50-Ohm controlled impendace

[spktu]

pcb ohm controlled

What is the practical way to achieve 50-Ohm controlled impendace of a trace on a 4-layer PCB?

 

[House_Cat]

You don't say what the inner two layers are on your 4 layer board.

If the top and bottom layers are signal layers and the inner two layers are planes, you acheive 50 ohm impendance by controlling the thickness of the dielectric between the top and bottom and the adjacent planes. The impedance is a function of the thickness of the trace, the width of the trace, the permittivity of the dielectric, and the distance between the trace and the nearest return path (the plane).

If you don't have power and/or ground planes, you will have to control impedance by routing your signals as either broadside or edge coupled differential pairs. In this case, the impedance is a function of the distance between the trace centers, the dielectric permittivity, the width of the trace, and the thickness of the trace. Sometimes the return path is done with ground or power copper flooding adjacent to the signal trace. The same rules apply as mentioned for the differential pair (the flooding becomes one side of the differential signal path).

 

[kobik]

ask your pcb manufactur for the board parameters
layer thickness, distance between layers, dielectric (er), trace width

because not all you want can be produce