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  1. #21
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    Re: microstrip low pass filter design issues with sonnet

    Why does Z0 of a microstrip line depends on W but not L ?




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    Re: microstrip low pass filter design issues with sonnet

    The two artificial lines don't need to be 50ohms. The goal of these artificial lines is to provide a way for Sonnet to de-embed them, so your reference planes do not include the artificial lines. If you have metal leading up to where the artificial lines start, it will have a certain width. You should then make your artificial lines the same width as the lines leading up to them.

    If you make them different widths, you introduce a discontinuity, and Sonnet may have a harder time de-embedding to give you the right answer.
    Someone told me the above with regards to the two extra rectangles nearest to the ports.

    What do you guys think ?

    What about in HFSS ?



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  3. #23
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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    Why does Z0 of a microstrip line depends on W but not L ?
    Z0 is related to the line cross section. For a lossless line it is sqrt(L'/C') where L' is inductance per unit length and C' is capacitance per unit length. So you can see that line impedance itself does not depend on the line.

    A coax line of 50 Ohm remains a 50 ohm line, no matter if you cut it into pieces of 1 meter or 1 inch.

    But don't confuse that with the effect of impedance transformation caused by a line - that of course depends on line impedance and length.

    ~~

    Just in general, we add 50 Ohm line as interconnect between RF blocks for purely practical reasons (keep blocks in layout at reasonable distance). There is no special RF effect or purpose, it's just an interconnect, with no impedance transformation. That's the special case: if source impedance = load impedance = line impedance = 50 Ohm, we have 50 Ohm at the input and output of the line, no matter what the line length is.


    - - - Updated - - -

    Quote Originally Posted by promach View Post
    Someone told me the above with regards to the two extra rectangles nearest to the ports.
    Yes, Sonnet can remove the effect of the feedline at the port if reference plane shift is used. Sonnet then calculates the line properties and removes them from the total result.
    That reference shift is then visible in the Sonnet editor, by arrows from the port up to the reference plane.

    This is useful if you want to have the S-parameters of the "inner" device without feed lines, but need to add some lines to keep that inner device away from the Sonnet box walls (metal).

    ~~

    I think you have to think (or read) about all this transmission line stuff to connect some more pieces of the puzzle. It's not that complicated, but you seem confused what lines are meant as simple interconnect (50 Ohm) and what have a special purpose (impedance transformation where Zload is different from Zline).


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  4. #24
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    Re: microstrip low pass filter design issues with sonnet

    Sonnet can remove the effect of the feedline at the port if reference plane shift is used. Sonnet then calculates the line properties and removes them from the total result.
    That reference shift is then visible in the Sonnet editor, by arrows from the port up to the reference plane.
    What do you exactly mean by reference plane shift ?



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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    What do you exactly mean by reference plane shift ?
    http://www.sonnetsoftware.com/suppor...embedding_.htm
    http://www.sonnetsoftware.com/suppor...d_feedline.htm


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  6. #26
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    Re: microstrip low pass filter design issues with sonnet

    I have inserted the feedline as shown in the figure below. May I know how to generate a DXF layout file (without the extra feedline structure) for actual manufacturing purpose ?

    By the way, is FR-4 with height 1.6mm and Erel of 4.2 common for chinese PCB manufacturers ?




