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Middlebrook/Wiedmann in Cadence parameter sweeps Openloop

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jgk2004

jgk2004

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Hello all,

I am trying to get my open loop response in a closed Middlebrook/Wiedmann setup, but I can't seem to figure out how to do a parameter sweep then combine two steps in my parameter sweep into one graph to get my Open loop gain. Here is my equation

-1/(1-1/(2*(I(Vi)@1*V(x)@2-V(x)@1*I(Vi)@2)+V(x)@1+I(Vi)@2))

In cadence how do you get the sweep 1 and 2? and how to do it in the calculator??

Is this possible? I am trying not to copy my testbenchs to get the different parameters....

Johnk
 

Re: Middlebrook/Wiedmann in Cadence parameter sweeps Openloo

I don´t know if it can help, but in the pdf attachement you find
the formulas and macros I always use in PSpice/Probe for the
loop gain simulation according to Middlebrook.
Good luck
 

Hello LvW,

Yes This is exactly what I want to do but how can I plot @1 and @2 from different sweeps into one plot in cadence?

Thanks
 

Re: Middlebrook/Wiedmann in Cadence parameter sweeps Openloo

jgk2004 said:
Hello LvW,
Yes This is exactly what I want to do but how can I plot @1 and @2 from different sweeps into one plot in cadence?
Thanks
Click to expand...

Sorry, I am not familiar with Cadence.
But note, in PSpice both runs must be performed with parameter stepping, that means not two different runs.
As a parameter I use z=0,1, whereby z=0 activates the voltage source (deactivates current source) and for z=1 vice versa. Then, in the graphical post processor I can ask for data of both runs and combine them according to the formula.
Regards
 

Hello LvW

Ok thanks for the input, Yeah I used to do this exactly like you are talking about in LTspice. I am just getting used to Cadence and finding things that don't really work the same. Thanks for the input.... Its looking like I will just have to copy my schematics into a testbench...

Johnk
 

Re: Middlebrook/Wiedmann in Cadence parameter sweeps Openloo

Like I write on my webpage https://sites.google.com/site/frankwiedmann/loopgain, Tian's method (as I prefer to call it) is directly implemented in Cadence's Spectre simulator as stability analysis (stb), so there is no need to use the complicated formula.

Regards, Frank
 

Well in the Stb I can't seem to get the plots of Mag and Phase? It just spits out PM and GM numbers but I feel you really need to look at the plots to get a full understanding of response? Frank do you know how to get it to plot everytime?

thanks
John
 

Re: Middlebrook/Wiedmann in Cadence parameter sweeps Openloo

Hi John,

all the alternatives (approximations resp. exact procedures) for plotting the loop gain of a system with feedback have been discussed in this forum several times (L-C method, Middlebrook, Tian,..). Try to use the search function. In addition, the document from F. Wiedmann contains short descriptions and relevant links.
Regards
 

Hello LvW,

You are correct this forum as answered all of my questions except for the stb simulation in cadence which is the Tians method. It is just incomplete due to it not plotting anything just giving numbers which is great but not complete.. or well I can't find the plots... I can't find anyone talking about this stb stuff. thats why I asked! also it is not on Franks pages.
Thanks

John
 

Re: Middlebrook/Wiedmann in Cadence parameter sweeps Openloo

Hi John,

Of course you can plot the loop gain (magnitude and phase) after an stb analysis. It is accessible from the Virtuoso Analog Design Environment under Results -> Direct Plot -> Main Form. Please note that the loop gain phase will start at 180 degrees and not at 0 degrees as usual (see https://www.designers-guide.org/Forum/YaBB.pl?num=1124688329 for details).

For questions regarding Cadence Spectre, I can recommend you the forums at https://www.cadence.com/community/forums/ and https://www.designers-guide.org/Forum/.

Regards, Frank
 

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