Hi, ADI

I download the BOM of ad-fmcomms3 board, the AC-Couple capacitor in the signal chain is TDK's C0603C0G1H180J;

I checked in TDK's website, found below specs of this capacitor:

We can see that **the self-resonant frequency of this capacitor is about 2.2Ghz**;

So I think the resonable working frequency range of this capacitor is less than 2.2Ghz;

**My question is why choose such capacitor for ad-fmcomms3 board?**

Thanks!

You can certainly optimize the value and type of capacitor for your application based on your frequency range of interest.

The capacitor is used as an AC coupling capacitor and its primary roles are to block DC and to provide a near short for the frequencies of interest. The fact that the capacitor starts to look inductive past 2.2GHz does not impact the performance since the impedance is still very low. However, at lower frequencies, 70MHz for example, the impedance is about 100ohms which can result in signal attenuation. In this case you would want a larger capacitor value, but typically the higher the capacitor value the lower the self resonant frequency. The answer may be a parallel combination of two capacitors where one would dominate the impedance at low frequencies and another capacitor that would provide a low impedance at higher frequencies.

Ideally, to arrive at an optimum solution, you would need to do an EM co-simulation which would take into consideration board layout effects, component and AD9361 input impedance behavior over frequency.