ST25R3911B: Lower the output power

[luigiantuono]

Hello,

I am working with an NFC reader with an ST25R3911B chip. The problem I am experiencing is the output power. The power emitted by the antenna is too high; therefore, my task is to reduce the transmitted power.
To do this I have relied on reading your application notes. From this extensive reading I realized that there are many possible solutions to reduce the output power, including:
- reducing the supply voltage of the chip;
- using a single-ended antenna instead of differential antenna configuration;
- designing an antenna with higher impedance;
- change the antenna driver output resistance;
- change the chip internal regulated voltage.
Unfortunately, I am working with an already defined differential architecture that excludes the first three options (because would be optimal to not change the current HW structure of the PCB). This allows me to hope for only the last two.
First of all I am assuming that these last two options do not require a change in the PCB hardware, if this is true could you give me a hand in figuring out how to implement these last two options at the firmware level (register addresses to overwrite and how)?
Thank you for your time.

Sincerely,
Luigi

 

[Ulysses HERNIOSUS]

Hi Luigi,

• driver resistance can be influenced by setting droff* bits - please be aware that when you are using protocols using AM modulation it you may also need to adapt dram* settings or use automatic adjustment
• regulator based power reduction: Use reg_s=1 and change rege_* bits.

BR, Ulysses

[luigiantuono]

Can you please confirm that the registers you are referring to are:

- for driver resistance : 0x27

- for regulator register : 0x2A

if so, I will try both of the approaches, and I will be back in case I need more help. Thank you. 

Kindly,

Luigi

[luigiantuono]

Thanks for your answers,

I am going to answer back in chronological order. So starting from @Ulysses HERNIOSUS :

- The tag is in the reader field during the dump of register's content while the reader antenna is a custom printed antenna. I am sure of the correct NFC pairing between the passive tag and the reader cause I set a blinking led feedback on the custom printed PCB that host the ST25R3911B.

- I am measuring the spectrum using a SA as in the setup shown in the attached image:

The setup is installed inside a metal box.

I am quite sure that this partially answers to @Travis Palmer 's questions anyway I would like to give to you some more details regarding the HW configuration we used for the ST25R3911B. This is only becauise now my main issue is to reduce the power associated to the second harmonic of the signal. After that I weill go on with the lowering the output power of the chip itself.

Assuming that we are using an antenna (for the NFC) that has this characteristics:

Using then a tool for the antenna matching I found out that the suggested matching circuit is this:

Which is not that different (at least for the EMC filter) with respect to the one used on the custom PCB I am referring to, which is the following:

So, assuming that the coupling circuit is well designed (I hope that makes sense to you as well), my final guess to try to solve the problem involves AM modulation.

In both scenario1 and scenario2 files that I attached to my previous message, I saw that the contents of register 0x09 has bit 5 set to zero. This implies OOK modulation which in the case of an ISO/IEC 14443-3 Type A tag is not working properly.
I was wondering, the Analog preset command, from the information obtained from the configuration register (0x03) should automatically set the correct modulation type for the specific tag but this does not happen. Assuming this is the problem (I'm not sure because I don't know for sure how the chip behaves and whether it changes the settings and then resets them with each coupling cycle), could you help me understand this properly?

Also, if you find any obvious issue with the HW matching circuit please let me know because it seems fine to me and I don't want to go in the wrong direction keeping you busy with my questions.

Hope you're having a good day.

Kindly,

Luigi

 

[Travis Palmer]

Hello Luigi,

 

Getting one step back, I try to summarize:

Initially you wanted to reduce the output power of the ST25R3911B. 

Measuring the field strength with a SA you have seen that the higher harmonics are quite high.

The explanation for this is, that NFC is based on magnetic coupling. Two loop antennas (some people are quite strict and claim that this are coils and no antennas) are communicating via magnetic coupling. The SA uses a normal E-Field antenna, which is most probably "only" sensing the electric near field of the PCB. 

In other words, I have doubt that this setup is appropriate to evaluate the magnetic field strength of NFC systems. I would recommend to measure the field strength according to ISO10373-6 chapter 5.3 or 7.1.1.

Do you have any issues regarding the 2nd harmonic - 27.12MHz and any emission test / emission standard? 

Please let me know, if this helps you.

br Travis

[luigiantuono]

Hi Travis,
unfortunately the latest RED 2014/53/EU related tests performed on this configuration have not been passed. So my task, as I write in the first posts of the topic, is to lower the power output from the chip and reduce the amplitude associated with harmonics of higher than first order.
I realize that the setup for measurement is not optimal but I need it to have an estimate, albeit a rough one but still an estimate, of the frequency content of the field produced. Unfortunately, following the setup described in Chapter 5.3 of ISO10373-6 would be too great a complication for this stage of development. This is the reason why I was asking for your help. In particular, I ask the question again, do you see any particular discrepancies in the chip configuration? Could these abnormal behaviors (second harmonic so high) be attributable to use of particular types of components? I thank you in advance for your time and if I can be of any help in providing any new information I remain at your disposal.
Sincerely,
Luigi

 

 

[Travis Palmer]

Hello Luigi,

 

could you share the part of the layout related to the NFC IC? Especially related to VSP_RF and VDD decoupling and supply filtering.

Another question is, how the device performs without the tag being placed on the antenna. Do you still see the same spurs. There are certain tags where the limiter inside can cause such emissions if the voltage on the tag antenna is too high. If it is not possible to change the distance in the final application, the antenna geometry could be changed to reduce the coupling.

 

BR Travis