[time-nuts] DDS - higher frequecies

Hal Murray hmurray at megapathdsl.net
Mon Nov 26 00:30:52 UTC 2012


Suppose I have an A/D running at 1 MHz.  The standard simple minded approach 
is that it will work for any input signal with a bandwidth up to 1/2 MHz.  We 
usually think of that in the baseband, but it also works for, say  1.25 to 
1.5 MHz.  The input signal gets aliased down into the baseband.  (and if you 
are unlucky, which is easy, some of the aliasing reflects back and overlaps 
so you can't tell X-y from X+y)

There is similar math for D/A, the reverse direction.  I think this applies 
for a DDS making higher frequencies than simple arithmetic would allow it to 
generate.

Does anybody have a good web page for how that works?  My simple expectations 
are that it would have to generate lots of harmonics and then go through a 
filter to get rid of all the wrong stuff.  I'm missing the step where all the 
harmonics come from.

Are they just really tiny and I have to do a lot of good filtering and 
amplification?

Do I need something other than a traditional DDS for this sort of stuff?



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