Let's try plainer plain Inglitsch:

A low pass filter will allow low frequencies through without much loss, will
tend to block high frequencies, and at some magic frequency determined by
the values of the components will attenuate the signal by about half.
Frequencies above this are attentuated more, below this are attenuated less.


A classic low pass filter (there are dozens of different types) is simply:

Vin--------R--------OUT
                    |
                    C
                    |
                    GND

This is useful in PICs to take a high frequency signal, say a PWM output,
and turn it into a smooth nearly DC signal, say for an input to a
comparator.

Calculus is handy for figgering out how this stuff really works, but it is
possible to understand it in a practical way without it.  I was building
stuff with low pass filters long before I ever studied calculus.


OK, what about sampled data?  An old fella named Nyquist said that you need
to sample your data at least twice the highest frequency present in the
signal for it to make any sense.  Another fella named Lile said that Nyquist
was an optimist.  A low pass filter is often used in front of a data
acquisition system of any kind to limit high frequencies.  If the high
frequency cutoff of your filter is 10KHZ, you need to sample at least 20KHZ
(per Nyquist) and realistically even faster (per Pessimistic Mr. Lile) to
get good useful data without aliasing and other problems.

Next you are going to ask how to calculate the cutoff frequency of such a
filter.  I'll bet there are some good cookbook methods that don't involve
any calculus.  I, being lazy, just chuck parts into BSpice and let it figger
the answer for me.  Anybody want to weigh in?

--Lawrence Lile

----- Original Message -----
From: "michael brown" <n5qmg@AMSAT.ORG>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, February 28, 2002 10:18 AM
Subject: Re: [EE]: What is low pass filtering


> See below
>
> > michael brown wrote:
> > >This may sound like a dumb question, but I have not run across a good
> > >"plain
> > >English" description of low-pass [...]
> >
> > A time-varying signal can be represented as a function of time, f(t).
> > The same function can be represented in "frequency space" via a linear
> <SNIP>
> >
> > Hm, this isn't really "plain English" is it? Try again.
>
> Not to me, it pretty much reads like the rest of the "explanations" I've
> seen.  ;-)
>
> > A low-pass filter lets the slowly-varying part of a signal
through,......
> <SNIP>
>
> DSP's are good.
>
> > Was this in the least bit useful? I hope so.
>
> Thanks for the reply.  It reinforces what I already understand about
> filtering electrical or electro-magnetic "waves".  I am looking for the
> definition as it applies to EE and sampled data.  Also I hear/see the term
> "integrate" and am not completely clear as to what this means in the
context
> of EE and data samples.  I hear these terms being used frequently, but
I've
> never taken any EE type classes, so I'm not sure what it means.  Never
took
> a calculus class either.
>
> michael brown (unedumacated foole)
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>

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