PN7150 NFC Antenna Simulation using RFSim99 Simulation Tool

[andy_kim]

Hello,

I've followed NFC Antenna Design training.

https://www.nxp.com/design/training/nfc-antenna-design:TIP-NFC-ANTENNA-DESIGN

And checked the simulation tool example of RFsim99 on it.

What I confused is the ground termination on C2 of the matching circuit in the simulation.

At the simulation on the Antenna Matching,

It does not apply the ground termination on C2 and the graph of smithchart is as it shows in the Antenna matching training.

However, when I attach the ground termination on C2 in the simulation tool,

The graph of smithchart is different from as it shows in the Antenna matching training.

When it come to reference schematic of PN7150, it has ground termination on C2.

That's why i put the ground termination on it.

Could let me know what is the correct simulation between no ground termination on C2 and ground termination on C2?

Let me attache the picture of them for your refer.

Thank you.

Best regards,

Andy

[Tomas_Parizek · Accepted solution]

Dear Andy, 

Thanks for your question! 

Let me try to explain it. There is a difference between the real circuit and the simulation. 

The "PN7150 Typical circuit" shows the symmetrical matching structure connected to the Tx1 and Tx2 outputs. On the real board, the center point must always be connected to the GND as shown in the picture (PN7150 Typical Circuit).

Unfortunately, the RFSIM99 does not support the symmetrical port (which would require the Ground termination on C2) but only the asymmetrical one. Therefore the 50 Ohm port is connected to the "TX1" and GND "TX2". The center of the matching (C2) is without GND connection in this case. If you keep the GND connection there. You short the center of the matching with the Tx2 and you get this second resonance "lobe" in the smith chart. 

So, the conclusion is the following: 

1. For the real PCB design, use the "PN7150 Typical circuit" with the GND connection in the center.
2. For the simulation in the RFsim99, use the "No Ground Termination on C2" because of the reasons I have described above.

BTW. I give you also some advice about the simulations. Use resistive losses for the EMC inductors and driver itself e.g. 1.5R. It gives you more realistic results. 

BR

Tomas

[andy_kim]

Dear Tomas,

Thanks a lot for your clear explanation and advices.

Best regards,

Andy