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AD9854, reduction technique of output spurious?

Question asked by shawn.J on Dec 7, 2010
Latest reply on Dec 7, 2010 by DSB

Hi,

I'm using AD9854 device.

 

Fclk is 285MHz, Fout is 131.5MHz

 

DDS outputs fixed spurs at 5kHz offset and 1MHz offset.

The spurs are more higher than any others and the level of spurs is about -70dBc(see attached files)

 

If Fout changes to 65.75MHz(=131.5MHz/2), fixed spurs change at 2.5kHz and 5kHz.

 

I expect that the fixed spurs are effected by quantization error.

So,Fclk is increased up each 1MHz. But fixed spurs don't change.

 

Also, I expect that the fixed spurs are effected by input voltage of DDS

So, I used 3.3V LDO regulator(not switching regulator), and check the noise of ragulator output using oscilloscope.

But, input voltage of DDS doesn't effect the spurs.

 

1. Why does the fixed spurs generates?

 

2. How do I expect the spurs that are close to Fout ?

 

3. How can I reduce the spurs that are close to Fout?

 

 

Please excuse my english.

Thank you in advance.

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