HMC778 RFOUT coupling cap (why 100 pF)?

Hello ADI,

Why was a 100 pF coupling cap used on the HMC778 RFOUT on the eval board schematic?  Since this part operates around 10 GHz I would have expected that a much smaller coupling cap would be used.  I swept a 100 pF, C0G 0402 cap at 10 GHz and the insertion loss = 0.6 dB and the return loss = 13.5 dB.  Does ADI want the coupling cap to work down below 1 GHz for some reason?

Thanks,

Eric

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    •  Analog Employees 
    on Apr 8, 2021 12:27 AM in reply to Eric-Hooker

    Hello Eric,

    Thanks for sharing those measurements.

    Practically the quality (family, vendor, etc.) of the capacitor is also important. But generally speaking you are right, a smaller value cap fit here better as your measurements suggests too.

    Even you may not need any capacitor at RFOUT because this pin is already internally AC-coupled as shown in the datasheet.

    Regards, Kazim

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