AD9371 receive path spikes


I have started to see spikes in the data from the AD9371 receiver and would like to understand the cause and eliminate them.

Initially I tested the AD9371 with the default centre frequency of 2.55GHz. The data was captured from JESD into RAM and analysed over of a range of test tone frequencies and amplitudes. The amplitudes were measured in -10dbm intervals down to -100dbm and no-signal. At the lower end the output was noise in the range of +/-10 counts.

Altering the centre frequency to 1.3GHz and re-measuring now shows single sample noise spikes in the input signal when significantly lower than 8-bits (eg/ +/-40 counts). Spikes are in the range -256 to -249 counts, and +248 to +255 appear over about 1% of the samples (example time domain plot of captured test data attached showing problem).

I am surprised that just changing the centre frequency would cause this, however now I am wondering about the LO/PLL and problems with it, and things like decoupling. The hardware is a AD9371 evaluation board.

Any ideas as to the cause of this and how to eliminate the source of the noise?

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