RF and Microwave filters are electronic components that are used to reject or allow only a specific selected frequency signal in order to eliminate noise or unwanted signals. For effective filtering of signals, the filter should have, the lowest Insertion Loss (IL) in the passband, the highest isolation/rejection out of band, the highest Q-Factor with reasonable size and power handling, excellent Passive Intermodulation (PIM) performance, and sharp transition slope.
RF filters are widely used in RF and Microwave wireless systems to filter and receive only the required RF signal from the wide range of transmitted RF and Microwave signals, by rejecting the unwanted signals. This article will cover the following topics.
What is RF Filter?
Types of RF Filters
Applications of RF Filters.
What are specifications need to consider in selection?
RF and Microwave filter Manufactures.
RF and Microwave filters are important component in wireless communication system like satellite, mobile, radar system, etc as it blocks the transfer of unwanted signal frequencies which degrade the quality of signal in the communication process.
RF (Radio Frequency) and Microwave filters are electronic components or devices, designed to operate in the megahertz to gigahertz frequency ranges which are used by most wireless communication systems. All the RF and Microwave devices will have some form of filters to pass or reject particular or multiple frequency bands.
RF and Microwave filters are generally made up of one or more coupled resonators. In general filter, design is similar to any technology used to make resonators. Filters can be made by using transmission lines, waveguides, lumped components like capacitors and conductors, etc.
The following books are good references for the design of RF and microwave filters.
RF and microwave filters are important components in wireless communication. RF filters are used in radio receivers to pass on the right kind of RF frequencies while filtering out other bands of frequencies received over the receiver. The filters are most frequently used in equipment such as broadcast radio, wireless communications systems operating in RF, Megahertz, and Gigahertz frequency ranges.
In general, RF and Microwave filters are made up of coupled resonators, Q-Factor (quality factor) of the material decides the level of filtering of the filters. Based on the type of material and technology used, there are many types of filters are available like cavity filters, dielectric filters, coaxial filters, planar filters, electroacoustic filters, etc.
In RF and Microwave filter design the simplest resonator structure is an LC tank circuit consisting of series and parallel capacitors and Inductors. These filters are very compact, but have relatively poor performance. Lumped-Element LC filters are mainly used to design LPF (Low Pass Filter) and HPF ( High pass filter).
RF and microwave filters can be designed by using Planar transmission lines, such as stripline, microstrip, and coplanar waveguide. Planar filters have better resonators, lower size, and better filter performance than lumped element filters.
The design processes of micro-strip filter circuits are similar to the processes used for printed circuit boards. Higher Q factors for the filter can be achieved by using low-loss tangent dielectric materials such as quartz and lower resistance metals such as gold.
Coaxial filter design is used for filters that need higher filter performance and higher quality factors than planar transmission lines. The coaxial resonators filters are designed using high-dielectric constant materials and they help to reduce the overall size.
Cavity filters are smaller in size and have higher quality factors than lumped element filters. In the microwave frequency range (I.e. above 1 GHz). In RF range like 40 MHz to less than 1 GHz frequency range, cavity filters offer high selectivity even in megawatt power loads conditions.
By increasing the internal volume of the cavity in the filters, we can achieve a higher Q quality factor and increased performance stability at closely spaced frequencies.
Filters designed using low-loss dielectric filters are called Dielectric filters. By using low-loss dielectric materials, we can design significantly higher-performance filters than other types of filters. High-dielectric constant materials help to design filters with lower overall sizes.
Filters designed using Electroacoustic resonators based on piezoelectric materials like quartz resonator is called electroacoustic filters. Electroacoustic resonators are smaller in weight and size than cavity resonators. Electroacoustic resonators are generally available up to tens of megahertz and for more than 100 MHz, thin-film technologies such as thin-film bulk acoustic resonator ( TFBAR) and surface acoustic wave (SAW) filters are available.
Filters designed using waveguide materials are called wave-guide filters. The waffle-iron waveguide filter is an example of this type of filter. Waveguides are hollow metal conduits through which an electromagnetic RF and microwave wave can be transmitted. Waveguide filters are designed using the Cavity inside the waveguide to allow some frequency of signals to pass, while others signals to reject.
Energy tunneling filters are the new class of microwave filters for highly tunable filter applications. The lower or higher frequency filters are implemented on waveguides, or on low-cost PCB with the help of switches inserted at appropriate positions in it to achieve a broad tuning range.
