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Problems with IIP3 with PSS and PAC analysis

Jesture

 Hi

I am working on a (passive like) CMOS attenuator and my problem is that when I do the IIP3 with PSS and PAC analysis my third harmonic is not increasing by 3dB. It is much lower. Me and my thesis supervisor did the transient analysis and did the calculations, we know that the third harmonic is increasing by  3dB but the PSS and PAC is not showing so. And the circuit is correct and this is a common problem with another collegue who is making a mixer with passive blocks. Can anyone please help and explain what might be wrong because if I have to generate the IIP3 from a transient analysis from a scratch it takes forever. And yes I have tried everything (low power, checked the simulation setup and checked the fuctionality of the circuit, tried qpss) it's the same.

  • You may want to view the paper (Reference Cadence Online Support Article 11454704)

    The paper  LINFET: A BSIM Class FET Model with Smooth Derivatives at Vds=0

    by Lawrence F. Wagner, C. Michael Olsen.  

    IBM Systems & Technology Group, Global Engineering Solutions.

     

    http://www.nsti.org/procs/Nanotech2007v3/7/W79.604

     

    describes this third order harmonic simulation problem.

     

    Because of a singularity in the transistor models around vds=0, the slope of the third order harmonic is not 3.  The discontinuity of the first order of the charge and current at vds=0 is a know issue for BSIM models.

     

    As a workaround, try letting vds be a value other than zero, e.g. vds = 1v.  Run your simulations with decreasing values of vds.   At the same time, measure the IP3.   IP3 is not reliable when vds is near or equal to zero, but you can extrapolate the IP3 using values far from vds=0.

     

    It is strongly recommended that you use the PSP, BSIM6, or HiSIM2 models for this application, as they do not have the inherent limitation of the BSIM3/BSIM4 models.

     

    Best regards,

     

    Tawna

  •  To start of, thank you so much for your fast response. I will definitely chck out the paper. I was guessing some thing similar as well now it just got a bit more clearer. Thank you once again.

    Best regards

    Shashwata.

  • Like I mentioned above that I am designing a passive like attenautors where the transistors are in their triode region acting as variable resistors the voltages I am dealing with are very low. Can anyone suggest how I can do the IIP3 as with out PSS or PAC it is getting very difficult to generate the curve IIP3 Vs Attenuation. 

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