The Series R-L Network

Author: N.H. Crowhurst

Action of the series RL divider

A combination of resistance and inductance will also separate frequencies. Whereas a capacitor takes a flow of charge (current X time) to change the voltage across it, an inductor requires that energy be used to change the magnitude of the current passing through it. At low frequencies, the current has plenty of time to change, hence the voltage across the inductor does not fluctuate appreciably. (Most of the fluctuation appears across the resistor.) At high frequencies, the rate of fluctuation does not allow time for current in the inductor to change, and the resistor voltage and current are almost constant. (Most of the input voltage appears across the inductor.)

At a middle frequency, some of the voltage appears across both components, because the current changes, requiring a voltage across the inductor to make it do so. When a combined signal is applied across the resistor-inductor combination, the low frequencies get concentrated across the resistor, and the high ones across the inductor. Either of these arrangements would be quite satisfactory to separate voltages of different frequencies.