An RF and microwave filter is a passive multiport device that blocks the unwanted signals and passes discreet frequencies or blocks a discreet frequency and passes other frequencies through it. There are many types of filters designed based on frequency, application and power handling requirements like Low pass, High pass, Bandpass, Band reject, etc.
Eg: Radio communication transmitters use low-pass filters to block lower frequency harmonics signals that may interfere with originally transmitted signals.
A Low Pass Filter (LPF) is a 2-port component that is used to pass only a range of frequencies below a specified frequency (Cut of frequency) and to suppress the frequencies above that specific frequency. Low pass filters can be constructed using Lumped Element, Stripline, Coaxial, etc. In general, Low pass filters are designed using the Chebyshev type.
In a low pass filter, any frequency that is below the cutoff level will be directed to port 2. This function is accomplished by instituting insertion loss on the signal power for the higher frequency when passing through the Low pass filter circuit.
A high pass RF filter function is exactly the opposite of a low pass filter. Any RF frequency signal above a specific frequency (designed cutoff frequency) will be allowed to pass through the RF filter to an output port and all the other lower frequencies will be completely rejected. High pass filters can be designed using technology like Lumped Element, Coaxial, Waveguide, Stripline, etc.
An RF and Microwave Bandpass Filter (BPF) is a 2-port component that is used to pass a specific range of frequencies coming from port 1 to port 2 and to suppress the frequencies coming on either lower or higher side of that specified frequency range. Bandpass filters can be designed using technology like Lumped Element, Coaxial, Waveguide, Stripline, etc.
Bandstop / Notch filter (BSF/ NF) will work the exact opposite function of the bandpass filter. The bandstop filter is a 2 port component that offers a very narrow band rejection to the signal in a specified band and passes all the frequency signals on both sides( Lower and higher) of the frequency band. Bandstop filters are designed using Cavity, Stripline, Waveguide, Lumped elements, etc.
A diplexer is an RF passive device to implement frequency-domain multiplexing, which enables sharing of a common antenna between two distinct frequency bands. A diplexer is a 3-port Radio Frequency (RF) filter component which used to multiplex two different frequencies of 1st and 2nd ports into a third port.
Diplexers can be constructed using a set of 2 Bandpass filters that multiplexed to a third port by suppressing out-of-band signals on both paths. Two disjoint frequency bands on ports L (1st port) and port H (2nd port) both transmit and receive multiplexed onto a third port ( Antenna port) and signals on L and H can coexist on the third port without frequency interfering with each other.
A duplexer is a 3-port Radio Frequency (RF) component that is used for sharing a common antenna between the receive and transmit paths of an RF and Microwave communications system. Size, Insertion Loss (IL), and Rejection among the Tx and Rx channels are the prime factors that need to consider in selecting the duplexes.
In duplexer Frequency Division Duplexed (FDD) accomplishes by isolating the transmitted signal from interference caused by the received signal and also suppresses out-of-band signals on both Tx and Rx paths. A duplexer is a passive electronic device that allows bi-directional communication in radar and radio communications systems.
RF frequency and 3 dB cut of frequency of the filter needs to select as per the application, There are many filters manufacturers available with a wide range of frequency bands. A Sharp cut-of-the-filter helps to avoid unwanted signals.
Different types of filters are available like co-axial, microstrip, waveguide, etc… filters need to select as per the application.
VSWR of the filter is an important factor need to consider in selecting the filters. The ideal VSWR is 1.1:1 for the full band of operation.
Need to select the filters with lower insertion loss in the passband and higher loss in the out of the band.
Amplitude tracking is the variation of amplitude between the input and output of the filter. f ± 0.1 dB throughout the band is an acceptable variation.
Power handling of the filters is one of the prime features that need to consider in the selection of the RF and Microwave filters. The power handling capability of the filter should be higher than the maximum power expected to pass through the filters.
The operating range of -40ºC to +85ºC is ideal for the filter operation. For the specified applications, need to select the filter as per the required operating range.
Filters will be available will most of the connectors like SMA, BNC, N, TNC… As per the application need to select
The physical size of the filters including the position of the mounting screw needs to select as per the design space availability.
RF and Microwave filters are one of the prime components of any RF communication system. The selection of the right filter in the circuit helps to improve the quality of the RF signal which is transmitted and received in the communication systems.
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