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Help with large spurs in output

Question asked by sc_thomas on Sep 5, 2017
Latest reply on Sep 10, 2017 by mcee

I am seeing pretty large spurs around my signal output. I am setting the modulating signal to 30 MHz in the AD9857 and using the parallel input to apply a variable frequency sine wave. In this application my output is quite noisy. It gets more noisy as I increase the frequency on the parallel input. However, even when I am providing unchanging I/Q data to the parallel data input I see spurs at +/- 288.462 kHz. These spurs have an amplitude of -40 dBm.  I think this is the source of the noise in my system. I am applying a 40MHz REFCLK and multiplying the clock up 4x to 160 MHz for SYSCLK. Also, the AD9857 is being used in Quadrature Modulation Mode. The other configurations are specified in the table below. I am only using Profile 0. I don’t see this frequency coming into the device on the power rails. I am using linear power supplies on the rails and the analog and digital are generated separately. I wanted to check to see if you know of any issues with the part that would cause this. Are there settings in the AD9857 that would help me eliminate the problem?


The image shows the spurs when I/Q data is unchanging.