Meaning
Generation of an unwanted signal at twice the fundamental frequency of the input signal occurs due to even-order non-linearities in a device or substrate. The measurement of second harmonic Distortion is a key indicator of asymmetry in the transmission characteristics of a radio-frequency circuit. This distortion is bounded by the power level of the fundamental signal and the balance of the differential circuitry, which determines the degree of self-cancellation in symmetric systems.
Substrate Contribution
Asymmetric charge distribution at the silicon-on-insulator substrate interface creates non-linear capacitive behavior. This non-linearity is a major source of second harmonic signals in passive radio-frequency devices. Implementing a trap-rich layer prevents the modulation of this interface capacitance and minimizes the distortion.
Circuit Design
Differential circuit layouts help cancel out even-order harmonics by subtracting the out-of-phase signals. However, any amplitude or phase imbalance between the differential branches will limit the effectiveness of this cancellation. Designers must maintain strict symmetry in the layout to keep these emissions low.
Standard Requirement
Mobile communication standards impose strict limits on these emissions to prevent interference with other bands. Automated spectrum analysers are used to verify compliance under high-power conditions. This testing is crucial for ensuring that the device does not violate regulatory emission masks.
