HMC778 RFOUT coupling cap (why 100 pF)?

Question

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

kpeker · Accepted Answer

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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HMC778 RFOUT coupling cap (why 100 pF)?