do really short traces need impedance control?

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I have an antenna line for a cell modem.  The modem manufacturer specifies a particular PCB layer stack up, seemingly with the implication that following that stack-up is good enough.  But the PCB manufacturer wants to know if the trace needs impedance control.  I don't have the emag skills to know if such a short trace does indeed require impedance control?

 

I've attached a small image showing the connector and trace in question (near the center of the image, refdes S2/CONN2).  

 

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Not only the track, but everything in that area relies on the layer stack up. There’s a coplanar waveguide in there. If you change the layer stack up, then that will need to be recalculated.

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Kartman wrote:
If you change the layer stack up, then that will need to be recalculated.

Indeed.

 

And it sounds like the OP is using the cellular module maker's reference design.

One of the key reasons to stick exactly with it is that it will greatly ease your approvals process - any deviation at all means that you will have to do the entire approval from scratch.

If you need to ask the question, you really don't want to be doing that!

 

 

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bkerin wrote:
The modem manufacturer specifies a particular PCB layer stack up

See above.

 

If you want support on this, then you really need to be speaking to that modem manufacturer!

 

Only they can give you the specific guidance you require.

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  You have two pads that form two capacitors to ground and a trace in between that forms a virtually zero inductance. The wire inside the chip  and the trace on the module itself are the predominant players.

 

  The only thing that you can do to control that line impedance is to adjust the width. Your antenna and the source impedance are likely 50ohm. There are many online impedance calculators that can help with it. You need the board's permittivity which in most cases goes from 3.5 to 4.5. You need the distance from the top layer to the next ground plane, and also the trace thickness. With these numbers you find out the trace width. For that distance i don't think is that critical. The area of those two pads is greater than the area of the trace and the length of the trace is smaller than the length of the pad on IC alone.

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angelu wrote:

  You have two pads that form two capacitors to ground and a trace in between that forms a virtually zero inductance. The wire inside the chip  and the trace on the module itself are the predominant players.

 

  The only thing that you can do to control that line impedance is to adjust the width. Your antenna and the source impedance are likely 50ohm. There are many online impedance calculators that can help with it. You need the board's permittivity which in most cases goes from 3.5 to 4.5. You need the distance from the top layer to the next ground plane, and also the trace thickness. With these numbers you find out the trace width. For that distance i don't think is that critical. The area of those two pads is greater than the area of the trace and the length of the trace is smaller than the length of the pad on IC alone.

 

I think you are right on here.  I've now read elsewhere that the trace impedance only starts to matter as you approach lambda/8, which for cell bands (generally < 3GHz-sh) is about 0.75 cm.  So probably the pads are the dominant factor and the fact that I forgot to clear the ground on the layer under the module and antenna connector pads is why my first prototype doesn't work :/  I've got a random pi network in there.

 

That or its something missing in module software which is OMG