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How can I choose Value of DC block capacitor at HMC547?

     I'm using HMC547 (electric switch).

It operates from DC to 20GHz.

From Datasheet, evaluation board has no DC blocking capacitor at input and output port.

But frequencies I'm interesting are dual band, 3GHz and 9GHz. So I need DC block capacitor.

What is the proper value of Capacitor to have best insertion loss(S11) property?

When I use 0402 size 2pF, I got bad insertion loss characteristic.

Do you know any method to find optimum value of capacitance?