NFCのトピックPN5190 Antenna tuning

PN5190 Antenna tuning

sinu52 — Fri, 04 Jul 2025 06:03:07 GMT

Hi,

I am trying to tune the antenna on my custom board featuring the PN5190B2.

I went to the following steps:

1: Measure the actual antenna L, Cpa, Rcoil with the VNA.

2: Use the NXP antenna tuning excel sheet calculator to calculate tuning components.

I would like to have a symmetrical tune at the recommended cutoff frequency of 14.5MHz.

this gives me the following values to tune with:

When measuring these values on the actual board i get this smith chart:

The tune is not quite symmetrical yet. I believe this is due to parasitics on the board that the excel sheet is not taking into account.

Now when following the guide in AN12549 chapter 4.4.2 suggests that i should increase C0.

When increasing C0 to 820pF. and increasing C2 slightly from 345pf to 348pF. I get a good enough match in my opinion:

The problem is that i am now worried that by changing C0 to 820pF my cutoff frequency has shifted from 14.5MHz to something lower. The Excel sheet calculates it to be 13.9MHz. But with the tolerance on components it might even be lower than 13.56MHz!

Should i revert back to C0 = 750pF and leave the tune unsymmetric or does my board need a higher C0 = 820pF to get a cutoff at 14.5MHz due to parasitics?

I am happy to clarify my problem further if needed.

Kind regards,

Remco

Re: PN5190 Antenna tuning

KaiLi — Sun, 06 Jul 2025 14:02:46 GMT

Hello @sinu52

C0 = 820pF to get a cutoff at 14.5MHz, this case should be feasible, and this can be used as the final design, I think.

Re: PN5190 Antenna tuning

sinu52 — Mon, 07 Jul 2025 08:44:47 GMT

Hello, KaiLi

Thankyou for your answer.

I dont understeand what you mean yet.

Will 160nH and 820pF cause a cutoff frequency on my board of 14.5MHz due to parasitics? When measuring this, the smithchart does give a symmetric tune.

Calculations show that 160nH and 750pF should give a cutoff frequency of 14.5MHz. When measuring the smithchart does not show a symmetric tune.

Kind regards,

Remco

Re: PN5190 Antenna tuning

KaiLi — Wed, 09 Jul 2025 07:26:53 GMT

Hello @sinu52

We can use online tool NFC - NXP Community to deduce the two sets of parameters you debugged:
Case 1, the deduced cutoff frequency is too low, this set of parameters is not recommended
Case 2, this set of parameters looks good, it is recommended to use this set of parameters
In addition, the capacitance value here does affect the cutoff frequency, but it is not due to parasitic effects.