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Simulators >> RF Simulators >> QPSS and QPAC versus Transient for IIP3 simulation
https://designers-guide.org/forum/YaBB.pl?num=1226039437

Message started by aaron_do on Nov 6^th, 2008, 10:30pm

Title: QPSS and QPAC versus Transient for IIP3 simulation
Post by aaron_do on Nov 6^th, 2008, 10:30pm

Hi all,

I have a very standard passive mixer that i was simulating and I found the IIP3 to be much poorer than i expected (around -5 dBm). I simulated the mixer using QPSS and QPAC analysis. Anyway I was not convinced that the simulation was correct so I did it again using PSS and PAC and I got virtually the same result. Anyway i still wasn't convinced so this time i ran a transient analysis and left it over the weekend. The IIP3 i calculated was significantly better. Around +15 dBm.

So my question...which simulation should I trust, and does anybody know what kind of IIP3 should be expected from a passive mixer?

thanks,
Aaron

Title: Re: QPSS and QPAC versus Transient for IIP3 simulation
Post by didac on Nov 7^th, 2008, 9:43am

Hi,
mmm, not sure if that discussion applies here:http://www.designers-guide.org/Forum/YaBB.pl?num=1207474039/9#9. It depends if your model it's symmetric or not. If the model isn't symmetric I wouldn't trust any of the simulations. If the model it's symmetric I would trust PSS and QPSS since transient sims for linearity can cause artifacts in FFT algorithm(although I don't know if spectre FFT algorithm it's a modified one that can take into account variable timesteps).
Hope it helps,

Title: Re: QPSS and QPAC versus Transient for IIP3 simulation
Post by aaron_do on Nov 9^th, 2008, 7:22am

thanks! very helpful... Just wondering about something you mentioned,

Quote:

which makes me wonder which model used in the papers that I read about "sweet spot" bias point(switch from moderate to strong inversion)-not taking into account the side effects associated with this biasing point and issues like feedback of the second harmonic in source degenerated devices-.
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what side effects were you referring to? Also what is the problem with feedback of the second harmonic? Do you expect the second harmonic to be strong at this sweet spot?

thanks,
Aaron

Title: Re: QPSS and QPAC versus Transient for IIP3 simulation
Post by didac on Nov 9^th, 2008, 8:05am

Hi,
Well I made a few experiments on my self guided with some papers and I found that "sweet spot" bias its so sensitive to all:process,input signal,load,degeneration...its too narrow that you can end up with less linearity,worse NF,worse gain and worse input matching all at once. The second order non-linearity effect with a degeneration basically causes the spike of linearity to disappear completely(depend on the magnitude of the degeneration), 2nd order products feedback and mixes with the fundamental and they create another 3rd order nonlinearity even if the third derivative of the current it's 0.
If you look at IEEE for high linearity LNA's people are using more complex structures that are more robust.
Hope it helps,
PS:a couple of references that I have at hand
"RF Circuit Implications of Moderate Inversion Enhanced Linear Region in MOSFETs",IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 51, NO. 2, FEBRUARY 2004,Bill Toole, Member, IEEE, Calvin Plett, Member, IEEE, and Mark Cloutier, Member, IEEE-->that paper I found very useful to carry out sims with my PDK.
"A Highly Linear Low-Noise Amplifier",IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 12, DECEMBER 2006,Sivakumar Ganesan, Edgar Sánchez-Sinencio, and Jose Silva-Martinez-->a high IIP3 LNA and revises two other approaches for high IIP3, also points the effect of second order nonlinearity and feedback.(in fact I did this little search on sweet spot because I've seen professor Silva in a symposium talking about this issues).

Title: Re: QPSS and QPAC versus Transient for IIP3 simulation
Post by aaron_do on Nov 9^th, 2008, 5:00pm

Thanks for the reply.

I was thinking the second order would also change the bias point... Anyway I suppose the additional IM3 created by the second order non-linearity effect would simply move the sweet spot which could be then found testing IIP3 over a wide range of VGS instead of plotting gm2. Of course its all kinda pointless given the process variation...

thanks for all the references too

cheers,
Aaron