how to calculate Cutoff Freq of passive HPF

fonik

hi all,

again i need some help/clarification with the maths:

f = 1/(2*pi*R*C)

so C = 1/(2*pi*R*f)

correct?

if i wanted a 1kHz filter using a 100k resistor i would have to calculate the following:

C(n) = 1/(2*pi*100000 Ohm*1000 Hz) = 1.59e-9n = 1.6n

correct? i just wanted to make shure i got the decimals right Wink

v-un-v · Posted: Wed Apr 21, 2010 5:40 am Post subject:

Fonik, I've got it down as f*R not R*f.

Dunno if that will influence your output?

the app screenshot is electronic toolbox;

http://www.electro-music.com/forum/topic-41036.html

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Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Hi fonik:
I alway use this site: http://www.muzique.com/schem/filter.htm to do the
calculations, the funny thing is that in that site, the calculated cap value is 2nF
I think the calculator approximates the value a lot.

fonik · Posted: Wed Apr 21, 2010 5:45 am Post subject:

frijitz · Posted: Wed Apr 21, 2010 11:46 am Post subject:

Your formula looks wrong, but you got the right answer. If you use Ohms and Hz then the answer is in Farads:
C(F) = 1.59e-9F = 1.59nF.

(Oh, and I hope that comment about algebra maybe not commuting was meant as a joke.)

Very Happy

Ian

fonik · Posted: Wed Apr 21, 2010 12:27 pm Post subject:

thanks for indication, ian. it was a typo... Laughing

Rykhaard · Joined: Sep 02, 2007 Posts: 1290 Location: Canada

v-un-v · Posted: Thu Apr 22, 2010 1:44 am Post subject:

Hey, thanks for replying there Rich! For a moment then I thought I had died and no one could actually see that I'd posted anything!! Shocked

Rykhaard · Joined: Sep 02, 2007 Posts: 1290 Location: Canada

andrewF · Posted: Thu Apr 22, 2010 4:55 am Post subject:

i found this software in a post from Ricko
but it is excellent
texas instruments freeware active filter designer
http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

blue hell · Posted: Thu Apr 22, 2010 6:32 am Post subject:

Scott Stites · Posted: Sun May 02, 2010 8:40 pm Post subject:

I've been getting a lot of mileage out of Don Lancaster's "Active Filter Cookbook" lately. As I understand it, it remains relatively the same even in later printings. It makes filter design pretty easy for mathematically challenged people like me. It *really* helped a lot in allowing me to understand filters to a far greater degree than I did before.

Take a look around, you might be able to find a used version of it somewhere. I got an autographed copy (an authentic swirl of a felt tip pin on the opening page) at MPJA here for something like $20.

Scott Stites · Posted: Thu May 27, 2010 2:44 pm Post subject:

Speak of the devil:

25% off

http://www.mpja.com/email/05-18-10.asp?r=195316&s=21

fonik · Posted: Thu May 27, 2010 11:32 pm Post subject:

thanks for the pointer, scott. unfortunately no off-price for me, they ship to the states only.

Scott Stites · Posted: Fri May 28, 2010 6:31 am Post subject:

Those bastages... Laughing