How to Design an RC Circuit to Smooth a PWM Signal

Written by j.t. barett
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How to Design an RC Circuit to Smooth a PWM Signal
An RC filter will smooth out a PWM signal. (oscilloscope waveform image. image by Ken Pilon from Fotolia.com)

Pulse-Width Modulation techniques can improve the efficiency of amplifier and other signal-processing circuit designs. An analogue 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 analogue content. To design the RC circuit, you need to know the pulse's frequency.

Skill level:
Moderate

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Things you need

  • Pulse frequency specification
  • Calculator
  • Pencil and paper

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Instructions

  1. 1

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

  2. 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.

  3. 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.

  4. 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.

  5. 5

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

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