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AD9518-3, Phase noise due to two Ref clocks.

Question asked by DavidLee on Feb 26, 2015
Latest reply on Feb 27, 2015 by DavidLee

Hi,

I'm using the AD9518-3 to generate a 1GHz clock for my DDS.

I provide REF1 in the form of a 10MHz TCXO, single ended.

 

The AD9518-3 produces beautiful 1GHz outputs. The phase noise is better than my portable HP spectrum

analyzer can measure. Nice! That's great! For this test, REF1 is the only clock connected.

 

REF2 will be an additional, external clock for this subsystem. When I keep REF1 connected

and then connect REF2, I see phase noise sidebands on my 1GHz signal at twice

the difference frequency of REF1 and REF2.

 

E.g., REF1 = 10MHz, REF2 = 9.997 MHz, I see sidebands on the 1GHz output

at 6kHz, 12kHz, 18kHz,etc. The first sideband is -30dBc! Wow! Not good!

 

Note that I see the same effect when I lock to REF2, rather than REF1.

 

Can't this chip run with 2 single-ended clocks connected without there being cross-talk phase noise?

 

Thanks,

David

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