[sdiy] Wanted: non mathematical description of the function of RC-filters

[Florian Anwander]

Hello,

I need your help for a kind of beginners introduction into filters. The 
readers have a quite basic understanding of electronics. They know that 
resistors and capacitors exist (only a few will know what they do), and 
they know the difference of AC and DC.

Now I am looking for a very(!) simple explanation of the working 
principle of a passive RC highpass and lowpass. There shall be no 
mathematics involved. I started like following, but I not really happy 
with it:

'A filter works basically with the fact, that a capacitor transmits 
alternating current, but blocks direct current. An electrical audio 
signal is an alternating current. The lower the frequency the more the 
signal becomes similar to an direct current. If we keep the first 
statement in mind, then a capacitor acts basically like a high pass 
filter: the lower the frequency of an audiosignal is, the more similar 
it is to DC, and the more the capacitor will block it.
To get a tunable filter one would need a tunable capacitor, but that its 
complex. A tunable potentiometer is much easier to build. So the RC 
filter was developed. It combines a resistor with and capacitor. The 
cutoff frequency can be regulated with the value of the resistor. The 
RC-Filter consists of two signal pathes: "through the capacitor" and 
"not through the capacitor". To get a low pass filter the signal path 
"through the capacitor" directs to ground. So high frequencies are 
grounded, while low frequencies are fed to the output. The high pass 
filter works the other way around: the signal path "through the 
capacitor" feeds the output, while the signal path "not through the 
capacitor" goes to ground, which meands low frequencies are suppressed.'

The original is written in german, and maybe my translation contains 
additional errors. But I think 95% are correct.
I like this description until the point with the two possible signal 
pathes, but the final explanation of high and low pass does not suit me. 
Any better ideas?

Florian


http://fa.utfs.org/