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Self Resonant Frequency

The self-resonant frequency (SRF) is the resonant point where an inductor’s distributed capacitance resonates with its inductance. At this frequency, the inductive reactance equals and cancels out the capacitive reactance from the distributed capacitance, resulting in zero overall reactance. At the SRF, the inductor loses its energy-storage capability and behaves as a high-impedance pure resistance—meaning the Q factor of the inductor becomes zero at this point.

Inductor Self-Resonant Frequency

As shown in the frequency vs. impedance curve above, the peak corresponds to the self-resonant frequency. When the operating frequency is below the SRF, the inductive reactance increases with frequency; it reaches its maximum just before SRF. Once the frequency exceeds the SRF, the inductive reactance decreases with further increase in frequency.

Key takeaway: The self-resonant frequency defines the relationship between an inductor’s inductance value, its inductive reactance, and frequency.