Common/differential mode impedances on coupled microstrip line for series stubs - Page 1
Forum section: electronics / rf-microwave

author: a59d1
author_rank: Regular Contributor
timestamp: April 01, 2019, 12:55:13 pm
content: Quick question for anyone who understands distributed element matching. I haven't found an answer to this in any textbooks, Pozar etc, so maybe there isn't one in the first place.TL;DRwhen designing a series stub, should I care at all (within reason) about anything other than keeping the differential impedance to10050 ohms? Does common-mode impedance matter particularly much for open or shorted series matching stubs?Suppose I have some load that I want to match to my 50 ohm transmission line and 50 ohm source. It has some non-zero reactance, so let's just assume that the normalized load impedance ZL= 22 + i15 ohms. Actual values don't matter. I'm just trying to add elements to it such that I bring its input to 50+j0 ohms, or make its S11 as close to 0 as possible.In the case of making an open or shorted series stub, do we assume that each "series" component of the stub is coupled to the other? Examples I've seen show the series stub as essentially a short section of coupled microstrip transmission line with an example impedance of 50 ohms. I have not, however, been able to find justifications for what actual trace sizes or common mode or differential impedances should be set to. The best guess I can get (from ADS' dimensions for coupled microstrip,here) tells me that the odd-mode impedance is about 25 ohms and the even about 32, for a common-mode impedance of 16 ohms and a differential impedance of about 50 ohms. It seems that nobody designs these parts without ADS or similar...I ask this because I assume the coupling between two microstrip lines would matter most in an open-circuit series stub... given that the signal can't really pass if it's a poorly coupled open circuit. If we have a shorted series shunt, i.e. the coupled segment is terminated with a short-circuit, it seems like the common-mode impedance might matter even more. However, if we diverted this series shunt (and made a large gap between the two conductors) such that the differential impedance is super high, wouldn't it behave just like an inductive element or delay line?Thanks in advance.

author: rfeecs
author_rank: Frequent Contributor
timestamp: April 01, 2019, 05:31:36 pm
content: The text books refer to ideal series stubs. In practice, in a circuit with a ground plane, a series stub is not easily realizable and rarely used. Perhaps best to just think of it as an ideal element that doesn't exist in reality.So for microstrip you would end up with two coupled microstrip lines as you said, and the even mode impedance will be very small and very significant. So you would model this as coupled lines, not as a series stub.You've probably seen coupled lines used often in filters. There was a popular book describing use of all the various combinations of coupled lines:https://www.amazon.com/Microwave-Impedance-Matching-Networks-Coupling-Structures/dp/0890060991At 1100 pages is it the definition of TL;DR.

author: a59d1
author_rank: Regular Contributor
timestamp: April 01, 2019, 08:57:39 pm
content: Thank you rfeecs! This is very helpful; I had seen what I thought was a series stub (e.g. on thisdatasheet) but I see now that it must be something else.