Meaning
Creation of unwanted signal components at integer multiples of the fundamental operating frequency occurs when a sinusoidal signal passes through an active or passive electronic device that exhibits a nonlinear response. This harmonic distortion generation is particularly troublesome in high-power transmitters where amplifiers are driven close to their saturation limits. The resulting spurious emissions can violate regulatory limits and cause severe interference to receivers operating in adjacent bands.
Engineers must characterize this behaviour across all operating temperatures and power levels to ensure reliable system operation. This characterization typically takes place during the initial laboratory validation of the RF front-end where spectrum analyzers measure the relative strength of the spurious signals.
Nonlinear Origin
Transistors and diodes operating in their non-linear regions are the primary sources of these spurious frequencies. During harmonic distortion generation, the current flowing through the device is no longer directly proportional to the applied voltage. This misalignment creates sharp peaks and valleys in the output waveform, which corresponds to the introduction of high-frequency harmonics in the frequency domain.
Spectral Impact
The energy diverted into these higher frequencies reduces the power available for the main communication channel and lowers the efficiency of the system. If the harmonic distortion generation is left unmanaged, the second and third harmonics can fall directly into the receive bands of other nearby co-located transceivers. This desensitization forces the system to retransmit packets, which degrades the overall network throughput.
Suppression Technique
Low-pass and band-pass filters are connected at the output of the power amplifier to attenuate the unwanted harmonic components before they reach the antenna. In advanced designs, digital predistortion techniques are applied to the input signal to pre-compensate for the non-linearities of the amplifier and suppress the harmonic distortion generation at its source. This active correction allows the amplifier to operate closer to its maximum output power without violating spectral mask requirements.
