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?



Parents Reply
  • The data above compares using 100 pF and 1 pF coupling caps on HMC778 RFOUT as the frequency was tuned from 10 GHz to 10.5 GHz.  Notice the output power was always greater with a 1 pF cap.  In addition, output power was flatter with a 1 pF cap as the frequency was tuned.  With 100 pF, output power varied 2.38 dB but with 1 pF, output power varied 1.21 dB over the 10 GHz to 10.5 GHz band tested.