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[dsp] Analog front end design

David Willmore willmore at optonline.net
Tue Apr 26 02:37:29 UTC 2005

> I've been using the dsPIC filter designer lite to design filters so far.
> I'm not designing for a PIC, but the program's not really PIC-specific.  It
> generates working C code that I can feed real data and then plot the results
> in Excel.  I'm still a bit lost when it comes to actually choosing the
> parameters.  I know the passband I want, but I'm not sure where to set the
> stopband, for example.  The ripple parameters I find don't really matter if
> I'm constraining it to a limited number of coefficients - I just take the
> best it can give me.  Not always sure what's 'good enough' though.

That is a nice program.  It's more useful for applications where the precise
filter performance is critical.  For some applications, it's just not that
critical.  But, it's still a good tool to show you what the compromises are
between filter types.

For a data filter, you want a nice gentle curve to the filter rolloff,
ripple doesn't matter all that much.

> It does do a good job of showing me the results of my design.  I can at
> least keep tweaking things until it looks like what I want.

Yep, it's great for that.

> I'm not so worried about clock recovery and such in the digital domain.  I
> can handle digital, I've done stuff like that before - decoding data off an
> XR2211, for example.  But I'm lacking a lot of the math background I need
> for the DSP portions and it's a steep learning curve.

Start with www.dspguide.com.  Buy the book once you're read through it
and relalize how useful it is. :)  I like it, but wish it wouldn't be
so shy with the math.  Complex numbers don't bother me and the text goes
to extremes to avoid using them.  I find them very elegant tools, but
I'm a little esoteric myself. :)

> Anyway, is what I'm describing with the autocorrelation (?) the same thing
> you're talking about?

Not autocorrelation, but a correlator would make a good data recovery filter.
You *know* what a data transition will look like, so correlate the data with
one and see what the output looks like.  With the CPU ability of a dsPIC,
you shouldn't have any trouble with that.

Come to think of it, the output of an autocorrelator could be fed into a
correlator and the first (lowest frequency) peak could be used as the data
rate.  That won't give you phase, but it'll give you frequency.  I just think
there must be better and less CPU/memory intensive ways.

With the amount of CPU available in a dsPIC, one should be albe to make a
completely kick ass (pardon the term) 1200 baud modem.  Even a good 9600
baud one.  Next time I get a chance to bug Bob (Robert McGuire) about this,
I'll see what tricks he had to use to fit this into the early DSP TNCs.


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