Circuits & Electrical Power · Resonance
Where the two reactances cancel
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Where the two reactances cancel

Inductive reactance climbs with frequency and capacitive reactance falls. Put both in one circuit and there is exactly one frequency where they are equal — and being opposite in sign, they cancel completely. That frequency is resonance.

f0=12πLCf_{0} = \dfrac{1}{2\pi\sqrt{LC}}, read aloud f-nought equals one over two pi root L C. f0f_{0} is the resonant frequency in hertz, LL is the inductance in henries and CC the capacitance in farads. Note there is no resistance in it at all: R does not move where a circuit resonates. Take the root of the product first, then let 2π turn radians per second into hertz — drop that 2π and you have ω0\omega_{0}, a real quantity in rad/s, but 6.28 times the number you were asked for.

Resistance decides how SHARPLY it resonates. Q=1RLCQ = \dfrac{1}{R}\sqrt{\dfrac{L}{C}}Q equals one over R, root L over C. QQ is the quality factor, a bare number with no unit, RR is the loss resistance in the loop in ohms, and LL and CC are as before. The root L/C\sqrt{L/C} is the circuit's surge impedance in ohms, and dividing by R makes the whole thing dimensionless. More loss, lower Q, blunter peak — which is why a Q figure is really a statement about resistance.

And Q sets the width. BW=f0QBW = \dfrac{f_{0}}{Q}B W equals f-nought over Q, where BWBW is the bandwidth in hertz between the two half-power points either side of the peak. A Q of 100 on a 1 MHz carrier gives a 10 kHz window; that is how a radio picks one station out of the air. Chain the three together — f0f_{0}, then QQ, then BWBW — and you have described a tuned circuit completely. It is also a rehearsal for the boss.