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AD9957 on single tone mode outputs with unexpected spectrum around +/- 300KHz, WHY?

Question asked by mikeyunhai on Dec 3, 2011
Latest reply on Dec 15, 2011 by mikeyunhai

Hi, everyone,

 

  I am stuck with this urgent application problem and I really need help.

 

  The AD9957 is set to operate at single tone mode and when I measure the output signal with the spectrum analyzor, I found that there are four unexpected spectrum around the aiming spectrum, two on each side, where is about 300KHz and 600KHz.  I was looking for the methods to elimiate the unexpected spectrum and the possible factors making this problem happen.

 

  Hereby is my applicaiton parameters:

  - 40MHz external input AD9957 reference clock,

  - 24 times to reach 960MHz VCO,

  - loopfilter bandwidth 1.4M, assembled and calculated as officially-recommended

  - single tone output 70MHz

  - the AD9957 schematic circuit design refers to the official-released EVM schematics

 

  Below is what the problem looks like:

  - the unexpected spectrum power grows when output greater frequency, however they locate relatively the same to the main wanted spectrum

  - I changed a set of loopfilter paremeters and re-assembled R and Cs as well, the problem remain unchanged

  - snap-shot added below

  20111121294.jpg

  How did this happen and what factors may result in this problem? and are there any recommended solutions?

  Further infos may added if required.

 

  I am very grateful for everyone who would like to share a piece of mind here. Thanks very much.

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