rf 50 ohm impedance design in 4 layers nRF52840 dongle clone

[astella]

To the kind attention of Nordic support team,

I would like to do a question that is not strictly related to Nordic, but I hope your expert can kindly give me an answer, since I couldn't find a good source where to read up this information and I have the necessity to better understand this while doing a 4 layers custom clone of nRF52840 dongle. For a coplanar wave guide, the height of dielectric (about 1.6 mm for 2 layers pcb) is what enter in the formula to calculate the trace width to have a 50 ohm rf trace. But, what about a multilayer pcb? What is the proper height to consider? It is the total height of the pcb? Or the height betwwen the front layer where the antenna is located and the very first layer after that? The prepreg height? I'd love to have an answer from your wireless experts. Thank you for your attention and kindness 

[Andreas]

Hi,

 

Regardless if the board is 2-layer or multilayer, the dimensions for the coplanar waveguide shall be calculated by using the ground layer underneath as reference for the height of the dielectric. This is because the ground layer directly adjacent is acting as the return conductor. Using an internal layer in a multilayer board will mean the center conductor will have to be narrower to give 50 Ohm impedance, but it also lets you freely use any layers on the other side of the return conductor ground plane, as nothing should be placed/pass between the CPW central conductor and reference ground plane.

With a multi-layer board you can still use the bottom layer as CPW ground plane, but you then need keepouts in all the internal layers, in the area between the center conductor and the ground plane underneath.

 

Best regards,

Andreas

[astella]

Thank you very much