exponential rising

[Magnus Danielson]

Hi Patrik!

> >   An exponentially rising waveform across a cap.???  That's the inverse
> >   function to the caps' normal behaviour, isn't it? 
> >
> >I have no idea what you mean by "inverse", but a capacitor with a
> >resistive load will naturally do an exponential decay.
> 
> I meant the mathematic inverse-function of a logarithmic curve, which should
> be some kind of exponential one. The caps normal behaviour is, of course, to
> charge logarithmically, and discharge exponentially. So, one should be able
> to get an exponentially rising waveform as a result by inverting the voltage. 
> But, as you said, it will propably sound the same as an exponential decaying
> wave... 

I think you are missing some basics in simple filter electronics! So therefore
a minor crashcourse (I love that word:) :

An 1-pole RC lowpass filter:

 Input o---/\/\/\---o---o Output
             R      |
                  -----
                  ----- C
                    |
Ground o------------o---o Ground

This filter will have an rounded of initial part when being feed with 
squarewave
something like this:
     __         __
   ..  |.     ..
   .    ..    .
--|       |--|

An 1-pole RC highpass filter:

           || C
 Input o---||---o---o Output
           ||   |
                /
                \ R
                /
                |
Ground o--------o---o Ground

And it has the peaky curve (I hate to draw ASCII graphics.... especially 
curves)

The spikier the curve becomes, the more high frequency content is only passed
on. The more rounded and "duller" the curve becomes, the more low frequency
contents are allowed to pass.

Besides, both curves are exponential and none of the is logarithmic, what
differs is their polarity and what the offset levels are. This is quite a 
diffrent thing than saying a curve is logarithmic.

BTW. logarithmic and exponential is sometimes missused and interchanges or used
with the anti- prefix to add to the confusion.... quite anoying when you know
what is being ment. Should be even more anoying when you don't know but tries
to figure out and can't understand how words and practice match...

I could torture you with some nice complex expressions to go along with this
and they are available upon request, but I have to run to the store now (to all
it do concern it this list :)

Cheers,
Magnus