What is a Low Pass Filter?

Problem:
What is a Low Pass Filter?

Solution:
A Low Pass filter is a filter that passes low frequencies and attenuates high frequencies. The Knowledgebase discusses Single Pole and Two Pole filters. For filters with more poles, please contact Texas Instruments Applications.

The amplitude response of a low pass filter is flat from DC or near DC to a point where it begins to roll off. A standard reference point for this roll-off is the point where the amplitude has decreased by 3 dB, to 70.7% of its original amplitude. The region from at or near DC to the point where the amplitude is down 3 dB is defined as the passband of the filter. The range of frequencies from the 3 dB point to infinity is defined as the stopband of the filter.

The amplitude of the filter at ten times the 3 dB frequency is attenuated a total of 20 dB for a one pole filter, and a total of 40 dB for a two pole Butterworth filter. At higher frequencies, the amplitude continues to roll off in a linear fashion, where the slope of the line is -20 dB per decade (10 times frequency) for a single pole filter and -40 dB per decade for a two pole filter.

A very small detail that might slip by the beginning designer is that the initial roll-off in the region below the 3 dB frequency starts later for a two pole filter than it does for a single pole filter. Therefore, the two pole filter has "flatter" response at a higher frequency than the single pole filter. If the reader has not already discovered this - clicking on the figure will open a larger version that will show this detail better.



There is a strategy for increasing the roll-off of a two pole low pass filter, and one for making the group delay constant. These are discussed in Knowledgebase items covering Chebyshev and Bessel types of filter, respectively.

The phase response of a low pass filter doubles as well. The change in phase of a single pole filter is 45 degrees at the 3 dB frequency, and ultimately approaches 90 degrees at infinity. The change in phase of a two pole filter, however, is 90 degrees at the 3 dB frequency, and ultimately approaches 180 degrees at infinity.