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Emission of 3rd harmonics

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QLiu.1
Associate

‎2019-08-09 11:56 AM

Hello,

I am using the ST25R3913, following the application note and matching tool to define LC components for EMC filter and matching circuit.

Now the function test is quite good, but I have a high pulse at 3rd harmonics (40.8MHz), do you have similar case happens with emission issue?

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Travis Palmer
ST Employee

‎2019-09-13 05:32 AM

​Hello Qiang,

Low frequency harmonic emissions can be originated by:

1.) Exceeding voltage level specified in the electrical characteristics. This would activate the ESD protection structure and cut-off / distort the voltage on the antenna.

Critical pins would be: RFI (3V limit), trim_x (20V limit)

2.) Another source can be if the differntial signals are not routed differential. e.g. RFI / RFO have different length or different component placement.

3.) The return path of the negative VDD current is physically seperated from the positive one. Already at low frequencies the return current starts to follow the path of the supply current. If its way is blocked, an antenna is created and will radiate emissiones due to spikes / changes in the power supply. (AN5043 chapter 4.1)

BR Travis

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Travis Palmer
ST Employee

‎2019-09-13 05:32 AM

​Hello Qiang,

Low frequency harmonic emissions can be originated by:

1.) Exceeding voltage level specified in the electrical characteristics. This would activate the ESD protection structure and cut-off / distort the voltage on the antenna.

Critical pins would be: RFI (3V limit), trim_x (20V limit)

2.) Another source can be if the differntial signals are not routed differential. e.g. RFI / RFO have different length or different component placement.

3.) The return path of the negative VDD current is physically seperated from the positive one. Already at low frequencies the return current starts to follow the path of the supply current. If its way is blocked, an antenna is created and will radiate emissiones due to spikes / changes in the power supply. (AN5043 chapter 4.1)

BR Travis

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