RF and microwave filter
Encyclopedia
Radio frequency
(RF) and microwave
filters represent a class of electronic filter
, designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency
to extremely high frequency
). This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi
, etc...), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers and diplexer
s to combine or separate multiple frequency bands.
the filter can achieve. The book by Matthaei, Young and Jones provides a good reference to the design and realization of RF and microwave filters.
Lumped-Element LC filters have both an upper and lower frequency range. As the frequency gets very low, into the low kHz to Hz range the size of the inductors used in the tank circuit becomes prohibitively large. Very low frequency filters are often designed with crystals to overcome this problem.
As the frequency gets higher, into the 600 MHz and higher range, the inductors in the tank circuit become too small to be practical. An inductor of 1 nanohenry (nH) at 600 MHz isn't even one full turn of wire.
transmission lines (as well as CPW or stripline) can also make good resonators and filters and offer a better compromise in terms of size and performance than lumped element filters. The processes used to manufacture microstrip circuits is very similar to the processes used to manufacture printed circuit boards and these filters have the advantage of largely being planar.
Precision planar filters are manufactured using a thin-film process. Higher Q factors can be obtained by using low dielectric materials for the substrate such as quartz or sapphire and lower resistance metals such as gold.
transmission lines provide higher quality factor than planar transmission lines, and are thus used when higher performance is required. The coaxial resonators may make use of high-dielectric constant materials to reduce their overall size.
Physical length of conventional cavity filters can vary from over 82" in the 40 MHz range, down to under 11" in the 900 MHz range.
In the microwave range (1000 MHz (or 1 GHz) and higher), cavity filters become more practical in terms of size and a significantly higher quality factor than lumped element resonators and filters, though power handling capability may diminish.
materials can also be used to make resonators. As with the coaxial resonators, high-dielectric constant materials may be used to reduce the overall size of the filter. With low-loss dielectric materials, these can offer significantly higher performance than the other technologies previously discussed.
can be used for filters. Since acoustic wavelength at a given frequency is several orders of magnitude shorter than the electrical wavelength, electroacoustic resonators are generally smaller than electromagnetic counterparts such as cavity resonators.
A common example of an electroacoustic resonator is the quartz resonator which essentially is a cut of a piezoelectric quartz crystal clamped by a pair of electrodes. This technology is limited to some tens of megahertz. For microwave frequencies, thin film technologies such as surface acoustic wave (SAW)
and, bulk acoustic wave (BAW) have been used for filters.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) and microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
filters represent a class of electronic filter
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
, designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency
Medium frequency
Medium frequency refers to radio frequencies in the range of 300 kHz to 3 MHz. Part of this band is the medium wave AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers...
to extremely high frequency
Extremely high frequency
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...
). This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
, etc...), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers and diplexer
Diplexer
A diplexer is a passive device that implements frequency domain multiplexing. Two ports are multiplexed onto a third port . The signals on ports L and H occupy disjoint frequency bands...
s to combine or separate multiple frequency bands.
Filter functions
Four general filter functions are desirable:- Band-pass filterBand-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
: select only a desired band of frequencies - Band-stop filterBand-stop filterIn signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter...
: eliminate an undesired band of frequencies - Low-pass filterLow-pass filterA low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
: allow only frequencies below a cutoff frequencyCutoff frequencyIn physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
to pass - High-pass filterHigh-pass filterA high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
: allow only frequencies above a cutoff frequencyCutoff frequencyIn physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
to pass
Filter technologies
In general, most RF and microwave filters are most often made up of one or more coupled resonators, and thus any technology that can be used to make resonators can also be used to make filters. The unloaded quality factor of the resonators being used will generally set the selectivityElectronic selectivity
Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to and reject other signals nearby in frequency, such as another broadcast on an adjacent channel....
the filter can achieve. The book by Matthaei, Young and Jones provides a good reference to the design and realization of RF and microwave filters.
Lumped-element LC filters
The simplest resonator structure that can be used in rf and microwave filters is an LC tank circuit consisting of parallel or series inductors and capacitors. These have the advantage of being very compact, but the low quality factor of the resonators leads to relatively poor performance.Lumped-Element LC filters have both an upper and lower frequency range. As the frequency gets very low, into the low kHz to Hz range the size of the inductors used in the tank circuit becomes prohibitively large. Very low frequency filters are often designed with crystals to overcome this problem.
As the frequency gets higher, into the 600 MHz and higher range, the inductors in the tank circuit become too small to be practical. An inductor of 1 nanohenry (nH) at 600 MHz isn't even one full turn of wire.
Planar filters
MicrostripMicrostrip
Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components...
transmission lines (as well as CPW or stripline) can also make good resonators and filters and offer a better compromise in terms of size and performance than lumped element filters. The processes used to manufacture microstrip circuits is very similar to the processes used to manufacture printed circuit boards and these filters have the advantage of largely being planar.
Precision planar filters are manufactured using a thin-film process. Higher Q factors can be obtained by using low dielectric materials for the substrate such as quartz or sapphire and lower resistance metals such as gold.
Coaxial filters
CoaxialCoaxial
In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
transmission lines provide higher quality factor than planar transmission lines, and are thus used when higher performance is required. The coaxial resonators may make use of high-dielectric constant materials to reduce their overall size.
Cavity filters
Still widely used in the 40 MHz to 960 MHz frequency range, well constructed cavity filters are capable of high selectivity even under power loads of at least a megawatt. Higher Q quality factor, as well as increased performance stability at closely spaced (down to 75 kHz) frequencies, can be achieved by increasing the internal volume of the filter cavities.Physical length of conventional cavity filters can vary from over 82" in the 40 MHz range, down to under 11" in the 900 MHz range.
In the microwave range (1000 MHz (or 1 GHz) and higher), cavity filters become more practical in terms of size and a significantly higher quality factor than lumped element resonators and filters, though power handling capability may diminish.
Dielectric filters
Pucks made of various dielectricDielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
materials can also be used to make resonators. As with the coaxial resonators, high-dielectric constant materials may be used to reduce the overall size of the filter. With low-loss dielectric materials, these can offer significantly higher performance than the other technologies previously discussed.
Electroacoustic filters
Electroacoustic resonators based on piezoelectric materialsPiezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
can be used for filters. Since acoustic wavelength at a given frequency is several orders of magnitude shorter than the electrical wavelength, electroacoustic resonators are generally smaller than electromagnetic counterparts such as cavity resonators.
A common example of an electroacoustic resonator is the quartz resonator which essentially is a cut of a piezoelectric quartz crystal clamped by a pair of electrodes. This technology is limited to some tens of megahertz. For microwave frequencies, thin film technologies such as surface acoustic wave (SAW)
Surface acoustic wave
]A surface acoustic wave is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.-Discovery:...
and, bulk acoustic wave (BAW) have been used for filters.