May I ask here questions on impedance?
If I am having 5mil wide track, how should I do to control its impedance to 50 Ohm?
Because when I am referencing EVM6437 design, it found its Gerber files has specification that all signal lines needs to be impedance controlled to 50 Ohm, however I found in the PCB file that
Using a impedance calculation formula
Z0=(K/ Sqrt (Er+1.41)) * Ln(5.98*h/(0.8*w + t))
I took K=79 because 5<w<15 (87 if 15<w<25), and
Due to the table above, which resulted in 83.56 Ohm impedance as calculated by the formula.
There are several parameters in the formula and the one thing I am sure about is that 5mil width needs to be ensured, but how about other parameters? Do I have control over h, and is it the same as prepreg thickness?
I have another question over how “h” is defined. If the signal trace is adjacent to a internal power plane, is “h” the thickness of the prepreg or core layer between them? If the signal trace is not adjacent to any power plane, for example, signal layers on layer 3, 4 and 5, and a power plane on layer 6, then how should I define h for trace on layer 4, which are two signal layers separated between power plane 6?
And is GND plane also included in the definition?
This is the layer stack of the EVM.
The equations that you have been using are for microstrip transmission lines, where a signal trace is run over a ground plane. "H" is the inside distance between signal bottom edge and plane top edge. For a given trace width, thighness, and dielectric constant, impedance is adjusted by altering "h", or core thickness. The ground plane is typically used as the reference plane for the signal trace. The PCB design and fab company typically tries to maintain the impedance specified by the circuit designer. I am not sure if this particular EVM was designed for 50-ohm impedance.
Thanks for the answer. I need to study these concepts carefully.
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