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A harmonic is a signal or wave with a frequency that is a ratio of another reference wave or signal. Depending upon the integer multiple of the frequency to the original frequency, the respective harmonic wave can be termed as 2f, 3f and so on where f stands for the frequency of the reference wave.
The term harmonic is applied in various fields like music, acoustics, electronic power transmission, radio technology and many more to refer to waves of any form which are related by their frequencies (whole-numbered multiples).
The term harmonic is used to refer to waveforms that correlate with each other based on their frequencies. It is always applied to any member of a harmonic series. A harmonic is a signal whose frequency is an integral multiple of the frequency of a reference signal.
For instance, if the reference or fundamental frequency is considered to be some value, f, the waves having the frequency 2f, 3f, 4f and so on are considered to be harmonic waves. Thus, if the fundamental frequency is known, the successive harmonic frequencies can be easily calculated. Signals occurring at 2f, 4f, 6f and so on are considered to be even harmonics and those with 3f, 5f, 7f are considered to be odd harmonics.
The sound waves emitted by many acoustic instruments are mostly perceived as harmonic waves.
All signals usually contain energy at harmonic frequencies in addition to the energy at the fundamental frequency. Only perfect sine waves have all their energy contained in the fundamental frequency. Some waves like square waves, sawtooth waves and triangular waves contain large amounts of energy at the harmonic frequencies also.
In wireless communication systems transmitters need to be designed in such a way that the least amount of energy is emitted at harmonic frequencies, as high energy at harmonics could disrupt the wireless services.