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    Need help on I/O single stub match for LNA.

    Since I am designing a single stub match at the input and output of my LNA design. Is it okay to add a small amount of extra line at the end, so when I connect the SMA connectors they can use the extra small amount of line to solder onto and also so the upper ground left and right bars of the SMA do not touch the stubs (since my stubs will be at the ends of the lna pcb)? Or would you consider using two section tline?

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    Difference between a two section tline and a single stub?

    I am trying to design a single stub microstrip line on pcb, but I'm confused about the diagram and feel the drawing might be misleading as shown here: https://www.google.com/search?q=sing...3wU4125vAl-K4M
    When you design a single stub matching network, all that is needed is d(distance of the main line), l(length of the stub), and the load, but how do you know the length of the line before the stub in the drawing? Wouldn't the T shape of the line actually be the same as a two section tline instead? Or can the main line (d as shown in the Google image above) vary between the stub, such as shown in this picture: https://www.google.com/imgres?imgurl...1&vet=1&w=1280.
    Last edited by skatefast08; 6th December 2018 at 21:53.



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    Re: Difference between a two section tline and a single stub?

    Look thoroughly at the drawing. One side of the "main line" is terminated with Z0, the other with an arbitrary impedance ZL which has to be matched to Z0. The ZL side acts as impedance transformer, its length matters.



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    Re: Difference between a two section tline and a single stub?

    So, right before the stub (on the Z0 side) can be any length of line, but has to equal Z0 (50 ohm)? Or would it have to be the 50 ohm SMA connector and if so, how much extra copper line can I add before the SMA connector?



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    Re: Difference between a two section tline and a single stub?

    Here is not about "how much extra copper line can I add before the SMA connector".
    You can make initially the line to be 50 ohms, and in your simulator tune its length (and position in regards to SMA connector) until you get the desire performance (S11, S22, S21, etc).
    Sometime to get the performances the circuit might need to tune the line width as well..



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    Re: Difference between a two section tline and a single stub?

    Quote Originally Posted by vfone View Post
    Here is not about "how much extra copper line can I add before the SMA connector".
    You can make initially the line to be 50 ohms, and in your simulator tune its length
    How bout theoretically?



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    Re: Difference between a two section tline and a single stub?

    Quote Originally Posted by skatefast08 View Post
    How bout theoretically?
    Theoretically, use reflection coefficient and smith chart using wavelength and frequency of interest as inputs. You need to make your reflection coefficient as good as possible to reduce the reflections between Z0 and Zl ( hence low Standing wave ratio )
    Last edited by ktr; 11th December 2018 at 10:17. Reason: forgot qutoe



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    Re: Difference between a two section tline and a single stub?

    The whole concept of "Impedance Matching" was developed so that you could add arbitrary lengths of line at the input and output of a circuit component, and not see any performance degradation. But to do that the component, an LNA in this case, has to first be matched to the characteristic impedance of the line you want to use. Normally that is 50 ohms, so do your match at the input to 50 ohms, and the match at the output to 50 ohms, then you can add all the line length of 50 ohm microstrip you desire!

    In some rare cases, you might not match to 50 ohms...like if the LNA is driving a bandpass filter, and the bandpass filter components are easier to realize in a 20 ohm system....



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