# understanding pass filters

Discussion in 'The Projects Forum' started by aac9876, Feb 3, 2008.

1. ### aac9876 Thread Starter Active Member

Dec 9, 2006
124
0
I have been looking for an applet on the web that will help
me understand how an inverter filter turns a square wave into a sine wave. I found this..
Anyone get me pointed right with the settings to see how this happens.??Or any info based on what you see..thanks

2. ### rwmoekoe Active Member

Mar 1, 2007
172
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wow what a simulation .

but i think that applet wouldn't help much in understanding how a filter turns square into sine waves.

the point is, a square wave have almost all the harmonics of it's prime freq.
you, know... harmonics are higher frequencies than the prime freq right?
by filtering the signal just above it's prime freq with a steep-sloped filter, you got the prime freq alone, almost without it's harmonics.
it's a pure sine.

3. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Or, another perspective is the concept of the fourier transform. Any periodic signal can be written as an infinite series of sines and cosines. For the typical bipolar squarewave originating at t=0, we have an infinite series of sinewaves of various frequencies. The most powerful of all these is the fundamental (or rwmoekoe's prime frequency), then as you add more harmonics, the waveform becomes more like a square wave.

So, when you are low-pass filtering, you're just removing all harmonics and keeping the fundamental.

well, wikipedia says it better than I ever could, check it out

http://en.wikipedia.org/wiki/Square_wave

Steve

4. ### aac9876 Thread Starter Active Member

Dec 9, 2006
124
0
I saw one schematic relating to this in some archives..
it seemed to be a multiple cut off circuit, where as the signal goes up it gets cut off at multiple places on it way up.
Does this sound right. Does this mean that as the voltage
goes up it can be thought of as a different frequency.??
the cut offs are rated by frequency..??
There also seems to be a problem ,in the related articles,
of the current draw staying consistent as more current is drawn and requires more stuff to help with that.
I would like to build one for practice but would need a video I think to follow exactly.