This is one of those hardware questions that's making me bang my head against a wall.

About six weeks ago I spent a day tuning the NFC antenna on my board. I ended up using ~71pF of shunt capacitance (a 24pF and a 47pF in parallel). I tried 24/47pf configuration on a fresh board and, lo and behold, it worked! So I put it aside. Now, coming back to it, I've put the same values on yet another fresh board and it's not working.

Could it be the manufacturer? The components I'm using now are from a different manufacturer than the ones I used before. I've heard that this stuff is sensitive enough that even sourcing components of the same value from a different manufacturer can mess up your design. Or could it be some ambient factor like temperature or humidity? It's gotten a lot warmer here over the last few weeks. I'm having trouble believing that it's the board. These are all boards from the same manufacturing run, and should be more or less identical.

How can I tune the NFC antenna reliably?


This is the RF chip I'm using.

And here is the application note for tuning its antenna (including a circuit diagram).

The old caps I was using (I think - I tossed the packaging when I finished them, but the other caps in the tuning box were Murata):



The new caps I'm using (not working): this & this

  • 2
    \$\begingroup\$ How about a circuit diagram, or links to the datasheets of the capacitors (those that worked, and those that didn't.) Capacitors often have an intrinsic impedance, and it can be vastly different depending on how different models are constructed. \$\endgroup\$ – JRE Apr 9 at 14:21
  • \$\begingroup\$ Hi JRE! I've added what you requested to the main post, above. \$\endgroup\$ – macinblack Apr 9 at 14:35

The Murata GJM parts are intended for use in high frequency RF applications. The datasheet gives information about performance over a fairly wide range of frequencies. Such as this:

enter image description here

The Yageo CC series datasheet says that they are general purpose parts, and doesn't mention RF characteristics at all.

Looks like you need to get hold of some more of the Murata parts, or equivalent Yageo parts.

Capacitors are never perfect, and therefore have some amount of inductance and resistance - it's not something that manufacturers add to them, it's a consequence of using real materials.

If the manufacturer puts in the effort, they can make parts that are closer to ideal. That effort costs money, though, so those parts are more expensive.

You end up with different product lines that have different characteristics.

In your case, you are replacing parts that are optimized for high frequencies with parts where no thought or effort was given to RF performance.

They could vary in numerous ways from the ones you used before. But, the manufacturer doesn't specify those parts for that use so you can't really tell where the difference lies.

They can differ in how much inductance they have, and in how much resistance they have. Since they are ceramic, even the amount of capacitance they have can vary depending on the DC voltage present.

  • \$\begingroup\$ This makes a lot of sense! Thanks for the help. I've just reordered some of the older Murata parts, so I'm going to test them tomorrow to make sure this works, but I'm hopeful this will be my solution! \$\endgroup\$ – macinblack Apr 9 at 15:27
  • \$\begingroup\$ Update: the old parts didn't work (assuming the ones I pointed to were the original ones I used). The weird thing is that the measurements of impedance that I'm getting from the board are different too now. I'm doing some trial-and-error right now trying to find a good component match. \$\endgroup\$ – macinblack Apr 10 at 20:38

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