    three-pole microstrip lowpass filter using open-circuited stub.son


    Code:
    FTYP SONPROJ 13 ! Sonnet Project File
    VER 15.53
    DIM
    ANG DEG
    CAP PF
    CON /OH
    FREQ GHZ
    IND NH
    LNG MM
    RES OH
    END DIM
    FREQ
    ABS_ENTRY 0.001 6.0 
    END FREQ
    CONTROL
    STD
    OPTIONS  -bdj
    SPEED 0
    CACHE_ABS 1
    TARG_ABS 300
    Q_ACC N
    END CONTROL
    GEO
    DRP1 LEFT LINK
    POLY 13 1
    0
    DRP1 RIGHT LINK
    POLY 15 1
    0
    TMET "Lossless" 0 SUP 0 0 0 0
    BMET "Lossless" 0 SUP 0 0 0 0
    MET "Copper" 1 NOR 58000000 1 0.01 
    BOX 1 36.1 12 722 240 20 0
          20 1 1 0 0 0 0 "Air"
          1.6 4.2 1 0.02 0 0 0 "FR-4"
    LORGN 0 12 U 
    POR1 STD
    POLY 13 1
    2
    1 50 0 0 0 0 8.78975 
    POR1 STD
    POLY 15 1
    2
    2 50 0 0 0 36.1 8.790025 
    NUM 5
    0 5 -1 N 11 1 1 100 100 0 0 0 Y
    5.010005 8.689745
    16.050005 8.689745
    16.050005 8.889745
    5.010005 8.889745
    5.010005 8.689745
    END
    0 5 -1 N 12 1 1 100 100 0 0 0 Y
    20.049995 8.690025
    31.089995 8.690025
    31.089995 8.890025
    20.049995 8.890025
    20.049995 8.690025
    END
    0 5 -1 N 13 1 1 100 100 0 0 0 Y
    5.01 9.38975
    5.01 8.18975
    0 8.18975
    0 9.38975
    5.01 9.38975
    END
    0 5 -1 N 15 1 1 100 100 0 0 0 Y
    31.09 8.19003
    31.09 9.39002
    36.1 9.39002
    36.1 8.19003
    31.09 8.19003
    END
    0 5 -1 N 17 1 1 100 100 0 0 0 Y
    16.050005 3.109975
    20.050005 3.109975
    20.050005 8.889975
    16.050005 8.889975
    16.050005 3.109975
    END
    END GEO
    OPT
    MAX 100
    VARS
    END
    END OPT
    VARSWP
    END VARSWP
    QSG
    IMPORT NO
    EXTRA_METAL NO
    UNITS YES
    ALIGN NO
    REF NO
    VIEW_RES YES
    METALS YES
    USED NO
    END QSG
    TRANSLATOR
    DXFEXPORT
    UseTLs false
    SepObj true
    SepMat true
    DivideMulti false
    Circles false
    CircleType inscribed
    CircleSize 0
    KeepMetals true
    KeepVias true
    KeepViaPads true
    KeepBricks true
    KeepEdgeVias true
    KeepParent true
    ConvertParent false
    AllEdgeViasAsVia false
    END
    END TRANSLATOR



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  7. #27
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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    May I know how to generate a DXF layout file (without the extra feedline structure) for actual manufacturing purpose ?
    If you have a license for Sonnet DXF option: File > Export > DXF
    will export the layout "as is", including those feedline.

    On PCB, you will need some length of 50 Ohm line to attach the connectors. The narrow (inductive) lines are most likely too thin to solder the connectors. And let me repeat: if the extra line is 50 Ohm (which I haven't checked) then it doesn't change your filter response (magnitude). It only adds some delay (phase).

    If you insist to export a different version of the layout (excluding the 50 Ohm lines), create a copy of the file and modify the layout.

    Quote Originally Posted by promach View Post
    is FR-4 with height 1.6mm and Erel of 4.2 common for chinese PCB manufacturers ?
    Yes, those are typical values. Specify the correct thickness.

    And make sure your layout includes bottom ground also. In Sonnet, you didn't draw that because the bottom of the Sonnet box is metal boundary. For your PCB order, you need to include that backside ground metal in your files. One possibility: You can draw a polygon on the Sonnet ground layer. This has no effect for Sonnet, but will be included in export.
    Last edited by volker@muehlhaus; 1st September 2019 at 10:38.


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  8. #28
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    Re: microstrip low pass filter design issues with sonnet

    For your PCB order, you need to include that backside ground metal in your files. One possibility: You can draw a polygon on the Sonnet ground layer. This has no effect for Sonnet, but will be included in export.
    I have added a polygon covering the entire box region at the GROUND layer.

