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Unknown Spur of AD9912

Question asked by Dr.Drew on Sep 22, 2010
Latest reply on Sep 22, 2010 by Dr.Drew


I have some troubles with AD9912.

I use this DDS with direct low noise 1GHz clock and look for output frequency ranges where SFDR in 20 MHz BW is greater than 95 dB. For this i determine a maximum spur order which level is smaller than -95 dBc. But i can`t do this properly because unknown spurious in output spectrum are being present.

For example:

- output frequency is 250,5 MHz;

- output level is 3 dB below maximum according to DAC FSC (Rset=10k);

- clock frequency is 1 GHz 6 dBm;

- DDS registers are: 0x1 Serial Config

                              0x90 PWD and Enable

                              0x1F or 0x00 N-divider

                              0x00 PLL parameters

                              0x1 HSTL Driver

I see an spurious with frequencies 250, 500, 750 MHz and so on. Could these spurious be in output spectrum or there is an mistake?

Here is an phase noise plot of output signal with spuruious at 0,5, 1, 1,5 MHz offset.