ST25R95 EMC 3rd and 7th harmonic failed

[JECKSON]

Hi ST members

I am using the ST25R95, following the application note and matching tool to define LC components for EMC filter and matching circuit. the function test is good, but I have a high pulse at 3rd harmonics 40.8MHz and 7th 94.92MHz, do you have similar case happens with emission issue? 

have any suggestion？

thanks！

[Travis Palmer]

Hello Jeckson,

In general it could be that a ESD diode is saturated and cuts off/limits the sine wave. I would like to ask you to check all the voltages, if they are inline with the datasheet specification.

Could you also share the layout related to the NFC part.

It would also be interesting, if you change supply filtering what the resulting change in the spectrum would be.

BR Travis

[JECKSON]

Hi Travis

Thanks for your help！

I will check all voltage tomorrow， please check my layout first

[JECKSON]

Hi Travis

I test VPS /VPS_TX is 3.3V

the ST_R0 is 8.2 ~8.5V （3PCS）

thanks！

[JECKSON]

Hi Travis

do you have any comment？

BR Jeckson

[Travis Palmer]

Hello Jeckson,

first I would like to highlight that the ST25R95 is NRND. There is an Applicaton note AN6143 available to help migrating your design from ST25R95 to ST25R200.

Which standard are you measuring against? The spectrum mask looks different from FCC or CE.

I need to consult our expert for the ST25R95 regarding this issue. I will feedback early next week.

BR Travis.

[JECKSON]

Hi Travis

I think it‘s VCCI standard. OK， I will wait for your responding.

BTW,I update some test data, the EMC data I posted before is come from our customer test ，we reproduced that test again，and find 7th harmonic is gone（maybe different test circumstance），but 3rd harmonic is still NG.

And we find that different cards（NFC tag）tested with our sample have complete different EMC performance.

this is Mirfare 7 card result：

 this is a Mirfare 4 card，40.7MHz keeps very low

why they behave so differently，it is due to NFC card performance？

BR Jeckson

[Travis Palmer]

Hello Jeckson,

If the card is put in close distance / close coupling on the reader, it will receive the maximum power. Most cards have an internal limiter which is rectifying and limiting the received voltage. Dependent on how this limiter is inside the card is designed, it can produce more or less harmonics. This is something the reader has no influence, since the hard is cutting off the tips of the sine and therefore creating harmonics.

You can try to improve the placement on the reader and the distance between reader antenna and card antenna.

How does the spectrum look like, when there is no card in the field, and only the reader device tested?

BR Travis

[JECKSON]

Hi Travis

this is the data without a card，40.7 and 94.9 MHz  are almost same with background

BR Jeckson

[Travis Palmer]

Hello Jeckson,

as you highlighted above, the reader on its own is perfectly fine, but different cards can cause, based on how the card is designed, distortions in the magnetic field and causing harmonics. Selecting the correct card might be the solution.

I would recommend to provide your customer a fixed setup, a specific card glued at a specific position on the antenna. 

BR Travis