selfresonant

Self-resonant describes the frequency at which a passive electronic component or structure resonates using its own parasitic elements, rather than an externally added resonator. At this self-resonant frequency, the component behaves like a small LC resonator, and its impedance becomes predominantly resistive with energy exchanging between reactive parts.

In inductors, the winding’s parasitic capacitance between turns forms a parallel LC circuit with the winding inductance. The self-resonant frequency is approximately f_sr = 1/(2π sqrt(L * C_p)). As frequency rises toward f_sr, the inductor’s impedance transitions from inductive to capacitive; beyond f_sr it ceases to behave as an ideal inductor. The quality factor often deteriorates near the resonance due to energy losses in the parasitics and core.

In capacitors, parasitic inductance from leads and internal structure forms a series LC circuit with the nominal capacitance. The self-resonant frequency is f_sr = 1/(2π sqrt(C * L_p)). Near f_sr, the capacitor presents a very low impedance, and above f_sr its behavior becomes inductive. This limits the usable high-frequency range of many capacitors, especially in small packages where parasitics are more significant.

Outside discrete components, self-resonance also describes distributed structures such as antennas, where inductive and capacitive effects along the length create resonance at a characteristic frequency. In practice, engineers account for self-resonance in RF design by selecting components with appropriate SRF, minimizing parasitics, or avoiding operation near the resonance. Measurement methods include impedance spectroscopy and network analysis to identify SRF and its impact on circuit performance.