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AD9789 image spurs

Question asked by Alexander on Jul 1, 2011
Latest reply on Aug 31, 2011 by Alexander

Hi,

I'm trying to setup the AD9789 eval board for generation of single-tone signals in a part of TVWS band from 512 MHz to 602 MHz (the center frequency is 557 MHz). In order to do this I activated QDUC mode, turned off channels 1 to 3, bypassed QAM mapper, SRRC and interpolation filters, wrote P/Q equal to 1. THe DAC clock in my setup has frequency of 16*(557MHz/4) = 2228 MHz. I hoped that by setting the BPF center frequency to 557 MHz and changing the NCO frequency between 0 to 45 MHz (positive offsets from 557 MHz) and FDAC/16-45 to FDAC/16 MHz (negative offsets from 557 MHz) I would be able to cover the whole desired band. Everything works as expected except that I see a strong image spur when the offset from 557 MHz is more than ~30 MHz. The level of the spur achieve -30 dBc at the band edges. I would be very grateful if anyone could tell me what am I doing wrong.

 

Regards,

Alexander

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