ST25RU3993 internal power amplifier matching to 50OHM

[SStad.3]

How to match the output of the internal power amplifier to 50OHM? The Datasheet doesn't provide the output impedance. I know the eval board provides a schematic for the matching, but i don't want to simply copy and paste. I would like to understand. Any help will be appreciated.

[Bart Herse]

Hi SStad.3

the matching circuit for the internal PA has been developed in order to provide optimal performance.

This was done over the course of testing many different loads that was presented to the output of the internal PA looking for a high output power and good efficiency.

It is recommended to take the matching circuit as shown an then tweak it to compensate for layout changes and/or component type changes.

Cheers,

B

[SStad.3]

Hi Bart,

Thanks for your answer. Do you have recommendations for the impedance of the differential pair? It seems like the eval Board uses around 40OHM Zdiff. Is that right?

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Greetings Sebastian Stadler

[Bart Herse]

Dear SStad.3,

It actually would be better to have the diff pair trace impedance at lower values.

But at some point it becomes impractical to further reduce the diff pair trace impedance as the trace dimensions become large quickly.

Therefore these diff pair traces should be kept as short as possible.

Again, staying with the trace dimensions / impedance as shown provide a good internal PA performance.

Cheers,

B

[SStad.3]

So my layer stackup is 0.17mm of fr4 vs. 0,25mm fr4 of the eval board. But you recommend staying with the same dimensions for the diff pair? Trace widht is 0,8mm and spacing 0,15mm for the eval board. I calculated a width of 0,64mm and spacing 0,3mm to have similar impedance with my board.

[Bart Herse]

Dear SStad.3,

yes indeed, this was a recommendation. If your final trace impedance is similar then you should be good.

Cheers,

B