How in import differential S-parameters in ADS

[torbai]

Hello.

I've simulated several rings model in CST using lumped ports which have no reference node. When I import the S-parameters into ADS, I found that all ports are single-end and connect to a reference node, the ground in most cases. However, I really need them not connect to a same reference and keep them differential. How should I do?

 

[volker@muehlhaus]

I've simulated several rings model in CST using lumped ports which have no reference node.

What you mean is that your ports do not have one common reference node.

When I import the S-parameters into ADS, I found that all ports are single-end and connect to a reference node, the ground in most cases.

It seems to be that way, but that is misleading. The symbol seems to have a common ground node, but that does NOT mean that all your different port grounds are connected. If you have a differential port, all current that flows into the (+) in must leave the port at the (-) pin, exactly. There is absolutely no current to any other (+) or (-) pin.

If you look from a more theoretical perspective at your S-parameters, you will see that they don't even have information about signal path between different reference pins.

However, I really need them not connect to a same reference and keep them differential. How should I do?

It works (and is also correct in theory) to use the normal S-parameter block with one ground reference, and connect the loads at the differential ports (in EM) to the global ADS ground. This only means that the load is connected between the (+) and the (-) pin of the differential port, and does not create any short circuit between different isolated (-) pins.

It might seem unexpected, but this is how it works, in ADS and all other circuit simulators. The reason is that S-parameters are modal, not nodal.

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One example that you can try, to understand that method:

Simulate a line with a gap, and place a differential port across the gap. Now take the S-parameters to ADS. To create a connection across the gap, you need a connection between the (+) and (-) pin. This is a short at that port, and that is how you wire it in the ADS schematic: connect that port/pin of the SnP block to ADS ground.

It might look as if the port was shorted to the line ground, but it is not. There is zero volt across the differential port, which means there is a connection across the gap.

Hope this makes sense, just try it out. It is standard method with internal/differential ports for a long time ... and has always confused new users, in all simulation tools.