STEVAL-25R200SA Antenna parameters in ST25R Antenna Matching Tool

[Aeh1]

Dear..ST

I'm developing custom NFC PCB 4 layers 0.8mm based on 20x20mm main antenna example in STEVAL-25R200SA kit.

I tried to use the components in BOM for EMC filter L 0402DF-421XJRW 420nH 510mOhm and adjust frequency to

18MHz  and get EMC capacitor 186pF which is similar to the kit (180pF) I follow AN6065 as much as I understand.

However I don't know the Antenna parameters, Q value, of the kit, so the matching L, C, R seems not the same as the kit. So if you have these parameters, or smith chart of the main 20x20mm antenna, please share to me.

My application need very low power and the distance of the ST25DV64 tag and the reader is very close (5-10mm).

I will try to tune antenna for lowest current consumption so that the reader still read the tag.

Please advise Target Q, and Target matching Z, I should use with 20x20mm antenna.

The supply for the ST25R200 is 3.3V

(I read AN13219 from NXP as well, page 22 noted that Target Z 16 Ohm for 5V, 11 Ohm for 3.3V so I guess to reduce Target Z for lower supply?)

In addition, I found that Coilcraft 0402DF-421XJRW cannot buy easily and very expensive compared to other series.

Is document AN6065 antenna parameters in tool based on X-NUCLEO-NFC10A1 or N-NUCLEO-NFC09A1?

Both kits has the same antenna, but why the L, C matching circuit are not the same and not as in AN6065 tool?

Thanks in advance.

 

[Travis Palmer]

Hello Aeh1,

The typical flow is produce a small series of PCBs (10 to 20) and then measure the antenna parameter as described in AN6065. When measuring the antenna parameters the device should be in the configuration (which means housing, other PCBs metal parts). The measured antenna parameters can then put into the antenna matching tool. As you correctly stated, the target matching impedance is the main criteria for the power consumption.

Matching Inputs:

Feel free to select other EMC coils. You may want a lower value: 100nH to 270nH, a low DCR and a high Q-Factor.

The DCR and Q-Factor are key for efficiency of the inductor. You can also experiment and compare different types.

EMC filter. Select a value around 18MHz. Ending up with 18.1MHz or 17.9Mhz is not a big deal. You can tune it to get nice coil and cap values.

Target matching impedance: I would start with ~35Ohm and then based on performance and power consumption either increase or decrease it.

Target Q-Factor: You can choose it based on the highest bit rate you want to be able to communicate. For ISO14443 Type A 106kbps a Q-Factor of 30 to 35 gives the best performance but you will not be able standard compliant for 848kbps. 

Assemble the matching on a few boards and follow chapter 7.4 of the ST-Appnote (AN6065 - Rev 4)
.

It is also important to go through the following chapters of the AN. 

Especially: 8.2, 8.3, 8.4, ! 8.6 ! , 8.7.

Please let us know if you have any further questions.  

Please feel free to share your low power design experience with the community. I think it might be quite interesting for other members too.

BR Travis