RESONATICA
RF and resonant circuits

LC resonant circuit calculator

Enter two of the three values, the third is calculated. Typical question: which inductance do I need with my capacitor for a given frequency?

Free, no account needed

Calculate
µH
pF
MHz
Result
f = 5.033 MHz
Characteristic impedance √(L/C): 316.2 Ω · Period: 198.7 ns

Formula

f = 1 / (2π · √(L · C)). Rearranged: L = 1 / ((2π f)² · C) and C = 1 / ((2π f)² · L).

Example

10 µH and 100 pF give 5.03 MHz. For the outer ring of a Lakhovsky antenna (≈ 45 MHz) with 25 pF you would need about 0.5 µH.

Notes

  • Wiring and stray capacitances (leads, parts, a hand nearby) are often a few pF and pull the frequency down, noticeably with small C.
  • Component tolerances of 5 to 20 % are common; measure the real resonance with a dip meter, NanoVNA or spectrum analyser.
  • The formula holds for a low-loss circuit. Losses (quality factor Q) change the bandwidth, hardly the position of the resonance.