How to Build a Low-Pass Filter

Written by carlos mano
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How to Build a Low-Pass Filter
Low-pass filters allow frequencies below a certain limit to pass. (Digital Vision./Digital Vision/Getty Images)

Some electronic filters allow part of a signal to pass through while blocking other parts. These filters always have the word "pass" in their names: low-pass, bandpass and high-pass. Low-pass filters allow the lower frequencies in a signal while blocking sound waves above a certain limit. But this "clipping" is not completely clean, as most of the low frequencies get through and all of the high frequencies are greatly reduced. All frequencies are reduced somewhat unless an amplifying device -- such as a transistor -- is included in the filter.

Skill level:

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Things you need

  • Resistor
  • Capacitor or coil
  • Transistor (optional)

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  1. 1

    Construct a low-pass filter based on a couple of parameters. One of is the cut-off frequency, below which you pass the signal. The other consideration is "passive" or "active." Active low-pass filters include a transistor so signals passing through are not reduced. Passive filters come in two styles: RC and RL. Passive filters always reduce signals coming through, though only by a small amount if the signal is below the cut-off point. Reduction by a passive filter is almost complete if the signal is above the cut-off point.

  2. 2

    Build a low-pass filter that has cut-off frequency below 100,000 Hertz with a resistor and a capacitor -- an RC filter. The resistor is in series with the signal. The capacitor is across the signal, just before the resistor. High frequencies take a short path through the capacitor and avoid the resistor. The value of the capacitor determines the cut-off frequency. When the frequency is high enough, the capacitor offers little resistance to signals. If the frequency is low enough, the capacitor offers more resistance than the resistor, and the signal will go through the resistor.

  3. 3

    Build a low-pass filter that has a cut-off frequency above 100,000 Hz with a resistor and a coil -- an RL filter. The coil is in series with the signal. The resistor is across the signal, just before the coil. Low frequencies go straight through the coil because, at low frequencies, it offers little resistance. The value of the coil determines the cut-off frequency. Above the cut-off frequency, the signals find it easier to take a short return path through the resistor.

Tips and warnings

  • Neither style of passive filter sharply clips the higher frequencies. If better filtering is more important than expense, you can double the filter -- two resistors and two capacitors or coils -- to do a better filtering job. Increasing the size of the resistor also improves the filtering.
  • Doubling the filter or increasing the size of the resistor makes the filtering job better, but reduces the level of the desirable frequencies that do get through. Either enhancement makes it almost necessary to add a transistor to the filter for signal amplification.

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