What are Spurious and Harmonics?

Spurious and Harmonics are spikes of unwanted signals that are found at the output of amplifiers. Ideally, the output signal of an amplifier should be a replica of the input signal with a higher amplitude. However, this can only be achieved in ideal conditions and practically, some unwanted signals are introduced in amplifiers. These unwanted signals are called Harmonics and Spurious signals. They are usually caused by the use of non-linear active devices like BJTs, FETs, MOSFETs, etc. It is up to the design engineer to reduce these unwanted signals as much as possible.

Any unwanted signal generated by the amplifier outside the frequency band of interest can be called a Spurious signal. Harmonics are a subset of Spurious signals found at integer multiples of the fundamental frequency of the signal. For example, if the fundamental frequency of signal is ‘x’ then harmonics will be found at 1x, 2x, 3x, 4x, ---, nx frequencies. Generally, second-order harmonic is the most prominent and subsequent harmonics are neglected.

It is important to keep harmonic and spurious levels low to ensure high fidelity of the output signal. If spurious levels are not kept low then can cause a number of errors for instance, it may cause bit errors in communication systems if spurious levels get high enough. In military operations, they can cause the radar or other detectors to show false targets which is not desirable.

Harmonics and Spurious levels are measured in ‘dBc’ which is the ratio of input signal power vs the carrier signal power.