    However, I am not sure if I have actually added a ground metal plane.
    I could not see the ground plane using 3D viewer of Kicad PCBNew software.

    Could you advise ?

    Note: I have attached the dxf layout file as a zip file at the end of this post.






    three-pole microstrip lowpass filter using open-circuited stub.son

    Code:
    FTYP SONPROJ 13 ! Sonnet Project File
    VER 15.53
    DIM
    ANG DEG
    CAP PF
    CON /OH
    FREQ GHZ
    IND NH
    LNG MM
    RES OH
    END DIM
    FREQ
    ABS_ENTRY 0.001 6.0 
    END FREQ
    CONTROL
    STD
    OPTIONS  -bdj
    SPEED 0
    CACHE_ABS 1
    TARG_ABS 300
    Q_ACC N
    END CONTROL
    GEO
    DRP1 LEFT LINK
    POLY 13 1
    0
    DRP1 RIGHT LINK
    POLY 15 1
    0
    TMET "Lossless" 0 SUP 0 0 0 0
    BMET "Lossless" 0 SUP 0 0 0 0
    MET "Copper" 1 NOR 58000000 1 0.01 
    BOX 1 36.1 12 722 240 20 0
          20 1 1 0 0 0 0 "Air"
          1.6 4.2 1 0.02 0 0 0 "FR-4"
    LORGN 0 12 U 
    POR1 STD
    POLY 13 1
    2
    1 50 0 0 0 0 8.28975 
    POR1 STD
    POLY 15 1
    2
    2 50 0 0 0 36.1 8.290025 
    NUM 6
    0 5 -1 N 11 1 1 100 100 0 0 0 Y
    5.010005 8.189745
    16.050005 8.189745
    16.050005 8.389745
    5.010005 8.389745
    5.010005 8.189745
    END
    0 5 -1 N 12 1 1 100 100 0 0 0 Y
    20.049995 8.190025
    31.089995 8.190025
    31.089995 8.390025
    20.049995 8.390025
    20.049995 8.190025
    END
    0 5 -1 N 13 1 1 100 100 0 0 0 Y
    5.01 9.88977
    5.01 6.68973
    0 6.68973
    0 9.88977
    5.01 9.88977
    END
    0 5 -1 N 15 1 1 100 100 0 0 0 Y
    31.09 6.69002
    31.09 9.89003
    36.1 9.89003
    36.1 6.69002
    31.09 6.69002
    END
    0 5 -1 N 17 1 1 100 100 0 0 0 Y
    16.050005 2.609975
    20.050005 2.609975
    20.050005 8.389975
    16.050005 8.389975
    16.050005 2.609975
    END
    1 5 -1 N 46 1 1 100 100 0 0 0 Y
    0 0
    36.1 0
    36.1 12
    0 12
    0 0
    END
    END GEO
    OPT
    MAX 100
    VARS
    END
    END OPT
    VARSWP
    END VARSWP
    QSG
    IMPORT NO
    EXTRA_METAL NO
    UNITS YES
    ALIGN NO
    REF NO
    VIEW_RES YES
    METALS YES
    USED NO
    END QSG
    TRANSLATOR
    DXFEXPORT
    UseTLs false
    SepObj true
    SepMat true
    DivideMulti false
    Circles false
    CircleType inscribed
    CircleSize 0
    KeepMetals true
    KeepVias true
    KeepViaPads true
    KeepBricks true
    KeepEdgeVias true
    KeepParent true
    ConvertParent false
    AllEdgeViasAsVia false
    END
    END TRANSLATOR



  9. #29
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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    I could not see the ground plane using 3D viewer of Kicad PCBNew software.
    I'm sure you'll figure it out ... just look at imported layers one at a time.

    The DXF file created by Sonnet is 2D data, so don't expect different z position of signal and ground layer.

    ~~

    You seem to be changing the width of the feedline again and again ... make sure the width is designed for 50 Ohm. I didn't check your files, only looked at the screenshots.



