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    When simulating center tapped inductor in ADS

    Hello, I'm drawing and testing center-tapped inductor with ADS simulator. Center-tap will be connected to VDD.

    After drawing inductor,
    I assigned term 1&2 to two ends of inductor and term3 to center tap to simulate s-parameter simulation.
    Setting of term impedances are defaults ( 50ohm ).

    But I think I have to set impedance of term3 to 0 ohm because it will be connected to the VDD.
    Do I have to set impedance of term 3 as a default? or some other value?

    Thank you

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    Re: When simulating center tapped inductor in ADS

    Leave the port impedances set to 50 Ohm. The circuit simulator later takes care of the calculating the voltages and currents for the external impedances connected to the model. S-parameters don't work for 0 Ohm reference impedance.

    When you evaluate results for your inductor in differential configuration, the center tap is at 0V due to symmetry anyway.
    https://muehlhaus.com/support/ads-ap...uctor-em-ports



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    Re: When simulating center tapped inductor in ADS

    Thank you for your reply

    May I ask one more question?
    I've learned that Q of center-tapped is higher than not tapped one and I wanna exploit this feature.
    Also, in my idea, inductor of inductance of L1 is divided by factor of 2 when we use center-tap.

    However, my center-tapped inductor shows higher inductance and lower SRF.
    So, I'm confused and I'm not convinced with my simulation.

    If you have any idea about this phenomenon, please let me know. Thank you.

    Below figure is my simulation setting and S-parameter plot of it
    Click image for larger version. 

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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by jwonoh View Post
    Do I have to set impedance of term 3 as a default?
    Yes.
    See schematics in https://designers-guide.org/forum/Ya...m=1205240723#9



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by pancho_hideboo View Post
    There is no port 3 in your linked post. Which is fine.

    - - - Updated - - -

    Quote Originally Posted by jwonoh View Post
    I've learned that Q of center-tapped is higher than not tapped one
    I am working on these inductor simulations for many years, and never heard that. The center tap is a virtual ground when used with symmetric excitation. I think you might refer to differential vs. common mode Q factor.

    Quote Originally Posted by jwonoh View Post
    Also, in my idea, inductor of inductance of L1 is divided by factor of 2 when we use center-tap.
    I'm not sure what you compare, the full inductor (port 1 to port 2) vs. half of the inductor (port 1 to port 3)? Have a look at my appnote linked above. The important thing is to use the correct excitation - if you drive the inductor single ended from port 1 only, it behaves differently from the differential excitation. Also, that single ended excitation is not the typical use case. You should investigate differential operation.
    Last edited by volker@muehlhaus; 8th August 2019 at 14:37.



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by volker@muehlhaus View Post
    There is no port 3 in your linked post.
    Surely see schematics.
    https://designers-guide.org/forum/At.../Diff_Ind2.jpg
    Port-3 is connected to center tap.



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by pancho_hideboo View Post
    Surely see schematics.
    Port-3 is connected to center tap.
    Then I think your math is wrong for 3-port simulation. It is valid for 2-port simulation.



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    Re: When simulating center tapped inductor in ADS

    See https://designers-guide.org/forum/Ya...m=1205240723#9

    Quote Originally Posted by jwonoh View Post
    I've learned that Q of center-tapped is higher than not tapped one and I wanna exploit this feature.
    R0=50
    Sdiff = (s11-s12-s21+s22)/2.0
    Zdiff = 2*R0*(1+Sdiff)/(1-Sdiff)

    freq = xval(s11)
    Ldiff = imag(Zdiff)/(2*pi*freq)
    Qdiff = imag(Zdiff)/real(Zdiff)

    Z1 = 1/y11
    L1 = imag(Z1)/(2*pi*freq)
    Q1 = imag(Z1)/real(Z1)
    https://designers-guide.org/forum/Ya...m=1205240723#8

    Qdiff > Q1

    Quote Originally Posted by jwonoh View Post
    Also, in my idea, inductor of inductance of L1 is divided by factor of 2 when we use center-tap.
    However, my center-tapped inductor shows higher inductance and lower SRF.
    So, I'm confused and I'm not convinced with my simulation.
    Consider Ldiff not L1.
    Ldiff= 2*(Ldiff/2)

    Quote Originally Posted by volker@muehlhaus View Post
    Then I think your math is wrong for 3-port simulation.
    It is valid for 2-port simulation.
    It is valid for differential mode excitation.

    For evaluation of L1 and Q1, port-3 must not be connected.
    Last edited by pancho_hideboo; 8th August 2019 at 15:01.



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by pancho_hideboo View Post
    It is valid for differential mode excitation.
    For evaluation of L1 and Q1, port-3 must not be connected.
    I just tested it and confirm it works for 3-port simulation also.

    Click image for larger version. 

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    Re: When simulating center tapped inductor in ADS

    I couldn't see the proof of Qdiff>Qse ??



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by BigBoss View Post
    I couldn't see the proof of Qdiff>Qse ??
    My test was to check if pancho's equations also work to create differential data from 3-port S-params where the center tap is connected. That indeed works.

    Here is the comparison of single ended and differential Q for that exact same layout, from the same EM simulation:

    Click image for larger version. 

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    Last edited by volker@muehlhaus; 8th August 2019 at 18:33.



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by BigBoss View Post
    I couldn't see the proof of Qdiff>Qse ??
    We don’t see parasitic capacitance at crossing point, if differential inductor is drived differentialy.



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    Re: When simulating center tapped inductor in ADS

    If we consider the equivalent circuit of an inductor and when we make short circuit P2, corresponding distributed components are becoming short circuit.
    So, the difference which coming from SE and DIFF measurements might be result of this error. I'm saying that is an error because it does not sound very healthy to me.
    In my opinon , DIFF measurement looks like more healthy..



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    Re: When simulating center tapped inductor in ADS

    Quote Originally Posted by BigBoss View Post
    If we consider the equivalent circuit of an inductor and when we make short circuit P2
    What post and P2 do you refer to? In my post #11 the center tap is at Port 3 and that's floating in this comparison.

    The difference between single ended Q and differential Q is also seen for the 2-port case, not related to center tap. You are correct that it can be understood by looking at the shunt path in the equivalent circuit.

    Click image for larger version. 

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