Click here to purchase the entire book in PDF format. RL Filters We saw in Section 2.5 that we can build a filter using the relationship between the resistance of a resistor and the capacitive inductance of a capacitor. The same can be done using a resistor and an inductor, making an RL filter instead of an RC filter. Connect an inductor and a resistor in series as is shown in Figure 2.35 and look at the voltage difference across the inductor as you change the frequency of the signal generator. If the frequency is very low, then the reactance of the inductor is practically 0 $\Omega$, so you get almost no voltage difference across it - therefore no output from the circuit. The higher the frequency, the higher the reactance. At some frequency, the reactance of the inductor will be the same as the resistance of the resistor, and the voltages across the two components are the same. However, since they are 90$^\circ$ apart, the voltage across either one will be 0.707 of the input voltage (or -3 dB). As we go higher in frequency, the reactance goes higher and higher and we get a higher and higher voltage difference across the inductor. This should all sound very familiar. What we have done is to create a first-order high-pass filter using a resistor and an inductor, therefore it's called an RL filter. If we wanted a low-pass filter, then we use the voltage across the resistor as the output. The cutoff frequency of an RL filter is calculated using Equation 2.90. $$f_{c} = \frac{1}{2 \pi R L}$$ (3.90)