Impedance matching and RFO level definition

[PeterSPX]

Hello,

i want to design a new antenna layout with the ST253911b chip.

It is recommended to match the antenna across RFO1 and RFO2 to a load impedance of 10 Ohm.

Do i have to change the register "RFO normal level definition register" adress 27h to 10Ohm?

What happens if i set it to 1Ohm?

Best regards

PeterSPX

[Ulysses HERNIOSUS]

Hi PeterSPX.

Register 27h can be used to reduce output power, if you are for example field strength limits, etc. It does not need to match the load impedance. If you set it to 10 you are increasing the output resistance and as a consequence the emitted power.

Beware that 27h does not 100% translate into ohms but rather is a factor to the actual RRFO which is not exactly 1 ohm.

BR, Ulysses

[PeterSPX]

Hi Ulysses HERNIOSUS,

thanks for your quick answer.

Does I get reflection and overshoot when the load impedance unequal the RFO Resistor (EMC troubles)?

BR PeterSPX

[Ulysses HERNIOSUS]

Hi PeterSPX,

me being more on the software side I cannot give you a fully qualified answer. But my experience is: Most of the time we are using this register more or less freely.

AN4974 in chapter 3 talks about the considerations here and should be useful for you.

Regards, Ulysses