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RF filters are passive or active components commonly used in the signal chain of virtually all radio and RF sensing technologies. The main purpose of a filter is to significantly attenuate unwanted portions of the spectrum, or specific frequencies, while only minimally impacting the operating frequencies. RF filters are typically constructed of various resonator technologies arranged, coupled, and connected in such a way that a desired frequency response is produced. Due to physical limitations, no filter is ideal, and the degrading parasitics and mechanical tolerances result in substantial constraints and trade-offs for every filter design.
Filters are generally categorized by their filter response; low pass, high pass, band pass, or band reject (notch) filter response. A low pass filter typically attenuates a wide range of frequencies above a desired cutoff frequency, while a high pass filter attenuates a wide range of frequencies below a certain cut off frequency. A band pass filter attenuates frequencies around a set range of desired frequencies, or the pass band. While a band reject filter performs the same function in reverse, and heavily attenuates a “notch” of frequencies while minimally attenuating all other frequencies.
Given the nonidealities of filter designs, no filter has an ideal filter response, and filters are categorized based on their frequency range of operation. For instance, a low pass filter may appear to behave near ideally for a certain frequency range, but then the filter response degrades at some higher frequency and the attenuation may roll off making the filter a limited low pass filter up to a maximum frequency of operation. A RF filter may also be limited in performance by the interconnect used to connect the resonator/filter elements, which may place further constraints on the frequency response of the overall filter design. For instance, a coaxial connector may only be designed for transverse electromagnetic (TEM) wave mode operation up to a certain maximum frequency, effectively limiting the filter frequency response below that cut off frequency.
The main RF filter performance parameters are the frequency response, including pass band, insertion loss, and corner frequency. Other important RF filter parameters include size, weight, construction material, operating temperature range, reliability, frequency response drift over time, among others.
Low pass filters are often used to suppress RF interference and harmonics generated by active RF components, such as amplifiers and mixers. Generally, the operating frequency is within the cut off of the low pass filter, and the undesirable interference or harmonics are above the low pass filters cut off. An example of this application is the use of a low pass filter on the output of a digital-to-analog converter (DAC) to reduce the high frequency harmonics that would otherwise degrade the output signal quality.
High pass filters are commonly used in applications where there is substantial noise or interference at lower frequencies which would interfere with the operation of higher frequency circuits. For instance a high frequency high power amplifier (HPA) may have a gain response to frequencies well below the desired operating frequency range and would amplify those undesirable signals along with the desired signals. In order to avoid that and achieve greater efficiency, a high pass filter may be used to attenuate the lower frequency bands with undesirable signals.
More commonly though, a band pass filter is used for transmission and reception applications, as the frequencies above a desired range and below a desired range can be attenuated with a single device. Band pass filters are often used at the inputs and outputs of active RF devices, such as mixers, amplifiers, oscillators, receivers, and transmitters. Band reject filters on the other hand, are slightly less common and are generally only used in circumstances where a known undesirable signal is interference with a radio or sensing equipment’s operation. For instance, if a passive intermodulation distortion (PIM) generator only responds to signals in a specific frequency range, a band reject filter may be useful in suppressing those signals and mitigating the PIM response. Similarly, if a known interfering signal is present in an environment, such as a high power radio station nearby, a very precise band reject filter can be used to suppress that specific signal. It is also the case that a spectrum governing body may order a transmitter producing illegal interference to cease operation unless they can stop the interference, and a band reject filter may be custom designed and inserted in the transmitter output to prevent the frequency range of the signal from being transmitted.
