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How to reduce AD9957 ref-clock harmonic spur. ?

Question asked by ynien on Sep 29, 2014
Latest reply on Oct 30, 2014 by ynien

Hi,

 

I use AD9957 with my layout board and find some spurs in spectrum. The spurs level are around -80dBm, and they seem to be the harmonic of reference clock(24.576MHz). 

I try 3 different way(from the datasheet) to generator reference clock, but the spur. level seems the same. The 3 ways are (1)use xtal driven (2)single ended input direct drive with an oscillator.

(3)single ended oscillator and differential input.

 

Is there other way I can try to reduce the harmonic of reference clock?

 

2 spectrum are in the attach:

(1)I make a band-width 1.536MHz DAB signal, center frequency is 174.928MHz.  I mark clock harmonic 147.456MHz and 172.032MHz.

(2)When AD9957 Tx enable pin turns low, the spectrum still have reference clock frequency: 122.88MHz, 147.456MHz, 172.032MHz, 196.608MHz, 211.184MHz.

 

Thanks!

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