I'm routing an RF board ( above 1 Ghz) with microstrip lines. Our plan is to feed it from the backside, take it through the via onto the top layer and continue routing. as part of our rules we pull back the ground copper pour to ensure microstrip behavior.
Should we do the same for the other layers the via passes through? Or is the patristic capacitance low enough to not worry about? Or would the patristic capacitance cancel out the inductance of the via?
Answer
I would suggest treating the transition like an offset or eccentric coaxial line with the signal via being the center conductor and the ground vias the outer shield. Place the ground vias as close together as possible for the best performance. Also pull back all internal layers so that they are shielded by the ground vias.
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