RF filters and microwave filters are devices that pass or reject signals by frequency. The filter design determines the amount of insertion loss and phase shift for signals that pass through the filter. There are several basic types of RF filters and microwave filters. Bandpass filters are active or passive circuits that pass signals from a specific frequency band and reject signals from out-of-band frequencies. Examples of bandpass filters include surface acoustic wave (SAW) filters, crystal filters, and cavity filters. Band reject filters (bandstop filters, notch filters) are tuned circuits that prevent the passage of signals within a specified band of frequencies. Low pass filters allow low frequencies to pass while rejecting high frequencies. By contrast, high pass filters pass high frequencies and reject low frequencies.
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