Low pass filter hookup
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Posted by on May 18, 2008, 3:59 pm
Please log in for more thread options I created a low pass filter in orcad and trying to put theory into pratical. The circuit output is always showing the input at about one third the pk-pk voltage even at 30kHz and more, this is suppose to be a 3kHz lpf.. I think the wiring might be wrong. I took the black pin of the function generator as the ground reference point. Use two 6v batteries for positive and negative voltage and place the middle of the batteries at the ground reference point from the function generator. Here is a link with my circuit and a picture: http://www3.sympatico.ca/lerameur/LPF/ Or does any one has a specific instruction to a simple active LPF and wirinf connection, maybe I can start from there. I tested my OP Amp, and that is not the problem. k | |||||||||||||||||||
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Posted by John Larkin on May 18, 2008, 5:00 pm
Please log in for more thread options wrote: LM339 is a quad comparator. You need opamps. John | |||||||||||||||||||
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Posted by Bob Eld on May 18, 2008, 9:39 pm
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> Hello,
> > I created a low pass filter in orcad and trying to put theory into > pratical. The circuit output is always showing the input at about one > third the pk-pk voltage even at 30kHz and more, this is suppose to be > a 3kHz lpf.. I think the wiring might be wrong. > I took the black pin of the function generator as the ground reference > point. Use two 6v batteries for positive and negative voltage and > place the middle of the batteries at the ground reference point from > the function generator. > Here is a link with my circuit and a picture: > http://www3.sympatico.ca/lerameur/LPF/ > Or does any one has a specific instruction to a simple active LPF and > wirinf connection, maybe I can start from there. > I tested my OP Amp, and that is not the problem. > > k As was mentioned, the LM339 is a comparator not an op amp. Use an LM 324, LF 347 or a TL084 or equivalent op amp. The 339 will probably oscillate if used as an op amp and it needs a pull up resistor on the output of each amp section. It ain't the right animal. Be sure to check the pin out, these amps are very different than a 339. Secondly there is no true dc path to ground for the amps except through the generator. Place a 10K resistor from the input point on R5 to ground. This will establish amplifier bias even when the generator is un-hooked. Bypass the plus and minus power rails to ground with small caps, 0.1uF is ok. The circuit should work ok with proper amps as a six pole low pass filter with 36db per octave cut off slope. I calculate a cut off of 2kHz for two sections and 1364Hz for the first section. This will be sloppy filter with a main cutoff at 2kHz. How did you figure 3kHz? | |||||||||||||||||||
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Posted by Phil Allison on May 19, 2008, 1:29 am
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<mclesperance@gmail.com > Here is a link with my circuit and a picture:
> http://www3.sympatico.ca/lerameur/LPF/ ** All I see is a blank screen on a Tek scope. .... Phil | |||||||||||||||||||
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Posted by Eeyore on May 19, 2008, 2:33 am
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Phil Allison wrote: > <mclesperance@gmail.com
> > > Here is a link with my circuit and a picture:
> > http://www3.sympatico.ca/lerameur/LPF/ >
> ** All I see is a blank screen on a Tek scope. Same here. Graham | |||||||||||||||||||
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>
>I created a low pass filter in orcad and trying to put theory into
>pratical. The circuit output is always showing the input at about one
>third the pk-pk voltage even at 30kHz and more, this is suppose to be
>a 3kHz lpf.. I think the wiring might be wrong.
>I took the black pin of the function generator as the ground reference
>point. Use two 6v batteries for positive and negative voltage and
>place the middle of the batteries at the ground reference point from
>the function generator.
>Here is a link with my circuit and a picture:
>http://www3.sympatico.ca/lerameur/LPF/
>Or does any one has a specific instruction to a simple active LPF and
>wirinf connection, maybe I can start from there.
>I tested my OP Amp, and that is not the problem.
>
>k