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open shunt stub phase - RF Cafe Forums

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Post subject: open shunt stub phase Posted: Thu Jan 12, 2006 4:26 am

Hi i asked earlier about quarter-wave form Tlines and you guys helped out a lot. i've another question to ask: i used ideal transmission line models and it told me that I need 23 deg. electrical length for a line series tranmission line and i plotted the mag. and phase of s21 and saw yes the phase is 23 deg (in fact it was -23 deg, why is that?).

however, my main question is this: the model also told me that i need 47 deg. of electrical length for an open shunt stub for something. i then plotted its mag. and phase of s21 but saw that the phase was only 24 deg at the given frequency? why is that? i don't know the concept clearly to know why the phase was 24 deg.--something to do with open stub??. there is nothing special i am doing here. i just took completely ideal models in a CAD program and put two 50 ohms terminations and simulated them. your answer will be much appreciated. how wold be shunt short-circuited stub different? thanks



Post subject: Posted: Thu Jan 12, 2006 7:08 am

one more question: one can have open circuit single shunt stub but it said that when doing layout it is a good idea to have balanced shunt stubs, meaning half the length on either side of a series line, like a + sign. i calculated exactly how to convert one shunt stub to balanced stub...my question is when one simulates this should you get exactly the same response, what i am getting is that the phase are same but the magnitude are opposite. in the single shunt open stub i have phase starting from 1 (or 0 dB) and decreasing a bit throughout he frequency sweep but for balanced shunt open stub it start from 0 magnitude and approaches like .6 through its frequency sweep!! doesn't make sense. please provide explanation. my layout is i think ok, its like a + with both sides open and then placed ports on the sides where the signal enters and exits



Post subject: Posted: Thu Jan 12, 2006 2:09 pm

the phase are same but the magnitude are opposite

Is that what you meant to say? I am almost ready to answer all of your questions and more! After 3 more cups of coffee.



Post subject: Posted: Fri Jan 13, 2006 5:35 am

yeah....that's what i meant but i think i figure out the problem but would still like to know your feedback and the answer to the first one. thanks



Post subject: 23 or 24Posted: Wed Jan 18, 2006 2:47 am


Joined: Tue Dec 13, 2005 2:42 am

Posts: 5

Location: Czech Rep.

I think that this misunderstanding comes from the model/software you used.

1) Why -23 and not +23deg? ..I guess that the SW calculates the phase as decreasing quantity with increasing distance from the port on which the fictive signal generator is applied.

2) Why 24 deg and not 23 deg? (I HOPE I understood your question well - that after adding a shunt line the phase of s21 changed from 23 to 24 deg.) It is sure that some added line MUST change phase relationships in the system - if the added line has some significant length in comparison with the rest of the network. ...You can see this fact if you use Smith chart. Try to put some serial line with parallel line and see what will happen with the phase


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