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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?



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  • 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. 

  • fc  100 pF 1 pF Delta
    GHz dBm dBm dB
    10.0 9.59 10.12 0.53
    10.1 9.63 10.12 0.49
    10.2 9.41 9.90 0.49
    10.3 8.36 8.83 0.47
    10.4 7.63 8.09 0.46
    10.5 7.52 8.03 0.51

    The previous data was not recorded properly.  The table above is corrected data showing output power with a 100pF and 1pF coupling cap.  Notice the dB delta is now about 0.5dB as expected.

  • 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