The simplest low-pass filter circuit is nothing more than a resistor and capacitor:

Low-frequency voltage signals applied to this circuit emerge at the output terminal relatively

unattenuated, because the reactance of the capacitor is quite large at low frequencies. High-frequency

signals applied to the same circuit become attenuated by the capacitor, which tends to “short” those

signals to ground with its low reactance to high frequencies. The performance of such a filter circuit

is primarily characterized by its cutoff frequency, mathematically defined as

unattenuated, because the reactance of the capacitor is quite large at low frequencies. High-frequency

signals applied to the same circuit become attenuated by the capacitor, which tends to “short” those

signals to ground with its low reactance to high frequencies. The performance of such a filter circuit

is primarily characterized by its cutoff frequency, mathematically defined as

The cutoff frequency is the point at which only 70.7% of the input signal appears at the output (a -3 dB attenuation in voltage).

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