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    Re: microstrip low pass filter design issues with sonnet

    See https://github.com/promach/low_pass_filter

    By the way, what is the difference between open-circuited stub and stepped-impedance ?

    The open-circuited stub microstrip realization method is a modification of the stepped-impedance microstrip realization method. It approximates the series inductance as a high impedance transmission line while the shunt capacitance effect is simulated by an open-circuited stub using Richard’s transformation.
    See 2GHz Microstrip Low Pass Filter Design with Open-Circuited Stub



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    Re: microstrip low pass filter design issues with sonnet

    I am trying to understand why open-circuited stub has a better S21 response compared to stepped impedance according to the result at figure 5.4 below:

    this microstrip equation for Zo does not depend on length.

    Could anyone point me to a microstrip equation that describes the relationship between stub length (in this case, 5.78mm) and λ/4 at 5.6 GHz and substrate properties (dielectric constant and thickness) ?




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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    I am trying to understand why open-circuited stub has a better S21 response compared to stepped impedance according to
    You implement that shunt C by a transmission line, and have some freedom in choosing a width and then calculating the required length. It seems that the "open circuited stub" layout used a length of lambda/4 at 5.6 GHz, so that the open end transforms into a short at that frequency, creating a notch at 5.6GHz.



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    Re: microstrip low pass filter design issues with sonnet

    It seems that the "open circuited stub" layout used a length of lambda/4 at 5.6 GHz
    Is it possible to prove this mathematically using some microstrip expressions ?



  14. #34
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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    Is it possible to prove this mathematically using some microstrip expressions ?
    Yes, using the equation that gives input impedance as a function of line impedance, line length and load impedance.
    This is not specific to microstrip lines.

    Page 3, chapter "Impedance transformation"
    http://muehlhaus.com/wp-content/uplo...sion-Lines.pdf



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  15. #35
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    Re: microstrip low pass filter design issues with sonnet

    The wavelength is multiplied by 1/sqrt(dielectric constant); this does not hold perfectly true in microstrip because of the differing even-/odd- mode propagation velocities due to the substrate below the strip and the air (or free space) above having different dielectric constants.
    Someone else told me the above.

    What do you think ?



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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    Someone else told me the above.
    Correct, my statement refers to lambda/4 effective wavelength (90° S21 phase) for the actual substrate.

    Edit: Doing the math with the limited information that you supplied the S21 phase that I get is more like ~120° using ADS Linecalc, but the stub is rather wide and it gets a bit fuzzy due to parasitics. The math for notch length would be exact if you do circuit simulation with ideal elements. For actual layout, I guess length was adjusted to move the notch as required.
    Last edited by volker@muehlhaus; 5th September 2019 at 19:17.


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  17. #37
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    Re: microstrip low pass filter design issues with sonnet

    Doing the math with the limited information that you supplied the S21 phase that I get is more like ~120° using ADS Linecalc,
    Could you post the ADS linecalc window screenshot which shows 120° ?



  18. #38
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    Re: microstrip low pass filter design issues with sonnet

    Quote Originally Posted by promach View Post
    Could you post the ADS linecalc window screenshot which shows 120° ?
    Here it is, with 1.27mm thickness at er=10.8 we get 115 degree

    Click image for larger version. 

Name:	linecalc_115.jpg 
Views:	4 
Size:	154.6 KB 
ID:	155360

    but that assumes a thru line. Due to the parasitics of an open ended line and the narrow feed from the side, we actually get the notch at 5.8 GHz.

    This is what we get in Sonnet for the open ended line, feed with 200µm wide strips, reference plane at the open stub (feedline not included in results):

    Click image for larger version. 

Name:	sonnet.jpg 
Views:	3 
Size:	187.6 KB 
ID:	155364

    Zipped Sonnet model: ost_model.zip
    Last edited by volker@muehlhaus; 6th September 2019 at 13:23.


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