RF and microwave filters are used to remove undesirable signals and prevent them from entering a system. The increase of wireless standards in the existing frequency bands has caused filters to play an increasingly crucial role in minimizing interference. They are designed to operate at specific frequency ranges and allow (or attenuate) RF signals at different frequencies. There are two types of frequency bands in RF filters – passband and stopband. Signals within the passband are allowed to pass with minimal attenuation while signals falling in the stopband are strongly attenuated.
Main filter parameters
Technology type: depending on the application and size of the wireless system, there are several filter types – notch filters, SAW filters, cavity filters, waveguide filters etc. Each of them has different properties and form factors.
Passband frequency (MHz): is the frequency range on which signals can pass through with minimal attenuation.
Stopband frequency (MHz): is the frequency range on which the signals are attenuated. The higher the attenuation the better. This is also known as isolation.
Insertion loss (dB): is the occurring loss from the signals travel through the passband frequency range. Lower insertion loss means better filter performance.
Stopband attenuation (dB): is the amount of signal attenuation which exist in the stopband of a given filter. The level of signal attenuation can vary according to their frequency.
Types of filters
RF filters can be divided into four different types. Each type rejects or accepts signals in a distinct way. By using the correct type of filter, it is possible to accept desired signals and reject those that undesired. The four basic types of RF filter are:
Low pass filters only allow lower frequencies to pass. Normally it is flat until the cut-off point, after which it rolls off.
High pass filters are in many ways the opposite of low pass filters. They only allow signals to pass that are higher than the cut-off frequency. Above the cut-off frequency it is nominally flat, and below it the reaction falls away at a rate determined by the order of the filter.
Band pass filters only allow signals within a certain frequency range to pass. Signals above and below the cut-off frequencies are attenuated. Signals within the allowed band of radio frequencies pass through.
Band reject filters are the opposite of band pass filters, because they reject signals within a certain frequency range. This type of filter is often used to remove undesired signals that are present in a system.
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