Harping on Harmonics

Harmonics are spurious signal content (or spurious emissions) that are generated by a single frequency tone and nonlinear elements within a RF/microwave signal chain, or single-tone intermodulation distortion (IMD). The nonlinear elements create harmonics that are integer multiples of the fundamental frequency (often called the first harmonic). For instance, fo is the fundamental frequency and 2xf0 is the second harmonic, while 3xfo is the third harmonis, and so forth. 

Alongside harmonic spurious signal products are non-harmonic spurious content, often called spurs, though this can be confused as all spurious signal content is often referred to as spurs in some cases. Harmonic products are technically produced as an infinite number of integer multiples of the fundamental frequency, but the power level of the subsequent harmonics decline as the harmonic order increases. This results in the higher order harmonic products reaching the noise floor after the 3rd harmonic, or so, in most systems, and thus higher order harmonics are often ignored in single-tone IMD analysis.

Depending on frequency planning, harmonics products may be benign or troublesome. In some cases harmonics are easily filtered, in others harmonics may land within the frequency band of important communication signals. In the case of sensing applications, harmonics may be very undesirable as they may conflict with measurements. When harmonics occur in the receive band of a device and are self-generated, it is often called self-interference or self-quitting, as it may result in desensitization of a receiver.

The harmonics are often measured in terms of their relative signal strength to the fundamental frequency in decibels (dBc), or as a measure of the entire harmonic energy as a root mean square of the signal strength of all harmonic components. The harmonic product strength is a predictable fraction of the fundamental frequency signal strength. Hence it is relatively easy to calculate and account for harmonic distortion in RF systems.

Challenges of designing with harmonic consideration occur when dealing with complex communication systems with wide bandwidths and channels in large swaths of spectrum. This was the case with LTE and WiFi in recent years, and is becoming an increasing concern as additional spectrum is being allocated for cellular communications and wireless communications. In some instances, harmonics from these systems may be transmitted and interfere with very sensitive RF/microwave/millimeter-wave sensing and imaging apparatus. This is why there may be more stringent standards regulation on harmonic distortion in the latest wireless communication systems to prevent interference with their own sensitive receivers and other nearby systems.

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