NFC: Rmatch value for matching circuit

[asael_hernandez]

Hi.

I am designing the NFC antenna and using PN532 NFC controller for a project. I'm not a expert about NFC or RF circuits.

I have some questions about Rmatch.

About Rmatch I understand that need to be measure with an impedance or network analyzer. But, for prototype design, with what value of Rmatch I can obtain values for matching? What value of Rmatch should I start with?

Is there only a Rmatch = 50 Ohms or is there a range?

If there is a range, what is the range?, and, how affect a greater or lesser value of Rmatch to the NFC circuit?

Thanks for your replies

[Kan_Li]

Hi Asael,

Yes, for PN532, the recommended Rmatch is around 50ohm, and the range is 40 - 50 Ohm, the general tuning flow is like below, and please kindly refer to https://www.nxp.com/docs/en/application-note/AN1445_An1444.zip  for more details.

Hope that helps,

Have a great day,
Kan

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[Gladiator13]

Hi @Kan_Li ,

I am using a PN7150 in reader mode. Matching is done according to the application note right up to the RX path.

What I've seen is, that some NFC Tags have an extensive impact on our antenna characteristics.  This means that although our circuit is matched for unaffected scenario (and some smaller 15693 Tags), all 14443 tags I've tested made it perform considerably worse. When you look at a typical smith chart measured at both TX pins, the presence of an 14443 tag throws the 13.56MHz mark right down to the low resistance area (talking R<10ohm for a Z(Rmatch) = 40ohm circuit). This will increase current consumption for the duration and probably decreases EMC compliance. 
How should we handle that? Problem is, that we use both types of tags and have to pass booth EMC tests.

[asael_hernandez]

Thanks for the answer.

One more question. The impedance matching depends only on the NFC controller chosen?

[Kan_Li]

Hi Asael,

Actually the matching process is almost the same, while the target impedance depends on the NFC controller chosen.

Have a great day,
Kan

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- We are following threads for 7 weeks after the last post, later replies are ignored
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