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Simulators >> RF Simulators >> Contradictory results: pnoise+noise sparation v/s pac or transient
https://designers-guide.org/forum/YaBB.pl?num=1409168910

Message started by mixed_signal on Aug 27^th, 2014, 12:48pm

Title: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 27^th, 2014, 12:48pm

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?

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 28^th, 2014, 5:35pm

Hi,
The circuit is time variant.
Can we always trust transient analysis? Is it always superior to pss+pac or pss+pnoise ?

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by Ken Kundert on Aug 29^th, 2014, 11:21am

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

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 30^th, 2014, 9:33am

Hi Ken,
Thank you very much for your reply.

The system behaviour in a nut shell:
Noise at RF of transistor M1 gets downconverted to IF at port 1 and port 2 with opposite phase. When added the noise cancels at IF at the output.

Would you suggest any simulation to check if noise cancellation is occurring? I am worried because pnoise is contradicting with pac and pxf as well.

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 30^th, 2014, 10:51am

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)

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 30^th, 2014, 10:52am

pnoise outputs

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 30^th, 2014, 10:52am

pxf outputs

Title: Re: Contradictory results: pnoise+noise sparation v/s pac or transient
Post by mixed_signal on Aug 30^th, 2014, 1:52pm

Hi Ken,

The problem is solved!! :)
You are correct! pnoise result is correct and noise cancels! :)

The problem is a bug with vcvs model in analogLib. The ideal adder was implemented using two vcvs stacked. pnoise analysis preserved the phase information of signals during addition & gave correct result.

pac, pxf, transient+fft could not preserve phase info and always added the two TF irrespective of phase.

I replaced the adder with a real adder using MOS and it is resolved!!
Thanks again for your reply. :)