Hi,

I am simulating a circuit with nonlinearity, time variance & frequency translation. The output Vout=(V1) + (V2) .

I used pnoise+noise separation to find transfer function from drain noise of M1 to  V1, V2 and Vout i.e. TF1, TF2 and TF and I find that TF=TF1 - TF2 which I want because it cancels M1 noise.

But when I model drain noise of M1 with a current source as shown in figure, and perform pac analysis, I do not see cancellation and I find TF=TF1+TF2

I also modelled it with small sinusoidal current and did transient, performed FFT and found TF=TF1+TF2

I am doubtful which one to trust. Can pac and transient do the job of pnoise OR it is a mistake?

Always is perhaps too strong, but generally PSS/PAC is more accurate than transient. The reason is that in PSS/PAC the large and small signals are treated separately. The circuit is analyzed with only the large signals present, converted in to a linear time varying representation, and then the small signal is applied. In transient, both the large and small signals are present simultaneously, so errors created by the large signals can interfere/corrupt the small signals.

Your original questions are written in a confusing manner, and so are difficult to respond to. My initial response is that the problem is user error. Some where or some how you are mis-interpreting something and that the pnoise result is most likely the correct result because it is more canned and so involves less user involvement and interpretation. But it is hard to know.

-Ken

Hi Ken,

I have attached a revised block diagram. It is a mixer with flo=2.4GHz, frf=2.405GHz to 2.520MHz fif=5MHz to 120MHz

It cancels downconverted noise of M1 at IF.

pnoise was used to find noise source gain from M1 (RF)  to outputs (IF).

pxf was used to find transimpedance gain from modelled noise current source (RF) to outputs (IF)

pxf outputs