3
\$\begingroup\$

I would like to design a circuit around the W7002 antenna (Dataheet) and the NXP PN532 NFC Reader Chip. Since I never worked with antenna design before I do not have a VNA and I also don't have experience in antenna matching. The goal is to get an antenna which is able to read desfire tags through a plastic cover at around 7cm distance. Even a shorter distance like 4cm would be fine, but generally a longer distance is better. Fortunately for the antenna I want to use there are many values already provided in the datasheet so it shouldn't be that difficult to calculate the necessary values. I tried to use Adafruit's circuit as a "template" and I understand that I need to adjust C1, C2, C7 and C8. The damping resistors R3 and R4 might need to be changed aswell and lowered a little (to 1 Ohm each). What would be a good starting point to get the correct values for those Capacitors? I assume once I have a general idea I can try increasing/decrasing the values on a prototype and see if that increases or decreases the range? Also I am not sure what the purpose of C5, C6, C9 and C10 is, those are labeled "NC" so are these non-existent in this case?

Would I contact the antenna manufacturer or NXP as the chip manufacturer in this case for further assistance?

\$\endgroup\$
1
  • \$\begingroup\$ It would be helpful if you linked to all pertinent datasheets. You can edit your question any time to update it with new info. \$\endgroup\$
    – P2000
    Commented Jun 24, 2020 at 6:39

2 Answers 2

Reset to default
3
\$\begingroup\$

If you copy the adafruit design, you probably only have to change/tune the parallel resonance caps C7 & C8.

All the other components are just there for impedance transformation and you do not need to change that.

But C7 & C8 define the resonance frequency together with your (custom?) antenna inductance (L). This is the sweet spot.

Even if you had a VNA, you would not necessarily be able to get better results than the try and error tuning approach. This is because the RFID card heavily detunes the resonant circuit when approaching.

You will notice that a little variation of the parallel caps do not influence the reading distance that much, once you found a good reading distance. The size and windings of your coil have a much bigger impact.

\$\endgroup\$
2
  • \$\begingroup\$ Since the antenna inductance is known (I have the datasheet linked above) I could also calculate the theoretical correct values and then start moving from there (assuming they don't work good enough)? If I use an oscilloscope I need to change C7/C8 so the amplitude of the Signal is as high as possible? \$\endgroup\$
    – Wiers
    Commented Jun 24, 2020 at 12:40
  • \$\begingroup\$ It seems that L=0.65uH. So you can calculate C7/C8. Attention1: You do not want to tune the resonance for 13.56MHz, this is the frequency where your LC tank has highest impedance, i.e. there is no current in the coil. Your target frequency is somewhere between 14MHz to 19MHz. Attention2: Placing caps in series halfes the value. \$\endgroup\$
    – Stefan Wyss
    Commented Jun 24, 2020 at 14:23
0
\$\begingroup\$

If you have an oscilloscope and function generator with a similar frequency range you can manually sweep the frequency until you find the peak and trim from there

However if you are not exactly concerned about the best possible range and just want something that works. You can clone an existing demo board's antenna design.

One of my earlier projects did just that. Measured out the dimensions, trace thickness and length. And played with the pcb files until I matched it. Measured the existing capacitors to get the same values. And worked as well as the demo board. Based on later measurements with the right tools it was a little off the mark. But it functioned for my purposes.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.