[hpsdr] questions re: output spurs using an AD9954 DDS chip

[Graham / KE9H]

Steve:

I have some experience with the 9951, which is closely related
to the 9954, and did not experience a lot of close in spurs. At least
not above -85 dBc.  There are spurs everywhere below -95 dBc.

There are not specific application notes on reducing spurs, but there
is a lot of information "inferred" in the reference designs.

So the questions become:
What are you seeing, at what level?
What input reference frequency?  How clean is it?  Are you using the
reference multipliers, or direct input on the system clock frequency?
Are you running separate power systems for the analog and digital power 
feeds?
[And separate again for the uP?]
Separate analog and digital ground systems?
How do you cross the ground system boundaries for the output? Transformer?
How did you isolate the uP from the DDS? Separate power systems?

In other words, a copy of the schematic and PCB layout would be
helpful.

--- Graham / KE9H


Steven M. Simons wrote:
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> To the list;
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> Built up a uP driven DDS circuit (to obtain a 10>100MHz output) using 
> the AD9954 chip. Upon looking at the spectral output, I see lots of 
> close in spurs. As this is a 1^st attempt to use this chip, is there 
> anything written on how to clean up the output???
>
> Thanks,
>
> Steve
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> *KF6AJ*
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> *Steven M. Simons*
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