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
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.