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vias create impedance discontinuity, how are they used in high speed design?

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matrixofdynamism

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Vias turn at right angles and cause impedance discontinuity. It is difficult to get a 50 ohm impedance when signal path encounters a via. This can cause a disaster for high speed signals.

When doing high speed design, how do people deal with vias? Sometimes it may not be possible to avoid them, and I am not sure if using termination resistor is sufficient. It may be possible to carry out 3D simulation to determine how signals will behave when passing through a given via, but I am not sure when people would need to go so very far.
 

SunnySkyguy

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Every via has an inductance value.

Many micro vias has a reduction in L but the increase in effective width affects impedance.

We know Fringe effects from sharp corners and 90 deg bends affects leakage or radiation losses, so standard radius to widths are often used.

What performance do you need?
 

volker@muehlhaus

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Vias turn at right angles and cause impedance discontinuity.
Yes, a bend causes some extra series inductance. One possible solution is to compensate by adding shunt capacitance before and after the bend (pi network, low pass).

Just in general, you should think of signal and return (ground) path. You can design nicely matched transitions if you keep the 50 Ohm impedance in the vertical path, with proper placement of signal and return vias for controlled impedance in the via section.
 

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