Q factor for ST25DV

[bobbyjames]

I’m designing a tag coil for the ST25DV-04k. I am planning to use the ST25 to harvest NFC energy from a phone tap for a bioimpedance sensor (MAX30002). The ST25DV will then relay the sensor measurement to the phone via NFC again. I used an online calculator to design the tag coil, and these were the parameters I chose. 

Parameters for tag coil

However, I am unsure if my quality factor is too high. “59.77” is before coupling, and I assume it will be lower after that. I understand that a high Q is great for energy harvesting, but it can limit the bandwidth and garble communication between the tag and the reader.

Is my Q too high right now? If so, what is the optimal Q factor range? 

Anything else that stands out from my coil design?

[db16122]

UM2042 User manual Discovery kit for the ST25R3911B high performance HF reader / NFC initiator

[JL. Lebon]

Hello, 

You are correct, the higher the Q factor is, the better it is for energy harvesting as you will be able to transmit more power.

The counterpart off course is that you will decrease the power on side bands. Nevertheless, the ST25DV is ISO15693 and sidebands are at -/+423KHz, not at +/-848 like ISO14443 tags, so the impact is less than for a 14443 tag. Readers are usually very sensitive and can decode very low retro modulation in the tag’s answer, so I would stay with high Q for your kind of application.

That being said, I see in your screen shot that your target inductance for the antenna is 1.7uH. This doesn’t seem right to me. The target inductance must be around 4.8uH.

I also advise tuning the antenna a little bit above the 13.56MHz, more 13.8MHz. I guess that if you want EH you will be highly coupled with the antenna, so expect some de-tuning.

You can read those application notes for more informations:
How to design an antenna for dynamic NFC tags - Application note

How to design a 13.56 MHz customized antenna for ST25 NFC/RFID tags - Application note

 

Best regards.