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Quad MxFE (ADQUADMXFE2EBZ) analog Rx response

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Product Number: ADQUADMXFE2EBZ

Hi,

Can I ask what is the expected Rx analog response of the ADQUADMXFE2EBZ quad MxFe variant board?

The hardware details page on the wiki states "Rx Analog Input Frequency Range: Up to ~1.8GHz" but I cannot find any further information. Judging from the ADC front end in the schematic, it should be fairly flat up to 1.8 GHz with some high pass filtering (cutoff?) from the dc blocking caps.

I ask because during testing we observed a dramatic high pass filtering effect. We are operating 8 ADC at 4GSPS with a decimation of 2, leading to 2GSPS complex output. Sweeping a sinewave input across the range 0 to 1.8 GHz (using rohde and schwarz SMA100B sig gen), we see the following frequency response with an Rx NCO frequency of 1GHz:

WIth low analog frequencies (approx < 300 MHz) strongly suppressed.  Is this an analog issue? Or is there likely something wrong with our design? I see the same roll off regardless of NCO frequency, with low frequency input signals always suppressed.