This website is dedicated to the theory & design of analogue RF filters. Whether you are a student, a design engineer or a teacher – I hope you find valuable information here. This website runs since 2011 and is regularly updated and extended.
Filters come in many shapes and forms. There is however, a common underlying theory that is quite independent of the physical realisation.
In today’s wireless world, the spectrum limiting properties of filters are needed more than ever before. State-of-the-art SDR – Software Defined Radios still have and require analogue filters in their front end. Input spectrum limiting through filters is necessary because most antennas have no significant selectivity and receiver front-ends need a “pre-selected” band-limited rf signal spectrum in order to function properly. Portable equipment, like your mobile phone (cellphone) are often exposed to a hostile interference and RF-noise environment. If all spectrum received by an antenna would go straight to the active frontend input, clear reception is almost guaranteed to be impossible. Filters deliver a clean & limited input signal spectrum to the receiver, ensuring clear reception of even the weakest signals. Cellular base stations are often co-located with other services. Without filters, this simply would not work. Satellite transponders can share one antenna for transmitting and receiving thanks to elaborate manifold type filter multiplexing networks.
Electromagnetic simulators (CST, HFSS, Sonnett, etc.) provide us with the finest tools for filter design, but these tools must be used properly when good designs are to be obtained. Filter “wizards” and similar tools are useful but using them without a good fundamental understanding of filters seldom results in optimal designs and major difficulties and project delays are often encountered.
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“The first thing to learn about the theory of microwaves is that the idea of impedance cannot be used as a substitute for thought.” (Dr. W.W. Hansen, 1942)