[sdiy] Sallen-Key math question

[John L Marshall]

Jim,

It's just the way my pea brain looks at formulae. If I see a 1/x,
immediately I see a direct inverse relationship. I look further to see if
the relationship is exponential. That radical stuff has to go up by four
times for the relationship to be one half. I must double two variables under
the radical for the frequency to drop by one half. When I see 2 * pi, I see
a vector of radius r making one complete rotation. Those 2 * pi creep in
everywhere. It must be the cycle were in.

John

----- Original Message -----
From: Jim Patchell <patchell at silcom.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, October 16, 2001 8:08 AM
Subject: Re: [sdiy] Sallen-Key math question


>     It is just a different way to write the same thing.  Both expresions
will
> give the same result.
>
>     -Jim
>
> John L Marshall wrote:
>
> > I going from memory too isn't it Wc=1/SQRT(R1R2C1C2)?
> >
> > John
> >
> > ----- Original Message -----
> > From: Jim Patchell <patchell at silcom.com>
> > To: <music.maker at gte.net>
> > Cc: <synth-diy at dropmix.xs4all.nl>
> > Sent: Monday, October 15, 2001 10:15 AM
> > Subject: Re: [sdiy] Sallen-Key math question
> >
> > >     If the R's and C's are equal Wc = 1/RC,  If they are not equal
then
> > > Wc = SQRT(1/R1R2C1C2)
> > >
> > >     Also note Wc = Fc * 2 * PI
> > >
> > >     Please note, this was done from memory...
> > >
> > >     -Jim
> > >
> > > Scott Gravenhorst wrote:
> > >
> > > > I keep looking at the math for this filter in the
> > > > Lancaster's "Active Filter Cookbook" and frankly,
> > > > I just don't get it.  What I'm after is given a
> > > > cap of size C and a resistor of R ohms, what is
> > > > Fc ?  The diagram clearly shows how to calculate
> > > > R for a given Q value, but I don't see how to get
> > > > Fc.
> > > >
> > > > Is there a better source of info for this ?
> > > >
> > > > NOTE:  I am NOT an EE.
> > > >
> > > > =========================================================
> > > > - Government: The other religion.
> > > > - The media's credibility should always be questioned.
> > > > - Lambs who lie down with lions are lunch.
> > > >
> > > > -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> > > > -- Linux Rex         | RedWebMail by RedStarWare
> > > > -- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html
> > >
>
>