Resonance — The Sweet Spot
What is Resonance?
Resonance happens when you combine an inductor and capacitor, and at one specific frequency, their reactances become equal and cancel each other out. At that frequency, the circuit behaves as if it's purely resistive — no reactance at all.
This is hugely important in radio because it's how we select one frequency and reject all others.
The Resonant Frequency Formula
This tells you: for a given inductor (L) and capacitor (C), there's exactly one frequency where they resonate. This formula appears constantly in radio — learn it well!
Example: A 10 μH inductor and a 100 pF capacitor resonate at about 5.03 MHz — right in the middle of the 60-metre band.
Series vs Parallel Resonance
There are two ways to arrange L and C, and they behave oppositely at resonance:
| Series LC | Parallel LC | |
|---|---|---|
| At resonance, impedance is... | Minimum (just R) | Maximum |
| At resonance, current is... | Maximum | Minimum from source |
| Used for... | Passing a desired frequency | Blocking a desired frequency (or selecting it in a tuned circuit) |
Radio example: A series-resonant circuit in an antenna trap passes the trap's frequency through, while a parallel-resonant circuit in a receiver's IF stage presents high impedance at the desired frequency, developing a large signal voltage across it.
Q Factor — How Sharp is the Tuning?
The Q factor tells you how selective a resonant circuit is. High Q = very sharp tuning (narrow bandwidth). Low Q = broad tuning.
Think of it this way: A high-Q circuit is like a sniper rifle — it targets one frequency precisely. A low-Q circuit is like a shotgun — it covers a wide range.
For a series RLC circuit: \( Q = \frac{X_L}{R} \). So less resistance = higher Q = sharper selectivity.