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# Low pass Filter for Square wave

Discussion in 'The Projects Forum' started by balaji selva, Dec 2, 2013.

1. ### balaji selva Thread Starter New Member

Nov 29, 2013
20
0
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...

2. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
Why do you want to filter the square wave?

3. ### wayneh Expert

Sep 9, 2010
16,102
6,219
I believe a low-pass filter for a square wave is simply a direct connection to ground.

Mar 14, 2008
23,124
6,858
What?

5. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
An inductor is a low pass filter.

6. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
For the op, you can create a low-pass filter using an inductor or a resistor and capacitor.

But first, you have to say why you want to filter a square wave. What do you expect to happen?

7. ### wayneh Expert

Sep 9, 2010
16,102
6,219
Nevermind, it was me confusing voltage and frequency. Filtering out the higher voltage leaves you with zero volts. Presumably the OP is referring to frequency.

Perhaps he wants to convert PWM to analog?

8. ### LvW Well-Known Member

Jun 13, 2013
840
117
I suppose you need something like a Bessel response?
This lowpass filter is intended to preserve the squarewave form.

ScottWang and #12 like this.
9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
The first concept that you must grasp, is what, exactly, makes up an ideal square wave?

An ideal square wave is comprised of the fundamental frequency, plus ALL of the ODD harmonics of that fundamental frequency. Thus, an ideal square wave requires UNLIMITED bandwidth. Since unlimited bandwidth is not practical/possible to achieve in a real physical system, we have to settle for approximations of an ideal square wave.

If you run an ideal square wave through a low-pass filter, it will no longer be a square wave.

Have a look at the animations in the Wikipedia entry for Square_wave:
http://en.wikipedia.org/wiki/Square_wave

ScottWang and #12 like this.
10. ### #12 Expert

Nov 30, 2010
18,076
9,691
So, basically, there is no such thing as a square wave with only low frequency components. If you want a useful answer, you will have to tell us, how low do you want to go and how distorted can you put up with?

11. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
And if you filter out all the high frequency components you are left with a sine wave.