AD9957 output signal spectrum problem


I have a problem about the output signal spectrum about the QUDC of AD9957 . My output frequency is in the 280MHz range so my SYS_CLK is currently set to 960MHz. Assuming I set the interpolation factor rate to 50, my PDCLK frequency is 9.6MHz. When I set the Parallel Data I and Q vectors by "000000000000000000" and "011111111111111111", I think I would get a single tone signal. While, the signal spectrum I got is this(see the picture in the Attachment ), I'm confused about this. Some other modulated signals I generated also have this problem. I need your help.

  • The fact that I & Q are static values (0 and positive full-scale, respectively), means that your baseband signal is a static vector with a magnitude of 1 (normalized to the 18-bit input) and an angle of +90 degrees. This results in nothing more than a 90 degree phase shift of the carrier. Hence, you should only see the carrier frequency (280.0MHz) with no sidebands. That said, I do not understand the source of the spur at 120kHz offset.

    How, exactly, are you delivering the I & Q values to the device?

    Can you share a block diagram or schematic?