Hi,

I have been having problem matching simulated IM3 with lab data for a up-convert passive mixer+ buffer combo.  I am consisteantly seeing ~10dB differernce sim vs. lab (with sim being better of course).  Here are my questions:
1. for RF signal around 2 - 3G, what is an resonable expectation of matching the sim vs. lab for IM3?  3db? 5db? I assume 10db is just too much.
2.  does the matching expectation decrease as the IM3 performance improves?  i.e., for target IM3 performance =-40dBc, the matching should around 3dB, for IM3=-60dbc, the matching is relaxed to 5dB?  Is there a such rule of thumb?
3.  does down convert mixer have a better chance of matching sim vs. lab than up convert mixer?
4. how about active mixer vs. passive mixer?  i.e., does one expect to have better sim vs. lab matching for a active mixer than a passive mixer?

Any help/insight/sugestions/comments  are greatly appreciated.

Thanks.
Kelly

hi Aaron,

yes I am, but I ran transient with Two tone input not pss and qpss.  So won't my sim results still be ok?

thanks.
Kelly

The model issue Aaron is discussing is that the Id vs Vds has a discontinuity around Vds=0 in the BSIM3 model. The second derivative is not continous. That makes the IM3 simulation of a passive mixer go bananas.

Here is a link from cadence website on how to bypass the bsim model problem for passive mixer, it has been a while, I'll read it again (as soon as I can remember my password)

http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:ViewSolution;solutionNu...

thanks.
Kelly

You seem to have a fundamental misunderstanding about simulation. Modeling approximates the circuit to create a system of equations  that can be simulated. From there the simulator uses numerical methods to approximate the solution to the system of equations. If you make a mistake in the modeling and so create the wrong system of equations, there is nothing you can do in the analysis method to compensate from the fact that your are solving the wrong system of equations.

Perhaps the word 'discontinuity' is confusing you. Sometimes convergence problems are created by discontinuities in the derivatives of the models. That is not the issue here. In this case the discontinuities are in currents, not in the derivatives, and the problem is not a convergence issue, it is the fact that the model does not accurately represent the behavior of the circuit when Vds is close to zero.

-Ken

Hi all,

thanks for all your comments, so I guess I did misunderstand the problem.  what you are saying is you can't trust the simulation results even  from transient analysis if the BSIM model is used.   May I ask, when you used the correct model (such as PSP), do you get pretty close agreement?  what's the discrepancy between sim vs. lab typically, within couple dBs ?
Thanks.
Kelly

PS, I'll try to to see whether I can force the small corrent though the swtiches.....

My "limited" experience is that your correlation is only going to be as strong as the weakest link. If your RF modeling is off, then IM3 can be off too if the circuit is very "RF sensitive". Also, I see a lot of discrepancy when the transistors are OFF. I guess the model accuracy is not so good (PSP or BSIM) in this region.


cheers,
Aaron

Hi RFICDUDE,


sure. Obviously its application dependent, and in the design of high throw-count RF switches, the off-transistor nonlinearity is important. The RF switch is the last active circuit (sort of quasi-passive) before the antenna, so the HD from the switches cannot be filtered (that's only partially true). Therefore, the HD requirements for RF switches can be quite stringent.


regards,
Aaron