Applications for the ST25R3911B with increased power and read distance

[CFerr.1]

We have an ISO15693 application requiring read distances > 30". In reviewing all application notes, we have found a great deal of information for matching components but nothing to address the incorporation of an amp for increased field strength and distance. We have the X-NUCLEO-NFC05A1 as well as the ST25R3911B-DISCO. Interesting that the antenna loop is center-tapped on the latter and not on the NUCLEO. Inserting an amplifier for the RFO is straight forward but what about the RFI? Obviously we need to insure the divider keeps the input voltage within limits but how do get a clean modulated signal back from the higher-power RFO-loop?

[Travis Palmer · Accepted solution]

The easy solution would be to connect the RFI lines in a similar manner via a capacitive voltage divider to the antenna / amp-output. The divider has to be adjusted to the allowed maximum of 3V (better 2.7V) on the RFI input pins.

A more sophisticated way would be to add a 13,56MHz band-stop filter between antenna and RFI. It should be sharp enough to damp the 13.56MHz, but not (or less) the side bands (13.56MHz + 0.848MHz and 13.56MHz - 0.848MHz). Still the overall signal should be around 2.7Vpp on the RFIs.

The X-Nucleo-NFC05A1 uses the same antenna as the X-NUCLEO-NFC03A1. Since the antenna is not symmetric, it is not easy to add a center tap. The ST25R3911B-DISCO antenna is center-tapped. The tap can be opened by removing J404 on the bottom side. Removing J404 does not affect the performance or matching.

BR Travis