On EVAL board, the output frequency (DDS) is not exactly as my config.

Question

On AD9910 EVAL board with crystal mode (25MHz), I config N value in CFR3 for sysClk. Expect to output a DDS sine wave of 50MHz. 
 CFR1 = 0x00402002 
 CFR2 = 0x00400820 
 CFR3 = 0x193F4528 <--- N will be configured from 20, 24, 28, 32, 36, to 40. 
 FTW in Profile register will be set according to sysClk. 
 Question: I expected the output freq of 50MHz, but it was not. See below yellow mark (55MHz). Some are over 55MHz, some under 55MHz. 
 Is there something wrong with my config ? or wrong programming ?

Jules.Nikko · Accepted Answer

Hi PeterLu , 
 I can confirm that when generating signals using DDS is not the exact number. Example would be in the evaluation software, when generating 50 MHz (@500MHz external clock), it does not give the exact 50 MHz but rather only 50.00000005 MHz. There's a small difference but not to the extent that it would reach 55 MHz as per your experience. 
 Sorry I might have missed that part where you were changing the N values? When you changed your N values, did you also change your loop filter values? R1, C1, C2? I'm thinking this might be the reason. You have to have the exact values of your loop filter whenever you make changes to your feedback divider N. This might be the reason why you did not get the exact value of 50 MHz and instead got 55 MHz. 
 
 All the best, 
 Jules)

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On EVAL board, the output frequency (DDS) is not exactly as my config.

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