ST25R100 Tx power

[William_RnD]

I have successfully integrated a ST25R100 NFC reader with an ST25DV04K tag. Reading and writing data works perfectly.

But once you enclose the system in its housing, writing doesn't work, even though reads still succeed.

I presume this has something to do with that tag rely entirely on the RF field to power their EEPROM during write operations.

Can I increase transmitting power via software?

I can't find it the Documentation.

Any help greatly appreciated!

William

[Ulysses HERNIOSUS]

Hello William,

the software for our boards typically already sets the power to a high level. There is only little headroom (mostly improving power supply, using less drop on the regulator).

What you are describing really sounds more that you need to work on the matching and antenna. If the environment contains metal, displays and other conductive material then the matching needs to be done based on the final environment.

Best Regards, Ulysses

[William_RnD]

Hi, 

Hmm, well the way I designed this is I used the default matching network for the ST25R100, the one that is given by the NFC Tuning Circuit tool.

And the I remade the antenna, to fit in my enclosure with the same equivalent inductance.

How or what do I change best to make sure it can write through my enclosure, I know this is not a easy question and I probably need to measure a few things, if you know of handy sources I definitely have a read.

Is there like a procedure or something to define the values of the matching network?

Also attached a few pictures of the case in question. Its a plastic case .

Kind regards, 

William

[Travis Palmer · Accepted solution]

Hello William, 

It seems there is something in the housing which is influencing the NFC Field.

This could be:

• Metal Shielding
• Batteries
• other PCBs
• Keypads
• Display
• ...

I would recommend to measure the smith chart of the PCB alone and during the steps of assembly.

To measure with the VNA in the housing, you can either drill a hole above the EMI filter or use small wires which are considered during calibration of the VNA (port extension).

Big metallic surfaces in the housing would mean additional losses and de-tuning of the antenna. De-tuning can be considered when doing the initial matching. (If the resonance increase by 500kHz you simply tune the reader 500kHz lower when being unloaded).

To compensate losses you could add a ferrite sheet, e.g. between NFC antenna and metal part or change the geometry of the metal part.

Simply increasing the output power, without tuning the antenna properly would mean, that the device is running in a not ideal condition. Please have a look at the verification chapter of the antenna design application note.

I hope, this answers your questions.

BR Travis

[William_RnD]

Hi,

Yes that helps.

I will try to do these measurements.

Many thanks!

William

[William_RnD]

Hey, 

I have been experimenting with the tuning network, I have gotten the output to the antenna to 30Vpp, original was 17.5Vpp. I also have gotten one successful write, but this was holding the device under a weird angle, and it only worked once.

I also don't have a network analyzer sadly enough, I do have a spectrum analyzer and oscilloscope.

Can I tune it further to lets say 40Vpp? I feel like this is getting quite high.

Or should start looking into the ferrite sheet.

I have included my measurements, this is measured on the output of the tuning network.

Or do I have to mode my antenna farther away from the batteries?

Kind Regards,

William

[Travis Palmer]

Hello William,

30Vpp is already not too bad.

Can you share a scope shot of NFC field close to the reader using an ISO calibration coil? (if no calibration coil is available, a wire loop in ID1 size also does the job.

And maybe one where the pick-up coil is close to the tag (e.g. 4cm, 2cm and 1cm between tag and reader).

Trigger on the reader command (pulse width trigger, negative pulse, between 500ns and 15us).

You should see something like this:

I would be specially interested in the rise and fall time of the reader signal:

BR Travis

[William_RnD]

Hi, I did some measurements.

After these last changes I can read successfully, I just need to hold it longer still to the tag.

If I want to optimize this further should I change anything to the PCB design for the final sample?

Here are my measurement results:

No tag fall:

not tag rise:

Tag 4cm fall:

tag 4 cm rise:

tag 2 cm fall:

tag 2 cm rise:

tag 1 cm read:

Thank you soo much!!!

Kind regards, 

William

[Travis Palmer]

Hello William,

Is the antenna on all sides completely surrounded by metal?

Maybe you can introduce a cut/slot on one side to the boarder of the PCB.

You might need to re-tune the antenna but it should greatly improve the reading distance.

You can also see that your rise and fall time is quite sharp which hints to a low Q-factor that could be originated by high eddy currenty. 

BR Travis

[William_RnD]

Hi, 

No the case is all plastic, there is a small piece of metal on the left for the battery connectors.

Hmm all right, I'll look into it!

Kind regards,

William