[time-nuts] DDS - higher frequecies

Bob Camp lists at rtty.us
Mon Nov 26 00:38:25 UTC 2012


Hi

The output spectrum is modified by the usual sin(x)/x based on the actual speed of the DAC. It's like any digital signal, the rise time of the edge and the spectrum are related to each other.  Depending on exactly what sort of DAC architecture you have it may work ok, or it may not. If some bits propagate faster than others … not so good.

Bob

On Nov 25, 2012, at 7:30 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 
> 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?
> 
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
> 
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