The EMC cutoff frequency of ST25R3911B is defined as 11.75MHz with L= 270 nH, C= 680p. However, the antenna resonates at 13.56MHz. I can understand if the cutoff frequency is defined between 14 -18 MHz to suppress the 2rd harmonic mode. Could you give some idea why you select the 11.75MHz, how the 13.56MHz signal transfer the filter?
Hello Qiang,
We have seen a positive behaviour while beeing detuned by a card having the cutoff below the 13.56MHz. In an NFC matching network, the EMC filter cannot be seen seperated from all the other components. It is resonating as well with other components (e.g. antenna inductance, series and parallel capacitor). Therefore the whole system has to be seen as a network of 5th order.
Comparing two matching network configurations - one with emc cutoff above and one below 13.56MHz - using the same matching impedance, no change in antenna voltage can be observed.
The ST25R antenna matching tool comes with an QUCS template, which allows you to easily simulate the difference.
Simulating two NFC matching networks, one around 10MHz LP-Filter cutoff, the other around 17MHz LP-Filter cutoff give the following results:
17MHz:
10MHz:
As you can see on the pictures above, both have a S21 value of ~ -5.2dB at 13.56MHz.
BR Travis
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