#### Ian.Y.Jin

##### Newbie level 6

I'm currently doing an inductor Monte Carlo simulation in Cadence-Spectre (Cadence version: 5.10.41_USR5.90.69; Spectre version: 6.2.0.420). My objective is to see how the inductance and Q-factor (differential-driven) changes over process variations.

I set up a few expressions to convert 2-port S-parameters to 1-port differential equivalent as follows:

monteExpr( "S11" "sp(1 1 ?result 'sp)" )

monteExpr( "S12" "sp(1 2 ?result 'sp)" )

monteExpr( "S21" "sp(2 1 ?result 'sp)" )

monteExpr( "S22" "sp(2 2 ?result 'sp)" )

monteExpr( "Sdm" "0.5*(S11-S12-S21+S22)" )

The plots of first four expressions are working fine all the time, and I can see the effect of process variations. But the "Sdm" expression gives problem, sometimes it gives only one curve, and sometimes (without anything changed), it even gives error message as follows:

*Error* Evaluating expression ((0.5 * ((S11 - S12 - S21) + S22))).

*Error* ("eval" 13 t nil ("*Error* eval: unbound variable" S11))

I don't know what's going wrong. Does this problem ring any bell for you? Thanks.

Best Regards,

Y. Jin