Transmission Line Impedance on nRF52840-PCA10056 Development Board

[DebbyX]

Hi,

I am trying to design the transmission lines based on the reference board design from Nordic.

I opened up the Altium layout file for the board which I downloaded from the website. I measured the RF trace width to be around 30 mils, and gap between ground and trace is around 11.6 mils. I looked at the layer stack-up, and found the board is FR4 with about 55 mils thick. I suppose this to be a grounded CPW waveguide, as it has copper at top and bottom layer with substrate in between. However, when I put numbers in the online calculator, the impedance value is around 70 Ohms.

Second method to verify the impedance is that, I use the reference matching network and 50 Ohm ANT load, and calculate back to the source impedance. I also find the impedance is around 70 Ohms.

My question is, is the chip nRF52840 designed to be 50 Ohms load match? Should we design 50 Ohms TL to get the maximum output power efficiency? Why the transmission line I measured is about 70 Ohms rather than 50 Ohms?

Thanks for your help!

[Kaja]

Hi DebbyX, 

so the impedance looking directly into the radio is not 50 Ohm, this is why we use a matching network at the output, to transform the impedance to 50 Ohm (and also filter away harmonics). We do the same with the antenna, use for example a pi-network to match the impedance to 50 Ohm. The transmission line should be 50 Ohm after the matching network for the radio. from the radio to the matching network, you can just use the trace from the reference design. 

I have this picture of the impedance, that I made for the nRF52832 (so the matching network is different), but it illustrates the impedance you see when both the radio and antenna are match to 50 Ohm. 


Best regards,
Kaja