EMC Question of the Week: March 24, 2025
Which of these two-port networks is fully characterized by its S-parameters?
- (a)
- (b)
- (a) and (b)
- (a) and (b) and (c)
Answer
The best answer is “a.” If the S-parameters were determined by driving each of these circuits with an ungrounded differential source and terminating them with an ungrounded differential load, all three circuits would have the same S-parameters. In that situation, any of the three circuits could be used in a SPICE model to represent a device with those S-parameters. On the other hand, if the source and the load have a finite impedance to the same ground (or are connected in any way other than through the two-port network), all three of these circuits behave differently.
The ports in a circuit fully characterized by its S-parameters must have the same ground. Note that a differential signal applied to Port 1 of the circuits in (b) or (c) produces a common-mode voltage between the reference conductors for the two ports. In a system where the circuits connected to Port 1 and Port 2 are completely isolated, this common-mode voltage has no effect. On the other hand, in most real applications where another current-return path exists, S-parameters do not accurately model the behavior of an ungrounded device.
The equivalent circuit shown in (a) can be used in a SPICE model to represent any device that has the same S-parameters as long as the port references are system ground.
In systems where the circuits connected to both ports are isolated from each other and ground, all three of these circuits behave exactly the same. In other words, if grounding both sides of a network characterized by its S-parameters changes the simulation results, then that network cannot be fully characterized by its S-parameters.
Note: Two-port linear networks with references that are not system ground [like (b)], can be fully characterized as four-port networks.
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