Jim wrote:

Personally, I'd experiment with the AD9951 DDS chip, it's spurious
outputs are very low....

http://www.radioamatore.it/i0cg/add9951.html

However, if somebody wants a DDS for direct conversion experiments, the
AD9854 DDS has built in quadrature outputs which make it tempting. This
website discusses a complete design and, the kit can be gotten from Kanga.

It is pricy, but in my opinion, it has everything I would need.....

http://home.comcast.net/~aa0zz/index.html

The only drawback is that it's spurious outputs are higher.


Jim Pennell
N6BIU



If you want quadrature LO signals, and you are operating below half (or
ideally below quarter) of the DDS maximum output frequency, then you might
find that the best approach is to use the cleanest DDS chip that you can
find with a single output, then filter that to remove the harmonics, DAC
images etc. and then square it up with a LVDS receiver or comparator, and
divide by 2 or 4 using the flipflops of the divider to generate quadrature.
The challenging part would be ensuring that propagation delays don't spoil
the quadrature, but it probably wouldn't be any harder than making two
matched re-construction filters to use with an I-Q DDS chip. The fact that
you end up with quadrature square waves rather than quadrature sine waves
is a good thing (not a disadvantage at all) with most kinds of mixer.

Chris