Filter Basics Series Part 2: Designing Basic Filter Circuits

At the most basic level, filters are necessary in RF devices so that unwanted frequencies do not pass through the circuit and cause interreference. While filtering can become quite complex as operating frequencies increase, it can be made much less daunting by having an understanding of how basic filters are built using standard electrical components such as resistors (Rs), capacitors (Cs), and inductors (Ls).

Before we jump into types of filters though, let’s start by looking at how a voltage divider functions. A voltage divider is a passive linear circuit that produces an output voltage (Vo) that is a fraction of the input voltage (Vi). As shown in Figure 1, the voltage divider changes the V_i down to the V_o based on the values of the resistors used, which are R₁ and R₂ below.