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AD9578  Dither and Dither Scale

Question asked by cosmolittle on Sep 28, 2017

Dear Steve,

 

    I am trying to minimise spurii at offsets of up to 10kHz. I am using rational mode, and do not understand the explanation given for the Dither Scale setting. The explanation implies that this must be recalculated for every frequency programmed.

It refers to " feedback divider value"  which in fractional mode is 28 bits, however in rational mode it is 16bits.

 

  How does one calculated the correct value for the Dither Scale in rational mode, given a selected value for the Dither?

 

I am currently using the MASH setting of 3, and the dither setting also 3.

 

A typical frequency that I generate is 39.96MHz, with the following values:  output divider = 80

                                                                                                                          integer divider = 63

                                                                                                                           fractional divider = 117

                                                                                                                           modulus = 125

                                                                                                                           PD freq = 50MHz

I have spurii at offsets of 1.425kHz, 1.950kHz, and 2.975kHz.

 

best regards

 

Cosmo Little

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