low tolerance capacitor kit for antenna matching?

[k8943]

I'm trying to match an antenna for operation at 13.56Mhz using an iterative cycle of measurement (miniVNA pro2 vector network analyser) and simulation (RFSIM99) and seem to be going in circles. Have come to the conclusion (by including some tolerances in the simulation and exploring the extremes) that it may well be because the tolerances of my trusty Aliexpress 0603 ceramic capacitor kit are too wide (typically 5%(<200pF), 0.5pF (<10pF), 0.25pF(<5pF))).

There seem to be plenty of low tolerance (1% or less) 0603 ceramics on Digikey but have had no luck in finding an appropriate low tolerance kit (say 0.5pF - 100pF) anywhere. This is new game to me, any suggestions?

[David Hess]

I always did just fine with C0G/NP0 ceramics but if you need something better, maybe a small trimmer would be better.  The old standard was silver mica.

[k8943]

Thanks for the C0G, NP0 suggestion.

Browsing Digikey C0G, NP0 0603, arbitrary value “10pF” and quantity 50, find a range of tolerances:

https://www.digikey.be/products/en/capacitors/ceramic-capacitors/60?k=C0G%2C+NP0&k=&pkeyword=C0G%2C+NP0&sv=0&sf=1&FV=mu10pF%7C2049%2Cffe0003c%2C400005&quantity=50&ColumnSort=3&page=1&pageSize=500

There are +/- 0.1pF (1%), +/- 0.25pF (2.5%) and +/- 0.5pF (5%). Sorting by price, the least expensive 1% is a relative bargain at about 10cents i.e. 5USD for 50.

https://www.digikey.be/product-detail/en/walsin-technology-corporation/0603N100F500CT/1292-1469-1-ND/9355473

However odder values are likely quite a bit more expensive.

Do you remember what sorts of tolerance were you using?

My existing capacitor book has about 60 values (50 of each) in the 0.5pF to 100pF range and the 10pF is 5% tolerance:

https://www.aliexpress.com/item/Free-Shipping-0603-SMD-Capacitor-Sample-Book-90valuesX50pcs-4500pcs-0-5PF-2-2UF-Capacitor-Assortment-Kit/32254162310.html?spm=a2g0s.9042311.0.0.27424c4dMCOzzf

Not for want of looking am not finding capacitor kits with low tolerance in a useful range. Must be doing something wrong.

It’s going to be painful to ASSEMBLE a low tolerance kit by ordering parts separately - and expensive - though will do if that’s what it takes.

Found the NXP app note that describes exactly what I’m doing (p24-34) and there’s a paragraph (p23):

Note: Be aware of tolerances! The most critical tolerance in the antenna circuits appears at the antenna coil itself, but even for the matching circuitry tolerances of < ±1% are recommended.

https://www.nxp.com/docs/en/application-note/AN11706.pdf

That adds weight to the hypothesis that +/- 5% caps is the problem. Don't dare power up the chip until the antenna is matched in case the current runs too high....

[nctnico]

I think you should make the antenna less picky. AFAIK needing better than 10% matching isn't going to happen. There are also tolerances in the output impedance of the chip and PCB manufacturing.

[k8943]

Have made some progress.

Instead of replacing all the components with each iteration (as the spreadsheet would suggest), learned the approximate effect of each type of capacitor and changed them little by little, checking the results after each modification. In this way ended up with a reasonable tuning instead of leaping between seemingly unrelated Smith charts.

Trying to round the square, perhaps NXP recommend low tolerance caps for a production situation so that each antenna is similarly matched? Whereas if one has the time to steer ships into the harbour one by one with a lot of little nudging movements, then the result might be achieved with any sort of tolerance?

Thanks for your support. ;-)