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Tuned circuit

Ever wondered how on earth a radio (or a TV set) is capable of picking up the right channel out of hundreds of channels available? One way of doing this using a tuned circuit. A tuned circuit consists of a capacitor and a parallel-connected inductor. Usually, the capacitor can be tuned: you can change its capacitance by turning the attached knob. Actually, this circuit is the band pass filter from a previous section. So the bandwidth is equal to B=f/Q and the resonance frequency can be calculated using:

The notch must be small enough to block all other channels, but wide enough to allow the full signal of the selected channel to pass.

When the antenna picks up a signal close to this frequency, the tuned circuit will resonate on that frequency. Just look what happens if the antenna picks up a 159kHz radio signal:

Another radio station broadcasts at 149kHz, just 10kHz below the resonant frequency. The image below shows the signal for this frequency.

The difference is obvious. The detector (a circuit that demodulates a radio signal to audible sound) will only work on the 159kHz signal; the 149kHz signal is simply too weak.

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