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Simulators >> RF Simulators >> About Noise Value of time domain PNOISE analysis
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Message started by JC on Jul 4^th, 2007, 4:44pm

Title: About Noise Value of time domain PNOISE analysis
Post by JC on Jul 4^th, 2007, 4:44pm

When I use Pnoise analysis with noise type "timedomain" for a simple circuit (attached), I find the time domain value of output noise current through Vpulse is different with theoretical value sqrt(4KTG).

If doing Pnoise analysis with noise type "source", the PSD of output noise current is theoretical value 4KTG.

What does the noise value given by time domain Pnoise analysis represent?

-JC

Title: Re: About Noise Value of time domain PNOISE analys
Post by Frank Wiedmann on Jul 5^th, 2007, 1:10am

What you are seeing is aliasing as shown in figure 3 of http://www.designers-guide.org/Analysis/sc-filters.pdf. I would suggest that you read section 2.2 of this paper in order to better understand the meaning of the pnoise results.

Title: Re: About Noise Value of time domain PNOISE analys
Post by JC on Jul 5^th, 2007, 2:10am

Frank

Thanks for reply

I don't think there is aliasing or folding effect in the above circuit , because the thermal noise generated by the conductance is not modulated, neither the signal path from noise to output (Vpulse) is modulated.

Maybe I don't show clearly in the above circuit, which consists of a periodic pulse voltage source without noise and a resistor with the option "isnoisy=yes".

I have read that Ken's paper roughly but I haven't got the answer yet. I will read again carefully.

-JC

Title: Re: About Noise Value of time domain PNOISE analys
Post by Frank Wiedmann on Jul 5^th, 2007, 3:02am

Pnoise timedomain analysis includes an implicit sampling operation, this is where the aliasing comes from in your case. Because your circuit has infinite bandwidth, the noise you see should scale linearly with the number of sidebands you specify in your simulation setup.

Title: Re: About Noise Value of time domain PNOISE analys
Post by JC on Jul 5^th, 2007, 3:40am

Frank:

Thanks a lot! You are totally right. By simluation I found SpectreRF includes this frequency aliasing effect itself.

So I think due to this behavior of PNOISE timedomain analysis I should be very carefull to choose a proper probe to observe noise otherwise the noise obsvered may be not the real one!

-JC