[sdiy] Voltage divider with RC filter - help with the basics, please

[Neil Johnson]

Hi,

Russell McClellan wrote:
> Richie already gave a complete and accurate response, but maybe you'd
> be interested in a standard, general technique for this sort of
> passive network.

A quick test I often do when looking at a schematic is the DC-to-daylight check.

At DC: capacitors are open-circuit, inductors are short-circuit, and
resistors are resistors. If there is a path from input to output then
you have some low-pass action.

At daylight: capacitors are short-circuit, inductors and open-circuit,
and resistors are resistors.  If there is a path from input to output
then you have some high-pass action.

If you have low-pass only, then it is looking like a low pass filter.
If daylight only, then a high pass filter.  If both, then some sort of
notch or all-pass, or a low-pass or high-pass with some shelving.  And
if neither then bandpass.

Beyond that, then out with the Laplace and/or Spice. Although to be
anywhere near right you need to know about all the impedances
otherwise you just end up with wrong answers, or amusing little
surprises like RC circuits with voltage gain, and so on.

For those that like reading (the activity, not the town) I've just
found this Newnes book online via google:

muffinator.mit.edu/web/class/intuitive.pdf‎

Looks legit, from MIT, so download and enjoy!

Cheers,
Neil
--
http://www.njohnson.co.uk