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 is necessary because most antennas have no significant selectivity and receiver front-ends need a “pre-selected” band-limited rf signal spectrum. Portable equipment, like your mobile phone (cellphone) are often exposed to a hostile interference and noise environment. If all received spectrum 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, they simply would not function. Satellite transponders can share one antenna thanks to elaborate filter multiplexing networks.
Many universities teach filter theory and some do fine filter research work. This website may help students who want to study filters. 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 delays are often encountered.
“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)