Low Pass Filter

One of the uses of RC circuits is filtering signals. Dr. Fourier tells us that every wave-form is really a composition of sine waves of different frequencies. The low=pass filter lets us filter out the high=frequency components of a signal, letting us focus on the low frequencies we may be interested in.

A low pass filter looks like this:    

You can think of it as a voltage divider that's frequency dependent. Or, you can think of the high frequencies traveling straight through the cap to ground, and the low frequencies bouncing off the cap and getting sent to Signal Out.

The low frequencies keep most of their strength. The high frequencies are reduced. At a certain frequency, called f3db, the filtered strength of the frequency is exactly 3 decibels less than the original (or, about 70%).

We can calculate f3db with this formula:   f3db=1/(2*pi*RC)

So, if we know what frequencies we wish to filter out, we can choose an f3db accordingly. We can pick an R value to satisfy any impedance concerns, and then solve for the needed value of our capacitor C.

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