonics respectively. Note the the amplitude of each gradually decreasing as their harmonics order increases. Resultant distorted wave has smaller amplitude than that of fundamental wave.
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