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About AD9789 NCO

Question asked by Jaime on Jul 7, 2015
Latest reply on Jul 8, 2015 by Jaime

Hi Dan.


I hope you can solve me a doubt about the NCO.

It is documented in the data sheet figures that the frequency range of the NCO is from 0 to fDAC/16.


In other data sheet text it says that the output frequency can be placed anywehere from DC to fDAC/16.


This means that it is expected that the input is a baseband, but if it is an IF signal, both situations are not compatible.


I have tested the evaluation system already, and I think that there is no problem to input an IF, and so, the converter output frequency (before the block UC) is either fNCO+fIN or fNCO-fIN. Just to confirm from you, as I got that result anyway.


But the SPI interface is not a friendly one, and what I guess the NCO frequency form input do is to reduce the frequency you input to modulo fDAC/16. So I was able to get both sum and difference output frequency controlling the inversion.


But, on the other hand, if the NCO range is 0 to fDAC/16, does it means that the NCO runs at an internal clock of fDAC/8? This looks curious being most of this input part running at fDAC/16.


Thanks, and have a good summer...


Jaime Martin