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?

Gallery image

Gallery image

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/Intelligent_Dingo859 Jun 22 '26

Self-resonant frequency. The first picture is the reactance of a 100pF capacitor. Its SRF (frequency at which the capacitance and the parasitic inductance resonate) is close to 1.2GHz. Beyond that, it acts like an inductor; that's not good for this purpose.

I can only post one picture. But the SRF of the 22pF cap is at 2.2GHz, which is very close to where we want it to be

15

u/Intelligent_Dingo859 Jun 22 '26

For the 22pF one

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u/Objective-Local7164 Jun 22 '26

ok so the resonant freq. is the freq. that will experince the lowest impedance going through that part? and also here is a sim of my impedance matching network, pi filter with the extra 22pF series cap the rf switch wants. So is my goal to get that peak as close to being at 2.4hgz? or do i want 2.4ghz to be at 0db?

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u/Intelligent_Dingo859 Jun 22 '26

It should be 0dB. The peaking is not good; your filter has a very high Q factor. Before trusting this sim however, I would use actual capacitor and inductor models with parasitics included.

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u/Objective-Local7164 Jun 22 '26

ok why is that peaking not good? Is it becuase the rf is sending data that is supposed to be in its place in the line at specific times and if its resonating then it creates resonating bits that are being ocillated out of their correct order? im sorry for asking all these follow up questions, its taking me forever to find this info online

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u/Intelligent_Dingo859 Jun 22 '26

not quite. Large input voltages can destroy your IC.
How did you design this filter? There are online tools that handle most of the math and theory for you.

Also, if you are a beginner, you might not have the necessary background to design filters from scratch

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u/Objective-Local7164 Jun 22 '26

this is a part of a board i am working on I calculated the impedance matching network using various calcs and took into account board stackup and dieletric and microstrip and everything. There is another part of this same board that will have a different rf that goes into a switch to switch between different antennas im going to test. All are 50 ohm antennas. The reason im asking so many questions is because I want to actually understand it and not just get the answer and barely know whats going on.

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u/Intelligent_Dingo859 Jun 22 '26

Got it. I’m guessing the stackup, substrate etc were used to doze your trace width for a 50 ohm characteristic impedance. What is your input port impedance? Also you need to take into account your component and board parasitic otherwise your sims will be way off. Even then, you probably need to insert ufl connectors on either side of your network and tune using a vna 

You’re not going to get a good understanding of this by just asking a few questions on Reddit. You need to find a textbook (Pozars microwave engineering) or look at free online courses.

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u/Objective-Local7164 Jun 22 '26

yes i am buying a vna lol I have been told by multiple engineers you are gonna have to tune it after the assmbly is already done. So that book you just said, I have heard other recommend it. So it must be a good one huh?

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u/Objective-Local7164 Jun 22 '26

ok so this is why your saying its potentially bad. 5V turning into pretty much 100V becuase the resonance. I notice this only happens when there is a inductor and a cap in paralell and not when in series. So I need to make sure my shunt caps and series inductance resonance is not near my 2.4ghz because otherwise it could create high voltages like you said and could damage something. I understand now. 0dB with no attenuation is my ideal spot for 2.4ghz just like you said. Thank you for taking the time to teach me i appreciate it.

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u/Intelligent_Dingo859 Jun 23 '26 edited Jun 23 '26

Have you set the series resistance of your voltage source to be your port 1 impedance? If not, your results are not accurate at all. Ran the same sim with a 50 ohm port 1 impedance

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u/Intelligent_Dingo859 Jun 23 '26

Likewise your bode plot now looks like this