Recommendations - Flexi PCB - RF output filter and PCB layout - nrf52840 and nrf52833

[Joel]

Hi. I am starting design of anothe rproduct which will be on a flexi PCB wherein i'll use nrf52840 and nrf52833 in the two product variants. Nordic Semi has reference design files for rigig 4 layer boards for nrf52840. I was wondering if you have developed similar reference layout for nrf52840 and/or nrf52833?

My only main interest in raising this ticket is mainly the recommended layout on RF side of things and the values of the Pi filter. I'm assuming if I stick to similar layout as the rigid board and keep 50ohms impedance same Pi filter would suffice. 

I am open to WLCSP or QFN or any package recommendation.

Please advise.

Thanks

[Andreas]

Hi,

 

Our recommendation is to replicate the reference layout as closely as possible, regardless of stackup. The pi filter has to be tuned anyway, at least verified that the emissions are ok. To do this you need to measure the output power and harmonics, and tweak the components (resolder) and see how the behaviour changes. If you need Nordic can also assist with this.

 

Best regards,

Andreas

[Joel]

Thanks Andreas. At the RF port of the nrf52 chip is it close to 50ohms or do I need to measure it and then design a pi filter for harmonics etc with chip output importance matched to 50ohm line? 

[Joel]

Also, I had conductively measured after pi filler on nrf52 DK using smt connector provided and a vna and found that impedance is not very close to 50ohms. Please can you confirm this is the case? What return loss should I see at this connector on nrf52 dk before antenna and its match?

[Andreas]

Hi,

 

The impedance is close to 50 ohm, depending on the mode. What you see sounds normal. I would maybe expect up to 2:1, high-output TX has higher VSWR and e.g. RX has lower.

We normally do not tune the matching networks by matching the impedance to 50 ohm. No matter how good the impedance is looking you still need to verify that the actual emissions are good (especially harmonics), and chance is you will have to redo some by looking at the emissions. Therefore it's easier to just solely look at the emissions in TX while tuning. Cut the RF trace, or desolder a series component and solder on a coaxial cable (open-ended or a SMD connector), center conductor to the center conductor of the PCB transmission line and outer conductor to the ground plane.

 

Best regards,

Andreas