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[dsp] Analog front end design

David Willmore willmore at optonline.net
Tue Apr 26 01:42:51 UTC 2005

> Generally, wouldn't you want the sample rate to be a multiple of the baud
> rate to avoid ISI when you're filtering?  Or maybe I'm just designing my
> filters wrong.  Seems like it'd make clock recovery simpler, too.

I can see a lot of ways to do it.  If you rely on your sample rate being a
multiple of the data rate, you leave yourself open to a lot of errors if
those rates ever differ.  I see it as better to just admit that they'll
never be firmly related and deal with the consequences.

For clock recovery, just lock a DPLL to the square of the data signal?

> Anyway, I'm trying something along the lines of what you're suggesting, I
> think.  Right now it's just a test program on the PC.  It's multiplying each
> sample in the input waveform by a sample 90 degrees apart at 1700 hz.
> Haven't got the LPF implemented properly yet.  I set it up to output a
> digital value based on a couple of thresholds and I can plot the result in
> Excel and see the bits clearly.  It's not going to handle much noise that
> way, though.

The LPF is important. :)  All you really need is a second order IIR filter.
The DSPGuide should be able to help you design a simple one of those.

> Can you suggest a good book that would cover this?  Preferably something
> accessible to a relative newbie.  =]

Hmm, Sklar's book is good.  I'll try to find my copy and get you a
reference to it.  I think there's a second edition of it out.


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