Meaning
Sinusoidal frequency components generated by non-linearities in electronic circuits appear at exactly twice the frequency of the fundamental input signal. The second harmonic represents this specific frequency term, acting as an inevitable byproduct of signal amplification or switching in active devices. It applies to radio frequency transmitters and power conversion systems.
This component is bounded on the high side by the bandwidth of the system and on the low side by the linearity of the active components.
Signal Distortion
Active devices introduce waveform distortions when driven close to saturation. This behavior produces a second harmonic that can interfere with neighboring receiver bands if left unmitigated. This requires physical mitigation.
Filter Design
Radio frequency front ends use high-rejection filters to suppress unwanted out-of-band emissions before they reach the antenna. The second harmonic lies far enough from the fundamental frequency that it can be attenuated effectively using compact lowpass or bandpass filter networks. Engineers select passive components with low equivalent series resistance to construct these filters without adding insertion loss.
This filtering stage ensures that the output signal remains clean and complies with international communication standards.
Measurement Verification
Spectrum analyzers are used during product qualification to measure the power level of spurious emissions. When engineers test for the second harmonic, they measure its amplitude relative to the carrier signal to calculate the harmonic suppression in decibels. This testing must be conducted across all operating bands and temperatures to guarantee consistent performance.
It ensures compliance with FCC and ETSI regulations.
