I'm attempting to calculate the necessary trace width of a PCB transmission line in order to have a characteristic impedance of 50ohm. However, different online calculators are giving me slightly different results.
My PCB substrate is FR408 from OSHPark. So, the dielectric at my 5-6GHz frequency range is 3.66, the trace height is 1.4mil and the dielectric thickness to the first intermediate layer (this is a 4-layer board) is 6.7mil.
This Mantaro calculator tells me that I need a trace width of 11.98mil. Chemandy on the other hand tells me 14.72mil. 3mil seems like a substantial enough difference. Is one of these calculators more accurate than the other, or should I use a different equation altogether?
EDIT:
Looking at the equations, Chemandy provides an explicit calculation for the effective dielectric (due to the fact that one side of the microstrip is adjacent to air, as explained here), whereas Mantaro does not. If I use the effective dielectric provided by Chemandy with the Mantaro calc, the difference between the results roughly halves, but does not disappear. This might explain part of the discrepancy, but is not a full explanation (and still does not specify the right equation to use).
The reason provided in the edit is incorrect. I found the exact equation Mantaro uses in High Speed Digital Design by Johnson, where he explicitly states it takes into account the effective dielectric.
Best Answer
It looks like two simulators you used bracket the value given by FEM simulation of the stackup described in the OSHPark documentation.
Here is a simulation of the characteristic impedance of your microstrip structure swept over trace width.
You should remember, as some of the comments on your question point out, that there will be some variation in the geometric parameters of your board during fabrication including changes in line width and prepreg thickness.