- Lowpass filters do not allow sinusoids above a certain “cutoff frequency” (
*f*_{c}) to pass through them. - Highpass filters only allow sinusoids above a certain “cutoff frequency” to pass through them.
- The cutoff frequency of a filter is defined as the frequency at which the power transmitted is 1/2 the maximum power transmitted in the passband. This corresponds to a -3dB drop on a decibel scale. As the power of a signal is related to its amplitude squared, the cutoff frequency corresponds to an amplitude reduction of
.
- Bandpass filters only allow sinusoids above and below a certain frequency region to pass through them.
- Bandstop filters are designed to pass all sinusoids except those within a certain frequency region.
- Resonance filters accentuate sinusoids within a certain frequency region. They may or may not pass sinusoidal frequencies outside that region.
- Resonance filters are typically described in terms of their center frequency and quality factor (Q), which is given by the center frequency divided by the -3dB bandwidth. A higher
*Q*indicates a “sharper” resonance. - Allpass filters do not affect the magnitude characteristics of a signal (their magnitude response is equal to 1 for all frequencies) but have frequency-dependent phase characteristics.

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