PulseWidth Modulation techniques can improve the efficiency of amplifier and other signalprocessing 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 resistorcapacitor lowpass filter. It removes any highfrequency 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.
Things You'll Need
 Pulse frequency specification
 Calculator
 Pencil and paper

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

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.

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.

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.

Label the resistor on the sketch with the resistance value you calculated, and the capacitor with the capacitance value you chose.
References
 Photo Credit oscilloscope waveform image. image by Ken Pilon from Fotolia.com