PN7160 big SWR

[KLP]

Hello.

I have a small issue with matching the antenna to the PN7160 circuit. Everything seems fine, but not completely. The system operates in a single-ended configuration with the DXW21BN2511N# balun, in the setup as shown below.

X1 and X2 are inductors LQW18CNR16J00D (S1P models). X3 is the S1P measured using a VNA together with the balun. The simulation shows correct component selection:

The measurement in the actual circuit is very similar:

However, the SWR measurement indicates that the circuit is not well compensated.

The simulation shows the same as well:

The system works and reads NFC cards, but sometimes it freezes, generating a continuous 13.56 MHz signal, which leads to overheating. Where could the problem lie?

Krzysztof

[KLP]

Yes, I confirm. After changing the generator's impedance from 50Ω to 20Ω, the SWR now shows correctly.

I will try to measure the current, but it may take a while as this sometimes happens only after several hours of operation.

Krzysztof

[Tomas_Parizek · Accepted solution]

Cześć Krzysztof, 

I would say that the matching looks fine. You have almost exactly 20 Ohm. 

The reason why you see this VSWR is that you are using 50 Ohm impedance system for your measurement while the impedance of the circuit is matched to 20 Ohm. 

We don´t use the VSWR because the output impedance of the chip is not 50 Ohm. 

We rather use this approach :

And then the output power of the chip respectively its input current can be calculated as described here -> 

https://community.nxp.com/t5/NFC/PN7160-PN7161-DPC-Symmetric-Tuning-Values/m-p/1922063/thread-id/119...

About the overheating. Can you check the TXLDO input current (VDD(UP) pin) during the continuios 13.56 MHz ? This current shall not exceed 250 mA. 

BR

Tomas 

Customer Application Support Enginner 

[KLP]

Continuous current is 244 mA.

[Tomas_Parizek]

Hello, 

Thanks for your feedback ! As the limit is 250mA, you are already quite close. I would suggest to sligly increase the tuning impedance (from 20 Ohms to e.g. 23-25 Ohms) or decrease the TXLDO output voltage. 

This should decrease the continuous current to approx 230 mA. 

BR

Tomas 

[KLP]

Are the calculations from the spreadsheet the same when using the antenna with a balun as in the version without the balun? I have significant discrepancies between the spreadsheet, the simulation, and the actual circuit.

[Tomas_Parizek]

Hello KLP, 

Are you refering to this Antenna calculation excel sheet? 

Or the TX driver current calculation I shared before ?

BR

Tomas 

[vyacheslavguzhv]

hi Tomas,

Following your recomendations changes are made. However the issue still remains. Let me followup with more details. Please find below current customer's schematic. Kindly suggest what else can be modified.

[KLP]

YES. The measured current is 244[mA] and the calculated current is approximately 180[mA].

[KLP]

"What values of imaginary resistance are safe for the circuit?"**

[Tomas_Parizek]

Hello @KLP , 

Ideally, if the antenna is properly tuned. We recomenend to have imaginary offset +-3 Ohms from real axis for best efficiency. E.g. If your target tuning should be 20 Ohm. Then It is acceptable to have it as 20+-3i Ohm. 

If you detune the antenna by a card or a different object, the impedance will change to e.g. (7-40i) Ohm. Then the impedance seen by TX1 & TX2 is given by its absolute value. 

|Z| = √(R² + X²) -> √(7² + (-40)²) -> |Z|=40.6 Ohms 

So, it is always about combination of the real and imaginary part and their absolute value. 

The absolute value should not drop bellow approx 15 Ohms which might lead to TX overcurrent. 

BR

Tomas 

[KLP]

So this is a result I can consider quite decent?

[Tomas_Parizek]

Hello KLP,

Yes, this is perfect !!

BR
Tomas