Don Hyde wrote:
>Snip
>
>> Good stuff Dan!  Isn't your first example sort of a non-test, since it
>> violates the Nyquist criterion?  In other words, isn't it
>> already known that
>> you can't sample a 120KHz signal at 50KHz rate?  I.e., it
>> requires a minimum
>> of 240KHz rate.  Or am I missing something?
>>
>Snip
>>
>
>Read the chapter on Nyquist again, and ponder.
>
>What it says is that signal components between f/2 and f (the sampling rate)
>are folded back into the frequency band 0 to f/2, and their frequencies will
>be inverted i.e. a signal at (f/2 + a) Hertz will look like a signal at (f/2
>- a) Hertz.  It goes on to say that frequencies between f and 1.5f will get
>folded so that they look like frequencies between 0 and f/2 Hertz.  If you
>sample a 120 KHz signal at 50 K samples/second, it will look just like a
>signal at 20 KHz.


Exactly - the 1st figure Drew was referring to illustrates this
in the time domain. Combined with the 2nd figure, the two show
that the practical limitations of normal sampling by the PIC A/D
can be overcome using the ETS mode. Basically, the normal mode
with max 50khz rate will adequately display a waveform of only
about 5 khz max, while ETS mode pushes this out to 120 Khz.

Who'd a thunk it on a PIC? 120 khz analog signal acquisition.

- dan

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads