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Old 22nd August 2009, 09:22 PM   #1
neazoi is offline neazoi  Greece
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Default 1.8KHz low pass filter

Hello I need a schematic of a 1.8KHz low pass audio filter. I need it to be as sharp as possible.
Any schematics or links yould be appreciated.
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Old 23rd August 2009, 09:50 AM   #2
mjf is offline mjf  Austria
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[QUOTE=neazoi;1905023]Hello I need a schematic of a 1.8KHz low pass audio filter.

hello.
i presume you want to use it in a preamp or so.......
there is a schematic of an active third order butterworth filter.
you can use two filters in series if one is not sharp enough.......
example: with r= 10k and f= 1800hz
you will get around 12nf, 33nf and 2nf for c1,c2,c3..........
greetings............
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Old 23rd August 2009, 12:27 PM   #3
neazoi is offline neazoi  Greece
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Yes, it is for audio use in preamplifier levels.
It is basically used in a direct down conversion ssb receivers, to cut off the unwanted sideband of the downconverted (to audio) rf signal.
So this has to be as sharp as possible, to ensure that the higher audio frequencies will be attenuated as much as possible.
I did an extended search in the internet for a program to design such filters and I found FilterLab from Microchip company (the one that produces the PC microcontrollers).
I believe it is very nice program but I need your opinion.

I managed to design a filter using this software and I present you the results here to notify me if it is a sharp filter I have designed or not.

Note, I have no idea what phase slope means, but the responce slope seems sharp enough?
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File Type: gif smallfltschem.gif (12.4 KB, 117 views)
File Type: gif smallresponce.gif (19.6 KB, 114 views)
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Last edited by neazoi; 23rd August 2009 at 12:29 PM.
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Old 23rd August 2009, 12:30 PM   #4
neazoi is offline neazoi  Greece
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here are the attachments
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File Type: gif smallresponce.gif (19.6 KB, 107 views)
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Old 24th August 2009, 04:39 PM   #5
AndrewT is offline AndrewT  Scotland
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that roll off with aripple then fast slope flattening out slightly at higher frequencies looks like a Chebychev.

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

Try Googling for more info.
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Old 24th August 2009, 04:51 PM   #6
neazoi is offline neazoi  Greece
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yes it is a Chebychev filter indeed. I found it to have sharper cut off and the price you pay is about 1db ripple in the passband (with my attatched design).
I really cannot believe how simple it is to design an audio filter using filterlab, is it really so simple or have I done something wrong?

The way to test it can be done with a computer running an audio generator program and another computer testing the filter...
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