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Radio Tuned Circuits

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Tuned Circuit Properties


  • Tuned circuits consist of an inductor coil and a capacitor (often variable). The examples in the image are designed for high transmitter voltages. Much smaller ones are used in receivers.
  • A tuned circuit has a very high impedance at its resonant frequency (ideally = infinity).
  • At other frequencies, its impedance is lower.
  • Tuned circuits are used to select or tune in radio stations on a particular frequency and reject all the others.

At the resonant frequency ...

  • the voltage across the tuned circuit is at its maximum value
  • the capacitor and inductor have equal effect on the circuit because they have equal reactance. XL = XC
  • the current in the capacitor is equal to the current in the inductor
  • the current in the tuned circuit is in phase with the signal generator voltage (see diagram below)
  • the current (to ground/earth) through an ideal tuned circuit will be zero

Tuned Circuit Measurements



Q or Quality Factor



Below the resonant frequency


Above the resonant frequency


At the resonant Frequency


Tuned circuits work like this ...

Here is a water analogy.


Tuned Circuit Animation


Ferrite Rod Tuned Circuit



This doubles up as the antenna and tuned circuit in low cost radio receivers designed for the long wave (LF) and medium wave (MF) bands.

The addition of the ferrite greatly increases the inductance of the coils. This reduces the resonant frequency.


Ceramic Resonators and Quartz Crystals

These devices also exhibit strong resonant behaviour. They can be used in filter circuits and oscillators replacing the conventional tuned circuit. The advantages are small size, low cost and a high Q factor. However they can't be tuned like an L C circuit.




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