nRF905 on a PCB thinner than standard FR-4 1.6mm

[Witek]

Hello,

We have used the nRF905 in our products over many years. This time we have decided to design the circuit on a 0.8mm FR-4. The recommended layout and BOM suggest to use C11 (please check the "nRF905 RF and antenna layout" nAN900-04).

How do I choose the value for this capacitor?

Thanks

Witek

[JONATHAN LL]

Hi,

The C11 is optional and has never been determined so we have no value there. The C11 is optional and can be removed. If you need to calculate the differential to single ended matching network best option would be to just test a few components and see if it is needed or not and adjust accordingly.  

Regards,
Jonathan

[Witek]

Hi Jonathan,

Then, I guess, I just look at the signal strenth out of the antenna, to see what value works best?

Thanks

Witek.

[JONATHAN LL]

You could use a VNA to look at the 50 Ohm RF IO line where a antenna would be connected. You can then see how it effects the impedance and if the singel ended match is 50 Ohms. But do you need to use the C11 ? The result should be approximate 50 Ohms without C11. 

Regards,
Jonathan

[Witek]

Struggling with the range at this stage, so I am looking at all possibilities.

If I undersdand you correctly C11 should not make much difference?

[JONATHAN LL]

Are you able to measure the impedance of the line with a VNA ? There will be some change depending on what value that would would put on C11 but to what extent is difficult to say. Have you followed the reference layout closely? Have you tested the antenna before and know that it preforms well and has the antenna been tuned tot he device its on?

Regards,
Jonathan