How to Design an RC Circuit to Smooth a PWM Signal

By J.T. Barett
An RC filter, a PWM signal
oscilloscope waveform image. image by Ken Pilon from <a href=''></a>

Pulse-Width Modulation techniques can improve the efficiency of amplifier and other signal-processing circuit designs. An analog signal, such as from sound, modulates the width of a pulse having a frequency over 100,000Hz. The circuit then amplifies or transmits the pulse. The last step gets rid of the pulse using a simple resistor-capacitor low-pass filter. It removes any high-frequency signals, such as the pulse wave, leaving only the original analog content. To design the RC circuit, you need to know the pulse's frequency.

Step 1

Select a capacitor value with which you'd like to work. For example, you can arbitrarily choose 1 nanofarad.

Step 2

Calculate the value of the resistor in the lowpass filter. Use the following formula:

R = 1 / (2 x pi x C x Fp) where R is the resistor's value in ohms, pi is 3.1416, C is capacitance in farads and Fp is the pulse's frequency in hertz. If the capacitance is 1 nanofarad and the frequency is 100,000Hz, then resistance works out to 1,600 ohms.

Step 3

Compare the calculated resistance value to the available EIA E24 standard resistor values. Since 1,600 ohms, or 1.6K ohms, is already a standard value, you can use that. Otherwise, use the next closest value resistor.

Step 4

Sketch the RC lowpass filter circuit. Draw the horizontal zigzag symbol for the resistor. Connected to the resistor, draw a short horizontal line. Two inches underneath, draw a parallel line as long as the resistor and top line. Connect a vertical capacitor symbol between the two parallel lines.

Step 5

Label the resistor on the sketch with the resistance value you calculated, and the capacitor with the capacitance value you chose.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."