RE: rf 50 ohm impedance design in 4 layers nRF52840 dongle clone

astella — Wed, 23 Oct 2019 07:08:55 GMT

Thank you very much

RE: rf 50 ohm impedance design in 4 layers nRF52840 dongle clone

Andreas — Wed, 23 Oct 2019 06:57:51 GMT

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