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  1. #41
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    promach..
    You're trying to simulate your planar structure with wrong tool as HFSS.It doesn't mean that HFSS cannot be used for planar structures but it's really cumbersome tool and it's really efficient FDTD problems.
    All you have to do is to find a planar EM simulator such as ADS Momentum even Sonnet Lite and simulate your structure without trouble.If you're student, Keysight and National Instruments may help you to give a time limited but fully functional tools.Sonnet Lite can be used up to 4 ports (16MB memory) and it's absolutely free and it's very easy to use and very efficient for planar structures.
    HFSS is very powerful but hard to use tool..



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by BigBoss View Post
    Sonnet Lite can be used up to 4 ports (16MB memory) and it's absolutely free and it's very easy to use and very efficient for planar structures.
    Sonnet Lite is great, but he will hit the memory limits here. The combination of narrow gap and non-90° lines will need to much memory in Sonnet Lite.



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by volker@muehlhaus View Post
    Sonnet Lite is great, but he will hit the memory limits here. The combination of narrow gap and non-90° lines will need to much memory in Sonnet Lite.
    Yeah, possible.. But he can also request a evaluation from NI for Axiem. It's also very accurate and I have used few times for serious designs and it has never belied me on measurements.
    It's fast and accurate..



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    @volker@muehlhaus

    I think I know why HFSS gave completely different result compared to ADS for the same DXF file.

    DXF is only 2D.
    I suppose ADS automatically added the 3D height for the microstrip layout.

    In HFSS, the microstrip layout does not have 3D height added automatically unlike the via.




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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    I suppose ADS automatically added the 3D height for the microstrip layout.
    I created a stackup to map the 2D conductor layers to the correct z position, with the correct thickness. Post #30, second picture.

    Quote Originally Posted by promach View Post
    In HFSS, the microstrip layout does not have 3D height added automatically unlike the via.
    There is something else in your 3D view that looks weird: It seems that you don't have the backside (ground) metalization that is required for microstrip. I added that in ADS myself, because I know it is required for this design. In your DXF data, there was no ground metal.

    Can you show a side view of your HFSS model? It is difficult to see if the bottom end of the via is "hanging in the air" or connected to the bottom boundary.

    There are two ways how to get metal for ground: you can draw it on the ground layer, or you can make the bottom boundary condition an electric boundary with PEC (perfect conductor). That simulation boundary must hen be at the z-position of the bottom layer (ground).
    Last edited by volker@muehlhaus; 6th August 2019 at 10:39.



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    In your post #30 , you did not show the balun phase, S21 and S31


    Note that HFSS simulates the PCB with air below and above it, hence the large box enclosing the PCB within


    Overall view





    Side view




  7. #47
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    Note that HFSS simulates the PCB with air below and above it, hence the large box enclosing the PCB within
    You can do that, but then you need to draw the ground plane. From your screenshot, I don't see if you drawn ground plane metal, or if that is the boundary of the FR4 dielectric.

    Quote Originally Posted by promach View Post
    In your post #30 , you did not show the balun phase, S21 and S31
    My ADS license is busy with something else now, but I can add that plot later.



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    From your screenshot, I don't see if you drawn ground plane metal, or if that is the boundary of the FR4 dielectric.
    See the layer stackup at post #44



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  9. #49
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    See the layer stackup at post #44
    The stackup doesn't tell if you have a ground polygon actually drawn on those layers.

    We had that "via connected?" topic earlier in port #14/#15 but I'd rather want to see the ground polygon in the 3D view. It was missing in the DXF file, so I wonder if your model has the correct ground. Having a layer definition named "ground" means nothing if there is no metal drawn on that layer.

    - - - Updated - - -

    Here's the phase that you asked for. Note that all your lengths are different, so we don't expect 180° phase shift.

    Click image for larger version. 

Name:	march_phase.jpg 
Views:	6 
Size:	325.7 KB 
ID:	154809

    Click image for larger version. 

Name:	march_length.jpg 
Views:	0 
Size:	161.8 KB 
ID:	154810
    Last edited by volker@muehlhaus; 6th August 2019 at 16:32.


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  10. #50
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Let me investigate more about the ground layer.



    - - - Updated - - -

    Ok, I have made the balun works.

    The real reason is due to the via connectivity to the ground layer.



    - - - Updated - - -

    @volker

    Do you have a theoretical understanding on why balun needs via ?

