Filters are systems or elements that are used to remove substances such as dust or dirt, or electronic signals, etc. They play a critical role in many common applications, including power supplies, audio electronics, and radio communications. In signal processing, a filter is a device or process that suppresses some unwanted components or characteristics from a signal. Filtering is a kind of signal processing, the defining characteristic of filters being the complete or partial suppression of some aspect of the signal. In most cases, this means eliminating some frequencies or frequency bands.
However, filters do not operate exclusively in the frequency domain; especially in the field of image processing, there are many other objectives for filtering. Correlations can be eliminated for certain frequency components and not for others without having to act in the frequency domain. Filters are widely used in electronics and telecommunications, radio, television, audio recording, radar, control systems, music synthesis, image processing and computer graphics. Air filters are used to clean the air. Electronic filters clean electrical signals from unwanted frequencies in the audio and intermediate ranges, while RF and microwave filters serve the same function but in the radio and microwave frequency range.
EMI and RFI filters minimize or eliminate electromagnetic interference. Fluid filters remove debris from fuel filters remove contaminants from fuel, while hydraulic filters remove contaminants from hydraulic fluids. Water filters clean water, not only for drinking and swimming, but also in water treatment plants. Optical filters allow wavelengths of light to pass through and filter. Low-pass filters cut off all high-frequency parts of a signal, lower frequencies can pass the filter.
Low-pass filters are closely related to smoothing procedures, such as moving averages. BAW filters typically operate at frequencies from about 2 to about 16 GHz, and can be smaller or thinner than equivalent SAW filters. Notch filters eliminate a certain band from the frequency spectrum and are also made up of a combination of low-pass and high-pass filters. Although all of the above filters process fluids, fluid filters capture the less common forms of fluid filtration, such as food, wine and spirits, natural gas, automotive oil, etc. A filter circuit passes some frequency signals without any attenuation (amplitude reduction) or with some amplification & attenuates another frequency depending on the filter types. Hydraulic filters are primarily used to filter unwanted debris or contaminants from fluids for use in hydraulic cylinders or with other systems that utilize hydraulic fluids.
An alternative to transfer functions is to give the filter behavior as a convolution of the time domain input with the filter impulse response. Flow rate and pressure drop are important considerations when selecting fluid filters, since any filter element in a flow stream will lower the pressure of an incoming stream. Filters are usually replaceable and sold as complete sets of housings and filter elements, or are available only as elements.