n port in the spectre

[shanmei]

I import the s parameter touchstone file in the n port.

There are actually 4 pins for the n2port. How does those pins connection, the two pins with "-" symbol should be short to ground.


How about n8port, so there are totaly 8 pins, and 4 pins with "-"symbol. Those 4 pins are also connected to ground?

Thanks.

 

[volker@muehlhaus]

Yes, the (minus) pins of the S-parameter data all go to ground.

Now, the typical concern is: what happens to series inductance, series resistance etc. that might be physically in the ground path? Is it lost by shorting the grounds together? Answer: No, all this is included in the data and will show up in the "plus" path. This is possible because the currents in (plus) and (minus) are equal, with opposite polarity. Circuit designers who are used to nodes (not ports) have often difficulties to understand this, but it is perfectly valid because S-parameters are for ports (not nodes) where the currents in the (plus) and (minus) pin are dependent.

 

[BigBoss]

n-port means there are n times (+) and n times (-) pins to define "the Ports".Because one port consists of 2 pins.

 

[FvM]

Strictly speaking, s-parameters itself don't tell if a port is single-ended (negative side grounded) or differential, this has to be noted along with the data. In so far, a ground connection isn't obvious. But most network analyzers only support single-ended measurements and most RF devices (but not all) have single ended ports.

 

[volker@muehlhaus]

Strictly speaking, s-parameters itself don't tell if a port is single-ended (negative side grounded) or differential, this has to be noted along with the data. In so far, a ground connection isn't obvious.

I want to add something to this: S-Parameters are for ports, not pins. The current in (plus) and (minus) of each port is the same, with opposite polarity. The S-Parameter have no information about other modes, for example if you wire the S-parameter block to force current through the (minus) pins that is different from the current through the (plus) pins. Many simulation tools will show an open circuit for such "unsupported" modes with unsupported current paths, but in general it's just undefined.

Or to say it the other way around: no matter if you have single ended or differential S-parameters, connecting all the port ground nodes together is perfectly fine and doesn't physically connect the ground nodes.