2
\$\begingroup\$

I currently intend to measure the Plane Capacitance of a PCB I'm going to build. I do have the measuring equipment (an VNA from R&S) but am unsure how to design the testpoints I am going to use (which connect to the relevant planes). Is there some kind of industry-standard connector type or how would you go about hooking up the equipment to the PCB?

\$\endgroup\$
  • 2
    \$\begingroup\$ What is a VNA? What is an NVA? Who or where is R&S? We may not be able to help but we may be able to learn. Can you add explanations / links into your question? \$\endgroup\$ – Transistor Aug 8 '18 at 14:07
  • \$\begingroup\$ VNA = Vector Network Analyzer R&S = Rhode & Schwarz \$\endgroup\$ – EE_socal Aug 8 '18 at 14:58
  • \$\begingroup\$ Thanks. I think I missed it in the tags. NVA is a typo then. \$\endgroup\$ – Transistor Aug 8 '18 at 15:05
1
\$\begingroup\$

VNAs are typically connected to PCBs with high frequency coax connectors such as SMA. VNAs measurements are based on 50 ohm impedance, but PCB planes can have much lower impedance than that. You really need to do a 2 port measurement to get an accurate result. See this app note for details.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the appnote. So as you say, 1 port measurement is unusable for my case but a 2 port measurement could work. What I derived from the appnote as well: Length is the key. Minimize the length and you get more information on higher frequency ranges. Unfortunately I'm not quite sure what kind of probe I'd need for such measurement. They are talking about microprobes but googling that leeds to many sites talking about chemistry. Other sites offer RF probes. I guess this would be more what I'm looking for but wouldn't I need a special probe for the Kelvin measurement? The appnote shows a \$\endgroup\$ – Tom L. Aug 8 '18 at 19:56
  • \$\begingroup\$ coax cable attached to a PCB which seems a little bit clunky to me. Adding a SMA connector probably won't do the trick as well as that wouldn't allow me the two port measurement (or is there a probe just for that?). I'd really like a probe which can be mounted on a stand or similar and which then contacts via pads to the planes. \$\endgroup\$ – Tom L. Aug 8 '18 at 19:58
  • \$\begingroup\$ There are probes like this: gigaprobes.com But I am sure they are very expensive. \$\endgroup\$ – EE_socal Aug 8 '18 at 20:49
1
\$\begingroup\$

There are many ways to do this, depending on your purpose and your test capability. That said, I'd recommend fabricating a board with a test structure, then using EM simulations to do the rest. Build an EM model to match your measurement, then just do everything else in simulations. If you're just looking at a parallel-plate capacitor, it should be easy to get a sheet capacitance figure.

The test structure could be a simple plate capacitor (simple would be to pick a square of a reasonable size) in a coplanar waveguide (CPW) environment. Run the CPW line either to the edge of the board and use a bolt-on end-launch coaxial connector depending on your frequency range of interest (SMA is good to 18GHz, 2.92mm is good to 26.5GHz, 2.4mm good to 40GHz, etc...).

End-launch coaxial connector Source

You could also break out to a GSG pad structure and use a GSG RF probe. GSG probe on substrate

Source and a useful article on RF probing from Microwaves101

Edit: From your comments, it looks like you're mostly interested in the impedance of the power planes from the perspective of an IC (for decoupling purposes or similar). As I mentioned, the impedance "for [the] whole PCB" isn't necessarily the same depending on where you probe it. Likely what you'd want is a set of test points close to where you'll be decoupling your IC, and you could probably get by with just a GS or SG RF probe touching down on the two pads.

\$\endgroup\$
  • \$\begingroup\$ Wow, that site is great - I just spent an hour or so just browsing the topics. I like the idea of that probe, yet, I think we're talking of different things. As far as I understand you're talking about structures on a microwave design (e.g. a cap or similar). I am actually trying to do this for a whole board, whereas the planes span the whole board and their intent is to provide low inductance power paths (which I want to measure). I'm just thinking if I could use a similar probe for my task at hand. The frequency range I'm interested in is about 300k to 3GHz. \$\endgroup\$ – Tom L. Aug 8 '18 at 19:23
  • \$\begingroup\$ @TomL. Yeah, Microwaves101 is awesome. What exactly is it you're trying to measure? A network analyzer will give you S-parameters, from which you can extract things like capacitance, etc. If you were to, for example, connect an SMA connector to your PCB with the center pin on the top plane and the outer conductor connected to the bottom plane and measure the \$S_{11}\$, then you'd be able to extract the capacitance between the planes at the connector, which may or may not be accurate in other places on the PCB. This is why I think EM simulation is probably more suitable than measurement. \$\endgroup\$ – Shamtam Aug 8 '18 at 19:44
  • \$\begingroup\$ I should clarify when I say "at the connector," since I really mean "when excited at the connector." The important take-away is that the impedance looking at the ground planes is dependent upon where you look at them from. \$\endgroup\$ – Shamtam Aug 8 '18 at 19:46
  • \$\begingroup\$ That said, if you're trying to check the impedance of a path with two nearby pins, you could probably get away with a GS or SG coaxial RF probe touching down on two adjacent test-points. \$\endgroup\$ – Shamtam Aug 8 '18 at 19:49
  • 1
    \$\begingroup\$ See my edit. If the pads are big enough, you could probably fit two GS or SG probes on one pad and try a 2-port measurement that way. \$\endgroup\$ – Shamtam Aug 8 '18 at 20:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.