Problem with ST25R Antenna Matching Tool

[CT-55555]

 Hello everyone,

for my project I am using the X-Nucleo-NFC08A1 board. In addition, I am using an external antenna that I operate differentially. The antenna is connected to the board via an SMA connector. I know that I should also adjust the matching network to the antenna. For this, I wanted to use the ST25R Antenna Matching Tool, but I have a few problems determining the appropriate antenna parameters. I have also already read out the antenna via a VNA. :

 

The aim of the design is to match the antenna with the matching network to 50R at a frequency of 13.56 MHz, right?

[Travis Palmer]

Hello CT-55555,

Your antenna is quite de-tuned. The resonance frequency of the LC tank is too low.

I assume the parallel capacitor is too big.

There are dedicated antenna design appnotes for each product. 

For example AN5276 for the ST25R3916B. It describes how to do the antenna matching and what aspects to respect. The antenna matching tool provides the required calculation for the products and different topologies (differential, single ended, single ended with cable (50Ohm)). Additional information can be found here:

https://www.st.com/en/nfc/st25r3916b.html#documentation

To follow the process described in the antenna design application note, you should first disconnect the antenna and measure its parameter. Afterwards you input the antenna parameter into the tool.

The target matching impedance is setting the output power / power consumption. It is chosen based on your application and requirements (available power budget). 

BR Travis

 

[CT-55555]

Hello Travis Palmer, 

first of all thank you for you're answer. The picture shows the characteristics pf my antenna and its selfmade, so I dont have any data information. I followed the application notes and tried to use the tuning circuit calculator but the self-resonance-frequency is to low and I am not able to calculate the values of the other Devices. I tried to change some values to see what happens. The best version I got was this. Kepp in mind my antenna is connected to the circuit with long cable. 

I changed C305 and C308 from 680pF to 330pF and added  an extra resistor with 22 Ohm. Am I on the right track or have I neglected something? I try to do some fine tuning to get a better impendace. I also want to ask if there is a problem with the extra resistor because I will lose some power on it ?

Best Regards 

 

 

 

[CT-55555]

Additional with the help of the VNA i measured the antenna with the cable so the first picture of the post shows boath together.

 

 

[Travis Palmer]

Hello CT-55555,

AAT principle is to change the resonance frequency of the antenna by changing the parallel cap connected to the antenna. With a cable (single ended with cable topology) in between this is not possible, because only the matching circuit for transforming from target matching impedance to 50Ohm is on the same PCB. The antenna is most likely on another PCB. 

It would be possible if you have an additional line and VCC's (voltage controlled capacitors). You could then adjust the 50Ohm matching of the antenna itself (similar like on the differential matching).

You could also AC / DC couple the VCC controll line and NFC signal on the coax cable and extract it on the other side again. ( NFC Signal = AC, VCC bias voltage = DC). But the VCC signal must be filtered quite nicely i guess, otherwise you could get issues during FCC testing.

The Equivalent circuit of a coaxial cable looks like this:

Which I think is causing the un-symetrical wave shapes if also the shield is driven with the NFC signal. 

I did not yet have time to run some simulations about that, but the signal line has R + L in series, where the shield does not. So my assumption is, if you have a differential signal on both of the lines, the impedance of both lines are different and the sinus signal looks different (not differential anymore) on each line.

 

br Travis

[CT-55555]

Hello Travis, 

thank you for you're time and the detailed explanation. I tried single ended and it works very good. It's just some fine tuning. I would like to compare different antenna with different matching curcits. Do you have any tips for doing a comparrison ? 

Best regards