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More AD9578 questions

Question asked by cosmolittle on Apr 27, 2016
Latest reply on Apr 28, 2016 by SteveBeccue

Dear Steve,

 

Thanks for the reply. I have some further questions:

 

Crosstalk spurs.  I have noticed that the data sheet has no specification for crosstalk spurs when PLL1 and PLL2 are generating different frequencies. Also between outputs 1 and 2 when the output dividers are different. Is there any information about this?

 

Rational mode:  This is very interesting, as it seems to give a method of producing exact ratios between the output frequencies and the reference frequencies, which is not possible in normal fractional mode. I have tried the evaluation software, although I have not yet purchased the evaluation board, however it does not seem possible to control the feedback divider mode using this software. Could you please confirm this.

 

Considering the following example: Reference frequency 50MHz. Desired output1 frequency 79.2MHz .  I can choose any divider for output1 between 39 and 46 . The values that give a feedback divider closest to an integer are 40, and 45, giving dividers of 63.36 and 71.28. Using S=3, This would give register values of A=63,C=18,D=50 for a divider of 63.36, and A=71,C=14,D=50 for a divider of 71.28. Could you please confirm that with these values the output frequency will be exact?

 

An alternative using S=1 would be A=63, B=1, C=11,D=25 , again giving a divider of 63.36. Does this again give an exact frequency? Is there any advantage in using this configuration?

 

I do not understand the function of the S setting. Does this have some effect on spurs?

 

regards

 

Cosmo Little

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