Hi,

I'm designing a high-frequency circuit - 0.5-1GHz and i want to calculate the parasitics that the PCB traces will have.

In calculations, the dielectric constant, Er, is included. Eg. This online calculator

This is the stackup of the PCB that i'm using from the manufacturer i'm using.

The Er for FR4 is 4.2-4.8 and the Er for 2116 HR Prepreg is 3.6-3.8.

Signal traces are on layer 1 and and ground layer is on layer 2.

If i'm calculating trace parasitics for the outer layer, layer 1, should i use 3.6-3.8 for the Er or 4.2-4.8?

It seems intuitive to use 3.6-3.8 since it's for the medium between the signal trace and ground layer. If it's not 3.6-3.8, why is the core FR4 of the PCB used?

Thanks

 

Yes, use the prepreg Er. The ground place shields the signal from the lower layers.