Filter Basics
There are four common filter responses with respect to frequency:
Lowpass
A lowpass filter passes frequencies from DC to a specified corner frequency. Frequencies above this corner frequency are rejected. A lowpass filter is often used where harmonic filtering is required.
Highpass
A highpass filter passes frequencies above a specified corner frequency to some upper limit and rejects signal frequencies below this corner frequency. Highpass filters are used for rejecting low-end signals that may cause harmonics in band or for rejecting low-frequency noise.
Bandpass
A bandpass filter transmits a specific range of frequencies between lower and upper passband frequencies, rejecting frequencies both above and below these specified points. Bandpass filters are typically used for preselection or post filtering in frequency conversion applications to eliminate spurious and interfering frequencies on either side of the desired frequency.
Bandreject
A bandreject or notch filter rejects frequencies between the lower and upper corner frequencies and passes frequencies below and above the passband points. A bandreject filter acts to add a notch in an otherwise broad range of transmitted frequencies and can target specific undesired signals.
