I've recently been given a project to lay out some PCBs which will contain a few Mini-Circuits RF amplifiers, and their associated biasing components. However, since I need to have several stages of amplification, I need inter-stage AC coupling to prevent the bias from each amplifier affecting the other amplifiers in the chain.
That said, I basically therefore need to chose series capacitors for coupling signals in the ~1.420 GHz range, and I do not really have the experience to know what I should specifically be looking for in terms of specifications.
I know I need capacitors with high Q, and probably X7R or NP0, ideally low insertion losses (S21), but I'm really feeling my lack of experience and knowledge in RF when digging through datasheets.
What are the relevant considerations for choosing parts for this kind of application?
Best Answer
You should be looking for capacitors that exhibit series tuning resonance well above the frequency of interest - the self inductance of the capacitor twinned with its capacitance cause a sharp series resonance. Take a look at this graph from AVX for their MLCC NP0 range in 0805 size: -
More than likely the 10pF will exhibit resonance around the 2 GHz mark. I'd say you either make use of this (because, as an AC coupler it'll still block DC just fine) or find a better grade but for sure you are going to find a lot of caps resonant in the GHz region you want to operate at.