About variants and EMC

[mulfycrowh]

Hi everyone,

when you decide not to fit a bunch of components on a dedicated variant, your pcb on that area acts like a bunch of antennas.
What is your experience on that topic regarding EMC?
Is it better to unfit all components on the area or just the resistors from "Y"s allowing the choice between both variants?

[robertferanec]

Ideally this may need to be taken into account when creating schematic and pcb layout. They can not only be antennas, they can also damage your signals, so ideally your PCB should be designed the way the stubs are minimal - the components needs to be placed directly on signals or, they have to cut off almost whole signals. If needed, maybe you can add 0R and you can share pads (that is what I do in these situations). This may help: https://youtu.be/2LWXKOEnIJE

[mulfycrowh]

robertferanec I was speaking about variants and EMC.
The idea was when you don't fit some components on the pcb in the case of a variant, the tracks involved may act as antennas.
The attached screenshot shows an example where you have 2 graphic switches but only one is used.
Even if the 0R resistors cut the tracks assigned to the differential pairs, there are still remaining tracks like power suplplies that may act as antennas.

[robertferanec]

In your picture, if you don't use that interface, it should not have any active signals, so, that should not be problem for EMC.

[mulfycrowh]

robertferanecwhat do you mean with active signal? High impedance?
How do you manage power supplies? Do you use one 0R resistor you don't fit in that case?

[qdrives]

A power supply should be a stable DC voltage. No change --> no EMC problem.

The other remark is: how long is the track compared to the rise time? If it is short, then it will not be a problem. This would be the case both for EMC as well as SI.

[robertferanec]

active signal: I mean, if there are no signals running through that tracks (e.g. if you do not fit ethernet chip, there will be no signals running on the tracks between CPU and Ethernet). Or different words, the interface will not be active.

power: I would not worry about power supplies for several reasons e.g. power supplies are usually all over the board, fitting or unfitting one chip will not influence it. Or different words, if there would be a problem with power supply and EMC, then it would not be because of one chip is fitted or not fitted.