Low pass Filter for Square wave

[balaji selva]

is there a low pass analog filter for square wave which does not cause ringing effect or conversion to sine wave.....
or please say which is best filter for square wave...


if so please give the circuit diagram of that filter...

 

[chuckey]

If your square wave is +- about 0V, then it will end up as a sine wave whose amplitude decreases depending on the filter. If the square wave is from a 1 to a zero, then putting it through a low pass filter will result in a DC level that is proportional to the mean DC of the square wave.
Frank

 

[albbg]

Any periodic signal can be seen as a sum of sinusoids (Fourier). We have a fundamental frequency plus an number ideally infinite of harmonics weighted in amplitude. A low pass filter will eliminates (o reduce in amplitude) a given number of harmonics depending on how the cut-off frequency of the filter is far from the fundamental. If the reduced hamonics are far from the fundamental the signal shape will be close to the original. Moving back the frequency of the filter towards the fundamental, the edge of the square wave will became more and more smoothed. Finally, if the frequency of the filter is close to that of the square wave, we will obtain a sinewave. However you have to define where, in frequency, and how much in amplitude the filter has to operate. Furthermore you have to define what's the acceptable smoothing of the edges. Also the order of the filter has an effect.
About the ringing this is due to the high Q; you can reduce (or eliminate) it f.i. adding resistors in parallel with the inductors. It has a cost, of course, mainly in terms of filtering effectivness.

 

[LvW]

is there a low pass analog filter for square wave which does not cause ringing effect or conversion to sine wave.....
or please say which is best filter for square wave...
if so please give the circuit diagram of that filter...

What about a lowpass with Thomson-Bessel response?

 

[cmontoya]

1) You want a filter that provides a linear phase response for the first 5 harmonics of your square wave. So if you have a 1mhz sq wave I would pick 7mhz as my cutoff. 5 harmonics make an OK looking square wave.

2) You want a filter that does not shift the phase of those first 5 harmonics, more than say 2-3deg each. To get this you may need to move your cutoff out depending on topology

3) You will always induce ripple with a low pass on a square wave, the question is how much ripple can YOU tolerate.