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Impedance matching

IgorStojanovic , 06-08-2016, 07:24 AM
It is the first time I am doing a 4-layer PCB with Bluetooth PCB antenna, that should be 50ohm coupled with the corresponding IC. As my layer stack is TOP-GND (layer)-POWER (plane) - BOTTOM, Altium actually used POWER plane as a reference in the impedance matching. I just wonder is it valid calculation in my case, because, as I know from RF electronics, microstrip impedance is always calculated in reference to GND ? Am I supposed to put GND plane just below the signal layer (TOP in my case) where the matching trace is placed ? I initially make the GND as layer because I have separated ground regions (Audio-analog GND, Digital GND, RF GNF) connected to the single point, and as Altium requires at least one plane for impedance matching, I put POWER as a plane. I am not sure what is the regular option..
robertferanec , 06-08-2016, 07:37 AM
To keep the correct impedance, I always use the track geometry suggested by PCB manufacturer. I never rely on the numbers from Altium.

GND doesn't have to be the only reference plane (power planes sometimes can also be reference planes e.g. if you have a lot of decoupling capacitors between GND and Power plane). As I do not use impedance calculator in Altium, I can only guess, that Altium only recognize reference planes if you specify them as a plane, not as a layer. That is what is probably happening in your case.

And as you noted correctly, if you route your important track on the top layer (Layer 1), I would use GND as the layer below it (Layer 2) and that would be the reference plane.

, 12-22-2017, 03:56 PM
As you know, for the impedance matching, we calculated the width of track based in some parameters. One prarameters is substrate hight. Now, if ee use via and change the layer of routing, the hight will be changed, is it still valid the calculation?(for example the eidth was calculated for top layer respect 2nd layer, sp if we use via and go to for example 6th layer, how thewidth is changed?)
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