EMC cutoff frequency of ST25R3911B

[QLiu.1]

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?

[Travis Palmer · Accepted solution]

​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

[Almonleo]

Why is 10Mhz a better cutoff frequency than the 17Mhz?

[Ulysses HERNIOSUS]

Hi Almonleo,

please see below post from Travis. In simpler words: 10MHz EMC filter as opposed to 17MHz has little effect on the 13.56MHz carrier but helps to bring down the second harmonic which will be beneficial for EMC testing.

BR, Ulysses

[Almonleo]

Thank you Ulysses

Maybe I don't understand how we can measure/simulate the S21 of a 1 port system.

Are you looking at the antenna itself a the 2 port? Do you connect the Z0 to it?

Thanks

Alex

[Ulysses HERNIOSUS]

Hi Alex,

also not being very firm in this as software guy but what I get from my colleagues: The S21 is the transfer function from chip (with its impedance) into antenna. These are the two ports. I think also AN4974 should give you more details.

Regards, Ulysses

[Almonleo]

Hi Ulysses

Thank you for the reference to AN4974. It helped quite a bit. Unfortunately, I am not familiar with Qucs, so I am not sure ow S21 was calculated.

In the graph shared by Travis , S21 is negative dB. If S21 is the transfer function from the chip to the antenna, does it mean that the antenna voltage is lower than the chip output voltage?

Any description of how S21 is being calculated and why port 2 is connected in series with the equivalent parallel antenna resistance?

Thanks

ALex