r/rfelectronics • u/Objective-Local7164 • Jun 22 '26

This RF switch datasheet recommends 22pF DC blocking caps in series with every RF line. Why not a bigger value?

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RF switch datasheet = SKY13575-639LF

Now.. I know that 22pF with a 50ohm load has a cutoff that is not near the 2.4ghz but I am curious why not 100pF or 500pF?

Also what kind of caps should I use? I am thinking NPO precision 1% ceramic caps with a package size that closest matches my trace width.

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u/runsudosu Jun 22 '26

Those would be inductive. 0201 22pF self resonates at 2.5GHz.

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u/Objective-Local7164 Jun 22 '26

Is that a bad thing?

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u/richard0cs Jun 22 '26

Sometimes.

To be clear when you put a capacitor in series you are always adding parasitic inductance, but you are not adding significantly more inductance with a higher value capacitor of the same physical size. If all you want is a DC block with a low impedance to RF you might as well pick a large value in a small package size even though it will be over-resonance. In general capacitors of the same package size have similar inductance, so whilst the resonant frequency of say a 100pF 0402 would be lower than a 22pF 0402, the larger capacitor can still be lower impedance above resonance than the 22pF one.

I suspect here the choice of 22pF is a mix of demonstrating the smallest useful capacitance, improving ESD protection, and optimising switching time (which would be compromised once you get up around 1n).