X-nucleo nfc03a1 antenna measurement

[Andy b1]

Hi,

I'm trying to measure X-Nucleo-nfc03a1 with VNA:

i got completely different values for C11 C22 and C2 as is on nucleo board. I understood something wrong?

and when I remove Resistors 330ohm, inductor and 4 100pF capacitors 

I got like:

[Andy b1]

[Andy b1]

on tx1 and tx2

the question still what I measured wrong in first step that I didn't got the same capacitors as calculated in excelsheet? Any help are welcome

[Travis Palmer]

Hello Andy,

as far as I can see, you have changed R1 compared to the AN5248 from 50Ohm to 27Ohm.

Can you confirm that for the antenna measurement in your first post, you have disconnected the antenna?

BR Travis

[Andy b1]

Hi Travis,

thank you for your reply

-on the board IC is CR95HFV5 if I'm not wrong is 27Ohm?

 fig1:  only antenna I removed component  (with 4mm wires on SMA-F connector)

fig2 is not relevant I think because I measured at capacitor Ca with placed C8 and C15 other removed

fig 3 is measured on pads of  inductor on IC side

[Travis Palmer]

Hi Andy,

I would like to highlight that the ST25R95 as well as the X-NUCLEO-NFC03A1 are NRND.

In terms of sensitivity, output power and cost, we would have some interesting alternatives.

For example ST25R100/200 or ST25R300/210 which are far more advanced devices. 

If you still want to continue with ST25R95:

In your example you have changed the default EMI filter from 560nH / 200pF to 653nH / 180pF. 

I don't know which Rin has been chosen for the X-Nucleo-NFC03A1. When selecting an Rin of 50Ohm the excel sheet gives quite similar values to your picture. (I assume it is from the NFC03A1).

C11/12 = 57pF vs. 47pF

C2 = 84pF vs. 82pF

Maybe some finetuning was done on the NFC03A1 to compensate parasitic capacitance.

Also the used antenna parameter (Ra, La) are unknown. I have not found any document naming them. 

Maybe they have changed over time and the values you have measured are not the same as in 2014.

If I understood correctly, Rin defines your matching point (e.g. 27Ohm to 50Ohm). Changing Rin also changes C11, C2. So If you want to tune your device to 27ohm, it should look like this:

I hope, this answers your questions.

br Travis

[Andy b1]

thanks for reply.

I know is NRND but I need reference.

I double checked and impedance is 27R not 50R.  If I input all component on EVB board I didn't get the same

is there any reference Ra, La and smith diagram for X.Nucleo.NFC03a1?

[Henry Crane · Accepted solution]

Hello Andy,

As explained by Travis, the Rin entry in excel sheet corresponds to the pure resitive input impedance you want for your matching circuit.

As you said, the ST95HF/CR95HF output impedance is 27Ohm with Vps_tx=3V and 13Ohm with Vps_tx=5V.

In theory, the maximum power transfer between ST95HF and antenna is achieved if the matching circuit input impedance equals the  ST95HF output impedance. However, the power transfer remains very good in the matching circuit input impedance is higher than 27Ohm. 

for particular Energy harvesting application (you can refer to section 4.2 of AN4327 in https://www.st.com/resource/en/application_note/an4327-cr95hf-transceiver-antenna-tuning-circuit-with-emi-filter-stmicroelectronics.pdf ) it can be necessary to increase the matching circuit input impedance to maintain a good power transfert when receiver is stuck on antenna. in our M24lR-discovery kit we targeted 50Ohm. 

Coming back to the Nucleo-NFC-03 demo board, it seems that some boards are mounted with a ferrite bead instead of inductors for L0. impedance of these beads is close to 624nH instead of the 560nH expected from BOM.

Combined with boards parasitics that are not modeled in the excel sheet tool, this explains the discrepancy between the simulation and the measurement. As explained in AN4327, Please keep in mind that the tool intention is to give starting values for components, but measurement and adjustement of componenets are always mandatory (the tool can help in assessing effect of each component on tuning and help in finding final values).
another contributor to errors is the Q factor of components.

I made some step by step measurements to help you in your investigations (after removing all components on the board). Impedance are extracted @ 13.56MHz.
- antenna impedance: Za=1.19Ohm ~ 1.25uH (~means "in series with")
- ST/CR95HF input impedance on RX1/RX1: ZCR95HF= 16 ~ 22pF = 20kOhm//22pF
- Za // (ST95HF ~ 330Ohm ~330Omh)= 9.9Ohm ~ 1.35uH
- [Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF = 196~5.6uH
- [[Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] ~ 47pF ~47pF = 164 ~ 283pF = 190 // 24.3 pF
- [[[Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] ~ 47pF ~47pF]// (200pF ~200pF) = 41.7~157pF
- "Rin" = Zin = 47 ~ 177nH (+15j)

I measured the soldered components:
82pF=1.1~80.7pF
47p=1.1 ~47.2pF
560nH=1.4~566nH
100pF=1.1~100pF

and introduced the values in the excel sheet: step by step resuts are:
- Za // (ST95HF ~ 330Ohm ~330Omh)= 13.46 Ohm ~ 1.36uH (good match with measurement)
- [Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] = 238~4.93uH (slight deviation)
- [[Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] ~ 47pF ~47pF] = 239 ~156pF = 262 // 14pF (large deviation due to board parasitics/layout)
- [[[Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] ~ 47pF ~47pF]// (200pF ~200pF) = 35 ~ 131p (large deviation due to previous step deviation)
- "Rin" = Zin = 35 ~ 83nH (+7j)

if you use a tool like SimNEC and start the matching form the impedance mesured on the input of the two 47pF capacitances Z=[[Za // (ST95HF ~ 330Ohm ~330Omh)]//82pF] ~ 47pF ~47pF] = 239 ~156pF = 262 // 14pF, you'll see that the result is Zin= 45.3 ~13j which is very close to the measured value Zin = 47 + 15j.

To summarize, the issue you are facing comes from a BOM issue combined with the limits of the model used in excel sheet. As said by Travis, our ST25R reader IC family as to be considered for production but also because it comes with a more accurate and detailed  simulation model based on QUCS and available in our online tool eDesign Suite.

Feel free to contact us if you need more details.

Best regards,

Henry Crane NFC/RFID product support.