[time-nuts] DDS - higher frequecies

[Magnus Danielson]

On 11/26/2012 01:30 AM, Hal Murray 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?

I think you would enjoy digging up the Analog Devices DDS material, 
which goes into this and many other things.

http://www.analog.com/static/imported-files/tutorials/450968421DDS_Tutorial_rev12-2-99.pdf

See section 10.

Cheers,
Magnus