rc filter calculator
ringo888,
The formula you propose is the correct one to use for a first order low pass filter with a corner frequency of f. However, you don't mention the kind of "smoothing" performance that you need. You can not get a high quality sine wave by running a square wave through a first order filter. A square wave consists of an infinite sum of fundamental and odd order harmonics.
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For example,
The amplitude of the 3rd harmonic voltage component of a square wave is 1/3 times the amplitude of the fundamental voltage. A 1st order filter attenuates at a rate of approximately 3 dB/octave in the stop band. Since the 3rd harmonic is 0.585 octaves above the fundamental, the minimum power ratio of third harmonic to fundamental that can be expected for a first order filter[(1/3)^2](.585) = .065 dB. The minimum voltage ratio is the square root of this value, or .255.
Regards,
Kral