# Impedance calculation of traces with discontinuity

1. ## Impedance calculation of traces with discontinuity

Hi! I had read a lot of documents and posts about trace impedance on PCBs, but i'm still confuse about impedance calculation.

I know that the impedance of trace is calculated by width of trace, height of dieletric, space between traces, and constant of material of dieletric, this is for differential pairs. I know that has much than that, but it is enough for my doubt.

I'm using the Saturn PCB Design to calculate my impedances...

So to the point...

How to calculate a the impedance of a trace that has discontinuity?, I want to know the impedance of some trace that has at least 3 segments;
1 - from hdmi connector to a via_1,
2- from this via_1 to another via_2,
3 - from via_2 to a SODIMM connector.

Ok, so I think that the impedance should be the sum of all segments where each one can be understood as;
Z1: impedance of segment 1
Zvia1: impedance of the Via_1
Z2: impedance of segment 2
Zvia2: impedance of the Via_2
Z3: impedance of segment_3

So my total impedance should be Z = Z1+Zvia1+Z2+Zvia2+Z3.

I had a HDMI that is not working at 1080p, so i'm trying to know what cause this problem, and the impedance of 50Ohms of Single trace is required and 100Ohms to differential pairs, and i'm think that this two vias are problems, because when i calculate the impedance considering that is a continous trace i got 100Ohms to differential pairs.

So i suppose that this is at least one problem why it is not working properly.

What is the right way to calculate this kind of impedance?

Thank you.  Reply With Quote

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2. ## Re: Impedance calculation of traces with discontinuity

Get a system with a proper signal integrity function and a 3D field solver...

If you are getting discontinuities you are doing something wrong.

The impedance is not the sum of the 3 sections, each section will have its own characteristic impedance, these should be the same.
Use Saturn to work out the track and gap you need to get the correct impedance.

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Impedance seen by the signal is...
First trace, via, second trace, via, third trace.  Reply With Quote

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3. ## Re: Impedance calculation of traces with discontinuity

If you would design a HDMI system, I recommend you to use a PCB Signal Integrity Simulation Package like Cadence Sigrity or Mentor Graphics Hyperlynx ( or similar )
Otherwise you'll get a lot of trouble with PCB..  Reply With Quote

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4. ## Re: Impedance calculation of traces with discontinuity

High speed HDMI is 3.4 GBPS bit rate, same range as SATA or USB SS. Differential line impedance 100 ohm end to end. A short connection on your PCB, e.g. 50 or 100 mm should be no problem and can be designed without SI tools. You need continuous ground plane for the line, proper impedance calculation, as few vias as possible. If vias are used, drill, pad and anti-pad size must be appropriate.  Reply With Quote

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