Designing Filters

As we discussed in this article, filters are a key building block used for eliminating signals that can interfere with the operation of Software-defined radio. An FM notch filter for instance is used to eliminate strong FM signals from 88 MHz to 108 MHz that can otherwise saturate a software-defined radio. Similarly, bandpass filters ensure that only signals of interest make it in and out of the SDR and everything else is eliminated.

What goes into an RF filter design?

In this video, devttys0 discusses basic RF filter design.

Basics of RF Filter Design

While it’s relatively easy to design a filter using one of many available software tools, achieving the desired filter performance in practice can be tricky. For instance, there are practical issues of losses due to the capacitive and resistive attributes of an inductor. The resistance of the winding is reflected in the quality factor (Q) of the inductor. In fact at a certain frequency, called the self-resonant frequency, the inductor no longer behaves like an inductor and presents a very high impedance. Accounting for all these non-ideal effects makes filter design challenging in real-life.

At higher frequencies, typically above 300 MHz, discrete inductors and capacitors cannot be used on account of the parasitic effects of leads. Put simply, inductor and capacitor values at these frequencies deviate from their ideal value. The filter therefore does not behave as designed. At these frequencies, one approach is to use distributed elements. These elements are typically etched onto a PCB.

The PCB inside a 20 GHz Agilent N9344C spectrum analyser showing various microstrip distributed-element filter technology elements

In the video below, vk2seb shows how to design a high frequency filter using distributed elements consisting of copper tape etched onto a PCB and a software tool called QUCS. Very rapid prototyping involved here and very low cost with no components involved!

Microwave RF Filter Design

Filter design is a very interesting topic. It is specially relevant in the age of software-defined radio where you’re trying to maximize dynamic range while running these systems over wide frequency ranges. As the airwaves become more crowded, it increases the need for filters.

Once you get familiar with the basics, it’s very likely that you will want to delve deeper into this topic. At this point in time, we recommend getting yourself a text book for study. Filter design is very rich in complexity – many people have dedicated entire careers to this specialty.

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