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AD9081 eval board Rx at baseband


In our application, we wish to receive a baseband centred real signal (e.g. 1 GBd PAM-x) using the AD9081 (i.e. downmixer is external). I am currently testing the AD9081 in a loop back configuration (i.e. DAC0 directly connected to ADC0), generating an RRC shaped 1GBd PAM4 signal using the AD9081 DAC and a matched filter on the receiver. I am using the on board HMC7044 clocking solution.

When setting the DAC NCO to 1 GHz (or any other value such that the edge the LSB is >0 Hz), I am able to receive the signal with the expected SNR (>50 dB). However when settting the DAC NCO to 0 Hz (i.e. a baseband centred signal), I see a dramatic drop in SNR to around 20 dB.

After lots of experimenting I'm a bit baffled by this and wanted to check is there any reason why the AD9081 (or perhaps the eval board) might not be happy receiving signals at analog baseband? I see no reason why not - there is a bit of flicker noise around DC but I do not expect this to affect the SNR so drastically.