What about the self resonance frequency of caps with Gigahertz parts? For example, if I have an input coupling cap of 1000 pF
at 1 GHz, the SRF is lower than that. Is this a problem?
It is best to use a CAP below its SRF value. Especially on RF matching involved cases, like impedance matching or filter design etc.
When you are using the 1 nF CAP at input coupling or at DC bypass line, in most cases you will be fine for many reasons. The impedance that the CAP sees can reduce the SRF peaking, which can be 50 ohms for RF I/O pins and smaller/higher impedance for supply pins. And inductive at some cases due to the PCB traces.
Moreover, if you use the CAP above its SRF, you will start to loose CAP`s capacitic behavior. So if you do not need that you will be fine. Like the use of RF choke inductor at high frequencies, where the inductive characteristic is a bit demisable.
You can get the scattering parameters of the CAP and embed that into your circuit simulations and simulator will show you the response. Unexpected frequency responses can be due to the low SRF of CAPs used.
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