Jester
Full Member level 6
I realize a 2 layer board is less than ideal for this, however that's what were stuck with for this application.
I'm laying out a board that will make use of an open source Arduino board for the uC section, because the code is already written and tested on the MkrZero board. I imported the PCB files and when I perused the USB differential pair, first thoughts are this is not right. Trace width is 0.25mm (9.84 mils), and space is 0.2mm (7.87 mils). This is a 1.6mm board. Assuming FR-4
Viewed as an edge coupled external plane, Zdiff =148 \$\Omega\$
However, both the top an bottom are flooded with ground plane as shown.
Viewed as a coplanar waveguide, Zo is close to 90 \$\Omega\$, not sure about Zdiff?
I guess I need a more versatile impedance calculator.
Looks like they made some compromises on this board WRT the USB routing? Or does the top flooded plane significantly lower Zdiff?
The total length is only about 37mm (1.5"), so perhaps reflections are acceptable?
PCB house tells me to send it and they will tweak as required, however for 90 \$\Omega\$ based on edge coupled, the trace width would need to be closer to 40 mils and that's not a tweak, and 40 mil traces are not going to fit. I'm also not keen on using a 0.8mm board as there are some fairly heavy components in other parts of the circuit.
I'm laying out a board that will make use of an open source Arduino board for the uC section, because the code is already written and tested on the MkrZero board. I imported the PCB files and when I perused the USB differential pair, first thoughts are this is not right. Trace width is 0.25mm (9.84 mils), and space is 0.2mm (7.87 mils). This is a 1.6mm board. Assuming FR-4
Viewed as an edge coupled external plane, Zdiff =148 \$\Omega\$
However, both the top an bottom are flooded with ground plane as shown.
Viewed as a coplanar waveguide, Zo is close to 90 \$\Omega\$, not sure about Zdiff?
I guess I need a more versatile impedance calculator.
Looks like they made some compromises on this board WRT the USB routing? Or does the top flooded plane significantly lower Zdiff?
The total length is only about 37mm (1.5"), so perhaps reflections are acceptable?
PCB house tells me to send it and they will tweak as required, however for 90 \$\Omega\$ based on edge coupled, the trace width would need to be closer to 40 mils and that's not a tweak, and 40 mil traces are not going to fit. I'm also not keen on using a 0.8mm board as there are some fairly heavy components in other parts of the circuit.
