![Figure 1: Single-ended vs. differential signal "world views" of S-parameters Figure 1: Single-ended vs. differential signal "world views" of S-parameters](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNQgy-gkCrcCa9v7aAw5OQQEHI3E2Q7me4io9V6jGUWcEp6_7_47N9FLAw7nV7ZbyO8UxJZZFE8313GN8yKRrkiHMUTzhvZ1vIt5Dn85gPM4dtRIXuhMW3UKoKTFZAQIlebZLurUnVCZ7Q/w400-h178/Mixed+Mode+Fig1.png) |
Figure 1: Single-ended vs. differential signal "world views" of S-parameters |
We’ve treated single-ended S-parameters quite extensively in this blog. Links to several entries are listed at the bottom of this post. Now, we’re going to look at how we go from single-ended to mixed-mode S-parameters and what new information we can find in them. This will come in handy when we start looking at some of the MDI S-parameter tests that are performed for Automotive Ethernet compliance a bit down the road.
With single-ended S-parameters, we look at every combination of ‘going in signals’ and ‘coming out signals’. For example, two single-ended transmission lines and their return paths would yield a four-port S-parameter file. We take the complex ratios of each port combination to obtain the S-parameter value in the form of:
S_(OUT,IN) = V_OUT/V_IN
The bold typeface indicates complex quantities.
But what happens if we drive two transmission lines with a differential source? Figure 1 compares the single-ended and differential signal world views.