    I suppose this planar marchand balun works on the concept of EM coupling of two microstrip transmission lines ?



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Zin=Z0* [ ZL+j*Z0*tan(theta)] / [ Z0+j*ZL*tan(theta)]

    Someone told me to use the above expression to understand why balun needs via. Any advices ?

    Besides, why arg(S_21) - arg(S_31) alternatives between 180 degrees and -180 degrees ?



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    Let me investigate more about the ground layer.
    It's possible that the "negative" flag creates a solid ground plane if you drawn nothing.

    Quote Originally Posted by promach View Post
    Ok, I have made the balun works.
    The real reason is due to the via connectivity to the ground layer.
    Some mistake with the via is what I suspected starting from post 3.

    Quote Originally Posted by promach View Post
    Do you have a theoretical understanding on why balun needs via ?
    I suppose this planar marchand balun works on the concept of EM coupling of two microstrip transmission lines ?
    The via change the phase in the coupled line (reflection from open end vs. short), so you can't leave them out.

    Quote Originally Posted by promach View Post
    Besides, why arg(S_21) - arg(S_31) alternatives between 180 degrees and -180 degrees ?
    That's only a data display issue, the arg() data has that "jump" from -180° to 180° for each of the two phases. I didn't know how to "unwrap" the phase for continuous values <-180°


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  13. #53
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    The via change the phase in the coupled line (reflection from open end vs. short), so you can't leave them out.
    How is this related mathematically to Zin=Z0* [ ZL+j*Z0*tan(theta)] / [ Z0+j*ZL*tan(theta)] ?



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    How is this related mathematically to Zin=Z0* [ ZL+j*Z0*tan(theta)] / [ Z0+j*ZL*tan(theta)] ?
    Your equation gives the input impedance for a line terminated into ZL. It makes a difference if ZL=0 (via shorting line to ground) or ZL=infinite (open end, via not connected)


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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Why does the balun phase deviates away from 90 degrees and -90 degrees as HFSS sweeps up the frequency ?

    Can we do anything to the balun layout in order to make this issue less serious ?

    The zip file attached in this post contains the dxf layout file.



    - - - Updated - - -

    See post #50, I was referring to the phase drift below 4 GHz.

    After this, I will solve the phase drift above 4 GHz



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  16. #56
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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Here are results for your latest DXF layout:

    Click image for larger version. 

Name:	march_newlayout.jpg 
Views:	1 
Size:	317.9 KB 
ID:	154849

    Quote Originally Posted by promach View Post
    Can we do anything to the balun layout in order to make this issue less serious ?
    I would start from scratch and re-design the layout, without all those asymmetries. Equal lengths and a symmetric layout, instead of that 135° turn.
    Last edited by volker@muehlhaus; 7th August 2019 at 18:10.



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by volker@muehlhaus View Post
    Your equation gives the input impedance for a line terminated into ZL. It makes a difference if ZL=0 (via shorting line to ground) or ZL=infinite (open end, via not connected)
    Alright. I will redo the layout. To be frank, that 135 degree turn is to allow space for isolation circuit between port 2 and port 3. Please advise.

    Besides, how does the length of the coupled microstrip layout affect the value of theta in the expression ?

    Note: if we have a via (ZL = 0) , the expression reduces to Zin = j*Z0*tan(theta)



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    Quote Originally Posted by promach View Post
    is to allow space for isolation circuit between port 2 and port 3. Please advise.
    I have no advice for coupler layouts, because I've spent most of my time in simulation (not design). My idea was just to make it more symmetric. The 135° looks a bit suspicious to me, with possibly large parasitics due to current crowding at the extreme bend backwands, but if the layout is optimized that might work ... I don't know.

    Quote Originally Posted by promach View Post
    Besides, how does the length of the coupled microstrip layout affect the value of theta in the expression ?
    The equation is for a single line (not coupled), theta is the electrical length of the line (S21 phase)



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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    The equation is for a single line (not coupled), theta is the electrical length of the line (S21 phase)
    Yes, you are right.

    See New Design Formulas for Impedance-Transforming 3-dB Marchand Baluns

    I presume that we need to include both source impedance (Rr) and load impedance (RL) in EM layout simulation as well ?




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    Re: HFSS port 'Port1' does not have a valid reference plane above or below it.

    I have made both the coupled lines matching length and width. However, I am still not getting 180 degrees phase response.

    Why ?

    I have attached the dxf file as zip at the end of this post






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