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:
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.
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