Hi all,

Working on getting my head "around" the PIC a bit here, and have a
generic design question that may or may not come off as really dumb.

Not an EE here, so take any incorrect terminology with a grain of salt.

I'm looking to take an audio input to a PIC, first bandpass filtering it
heavily so that only the 50-300 Hz range is left over after a filter
(call it an anti-aliasing filter if you like) and then present that to
the PIC to do a frequency count.  Could even just use a low-pass filter
if that seems appropriate that rolls off sharply at anywhere from 250 to
300 Hz.

After reading quite a bit of information on the PIC's with A/D
converters, it appears I'm at a decision cross-road, or maybe not... the
group's input is definitely welcome and appreciated here.

It appears I have two choices... After the filter I can either...

Amplify whatever's "left-over" after the filter and rail-to-rail (or
even clip) the output from 0v to 5v with another op-amp circuit if need
be and add clamping diodes (oh wait the PIC's inputs are clamped... just
gotta watch the current, right?), and just count the pulses and average,
etc... using an interrupt routine on a standard input pin.

Or...

Use less filter and attempt to use one of the PIC's with a built-in A/D
and do a bit more math than the above idea to count the frequency of the
signal.   This approach seems more difficult for someone newer to the
PIC... but I'm willing to try it.

The audio input characteristics will be that it will be from a fairly
low-impedance source, 10V peak-to-peak, maximum, but can be lower.  It
will have an overall audio frequency range of typical voice
communications of about 0-3000 Hz, and could include quite a bit of
noise/static mixed in with all the other audio component.

So I *think* there may be some left over junk that might come through
from time to time at the 50-300 Hz range.  The audio will not be present
at all times, and the PIC will have to "decide" when a constant low
frequency component is there and when it's not, and what frequency it
is.

I guess I'm wondering a few things, not being much of an electronics
designer here... but having read a whole lot of stuff recently to try to
get up-to-speed, including the various Microchip application notes on
A/D and various app notes from all over on op-amp active filter
design... since I'm assuming that a one-to-three pole RC filter will
both not be enough filtering to get the higher frequencies out of the
signal to prevent aliasing and sampling problems, as well as it will
have other problems like affecting phase and lowering the overall output
to the point where I'll need to amplify the signal again anyway to make
it useful again... (?)... the questions are:

- Are the two options above both viable?
- Am I missing something obvious while I'm building my assumptions that
will lead to this circuit design?  (GRIN)
- Is one or the other of these "better" in your opinion?
- Any major advantages/disadvantages to either?

Way out on a limb getting ready to take the plunge... (GRIN)...
--
Nate Duehr, nate@natetech.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body