Well,
I guess these PC Based scopes aint all bad!
I have been downloading and trying some of the really cheap (AKA FREE)
ones that use the computer's sound card. I made up a little "Gizmo"
type interface with some diodes and resistors based on the information
from one of them.
My sound card is not a good one- it's about as cheap as they come I
would guess, but it appears as though I can sample at 44100 Hz
I have 4 outputs on the AVR project I am working on right now, each
running at a slightly different frequency.
I started probing some of the outputs and couldn't really understand
what i was see (and hearing if you leave the sound on to the speakers).
I could see what looked like an almost square wave and then spikes on
each one.
After playing around with all of this, using the "FREE" version of all
these I was able to determine the following....
The "Almost square wave" is actually ripple in the power supply. In
this case I am using the Atmel STK500 development board attached to my
project board. This pattern shows up on all the positive voltages.
So, I switched to a different power supply (My cheapo bench supply)
and those ripples went away.
So, that mystery is now understood.
Due to some bugs in the programming software I am using, 2 of the 4
outputs had to be programmed differently. What I found with the
horridly crude scope and probe is that those two outputs are out of
phase of the other two. The "Spikes" I was seeing are the actual
pulses.
I never could have gotten this far with my HP scope - my readings, if
I got any would have been all over the place.
I also think I understand that parameter for the USB scope having
50MSamples (or whatever the exact number was). Because I am sampling
at 44100 Hz with the sound card scope, my "ONs" and "OFFs" appear as
spikes as opposed to a square wave.
Atleast that is my very crude understanding of what I am working with
here.
More tinkering with this to do, but I have quickly gained respect for
these sound card scopes - so far it's working a thousand times better
than my HP scope has - not that HP scopes are bad, mine is just seen
much better days!
Chris
