This is used to add a DC ground to an RF line. If the resistance is high compared to your transmission line's characteristic impedance, it chokes off the RF. Such a capacitor is called a "bypass capacitor".Ī high-value resistor can also be used to form an effective choke. Here a capacitor forms an RF short circuit, which is transformed to an open circuit at the input. Series inductors are often used as RF chokes, as well as quarter-wave structures like the one shown below. But nothing is ever as easy as it seems in microwaves!Īn RF choke is what engineers call something that doesn't pass an RF signal, but allows a DC or low frequency signal to pass through. Sure, these look like very simple designs. To learn more about this important concept in microwaves, especially in filter theory, see our page on Resonances of RLC circuits. Resonance is a term used to describe the property whereby a network presents a maximum or minimum impedance at a particular frequency, for example, an open circuit or a short circuit. Often band-pass filters are followed by a low-order low-pass filter to dispose of the reentrant modes. In the figure you can see the passband at 10 GHz, and the reentrant mode at 30 GHz (3/4 wave frequency). It uses quarter-wave sections as couplers, they couple similarly at their 3/4 wave, 5/4 wave, etc. These are called reentrant modes.īelow is an example of an coupled-line filter. Sometimes when you design a band-pass filter for 10 GHz, it also passes RF at 20 GHz or 30 GHz or 40 GHz. Check out our multiplexer page for more info. It can be formed by cascading a LPF and HPF, or using resonant structures such as a quarter-wave coupled lines.Ī multiplexer is a network that separates signals from a common port to other ports, sorted according to their frequency. A series capacitor or shunt inductor or combination of the two is a simple high-pass filter.Ī band-pass filter has filter skirts both above and below the band. The opposite of a low pass filter, an HPF passes higher frequencies and rejects lower ones. Yes we will add some figures here soon!!! A series inductor or shunt capacitor or combination of the two is a simple low-pass filter. This is a filter that passes lower frequencies down to DC, and rejects higher frequencies. Here's a page on a new class of reflectionless filters. Another way to do this is to use a diplexer and terminate the unwanted band. One way to make a reflective filter into an absorptive filter is to add an isolator to the filter's input. Our page on transmission line loss will explain the difference between attenuation and rejection.įilters that are matched outside of their stop band are called "absorptive filters". Where does all the energy go? That's up to you as a designer to figure out, and a big reason why filters are typically located between attenuators or isolators. They rely on impedance mismatching to reject RF energy. Waveguide filters - how about someone out there contribute on this topic for us?įilter design approaches Commonly used terminology for microwave filtersįilters are typically two port networks. Will include: topologies, design considerations, tolerance effects, cover effects for microstrip filters, design equations, detailed design procedure, and references. Planar resonator filters for microstrip or stripline (coming soon on a separate page) Resonances of RLC circuits (separate page)įilter response types (moved to a new page) New video for November 2018 Reflectionless filters (new for October 2016)įilter schematic symbols (separate page, link fixed thanks to Rhian!) Got some filter data you'd like to share with us? This email address is being protected from spambots. Some stuff is on this page, some stuff is on other pages, and some stuff is still missing. For the near future we will concentrate mostly on planar band-pass filters, then follow up with some lumped element examples.īelow is a clickable outline for our filter discussion. We aren't trying to duplicate all this knowledge here our goal is, as always, to provide you with a basic understanding of the subject and hook you up with some vendors that can help you out. A classic in this area is "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by Mattaei, Young, and Jones available free from our download area. New for September 2016: we have a video explaining an exact synthesis technique from Keysight.Ī note from the Unknown Editor: many textbooks have been devoted to filter design. This page has a short video and links to design tools. New for November 2018: we have a separate page on the differences between Chebychev, Bessel, Butterworth, Gaussian and Elliptical filter responses.
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