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daisy

Junior Member

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Posts: 17

question about the noise simulation of sc circuit
Jun 18^th, 2008, 2:17am

Hi
I am simulating the noise of a sc circuit with hspice,and I have some questions as follows:
1.what is the meaning of the frequency of the flicker noise?Does it mean the frequency at which the circuit is operating?To simulate noise in hspice ,we have to add a ac soure,so the frequency of the flicker noise means that of the ac source?
2.As mentioned above,we have to add a ac source first before simulating the noise,but if the circuit is the reset part of a sc circuit as follows:
the nmos works as a reset mosfet,the voltage on its gate opens the nmos and its dc value is 5.5v,now if I want to simulate the noise of the nmos,I have to add a ac source on the gate ,but I'm not sure whether it is right.

I appreciate your help!

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Geoffrey_Coram

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Re: question about the noise simulation of sc circuit
Reply #1 - Jul 1^st, 2008, 5:47am

I think you're quite confused on the subject of noise analysis, and you should read something like Ken's "Designer's Guide to Spice & Spectre" (or perhaps your simulator's user's guide section on noise) before proceeding.

The flicker noise is not related to the circuit operating frequency. The ac source does not have a frequency. Instead the noise analysis is performed over a set of frequencies, and the noise sources have their power spectral density evaluated at the frequencies. The ac source is also used to compute the gain of the linearized circuit at the set of frequencies, so in some sense, the ac source's frequency is related to the noise frequency, but only because they are both set by the frequency sweep parameters of the ac/noise analysis.

For a switched-cap circuit, you probably need to do a PNoise or HBNoise analysis, and if you don't understand standard ac noise analysis, you've got some homework to do before you start running simulations.

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daisy

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Re: question about the noise simulation of sc circuit
Reply #2 - Jul 5^th, 2008, 1:39am

Hi
Thank you for your help!
I am planning to read Ken's "Designer's Guide to Spice & Spectre".

daisy

Geoffrey_Coram wrote on Jul 1^st, 2008, 5:47am:

I think you're quite confused on the subject of noise analysis, and you should read something like Ken's "Designer's Guide to Spice & Spectre" (or perhaps your simulator's user's guide section on noise) before proceeding.

The flicker noise is not related to the circuit operating frequency. �The ac source does not have a frequency. Instead the noise analysis is performed over a set of frequencies, and the noise sources have their power spectral density evaluated at the frequencies. �The ac source is also used to compute the gain of the linearized circuit at the set of frequencies, so in some sense, the ac source's frequency is related to the noise frequency, but only because they are both set by the frequency sweep parameters of the ac/noise analysis.

For a switched-cap circuit, you probably need to do a PNoise or HBNoise analysis, and if you don't understand standard ac noise analysis, you've got some homework to do before you start running simulations.

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