[time-nuts] DDS - higher frequecies

[Said Jackson]

Hal,

Check out the Analog Devices website. Good info on DDS Dacs there.

You want to stay a bit away from the 1/2fs Nyquist limit in your DA. The reason is the image coming down from your 1MHz clock.

If you output say 0.45MHz, you have an image at 0.55 MHz already (1MHz - 0.45MHz) so your filter has to be extremely steep to make that work and remove the spur at 0.55 MHz.. Check out the Mini Circuits LFCN low pass filters, they work at higher frequencies, and are very steep.. Your filter quality is going to determine how close you can get to Nyquist. 

Bye
Said

Sent From iPhone

On Nov 25, 2012, at 16:30, 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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