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https://designers-guide.org/forum/YaBB.pl Simulators >> RF Simulators >> Question about SpectrRF time domain noise analysis https://designers-guide.org/forum/YaBB.pl?num=1183920946 Message started by JC on Jul 8th, 2007, 11:55am |
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Title: Question about SpectrRF time domain noise analysis Post by JC on Jul 8th, 2007, 11:55am The phenomena: Use PNOISE with noiseType "timedomain" to analyze Mixer. Increasing the number of "Maxsideband" will increase the output noise power almost linearly (during simulation I set "Maxsideband" from 10 to 50). I am surprised why Spectre gives that result, since the noise transfer function has certain limit bandwidth. Very High order harmonics almost have no effect on the frequency of interest because conversion gain for that harmonic is very small. |
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Title: Re: Question about SpectrRF time domain noise anal Post by Frank Wiedmann on Jul 8th, 2007, 11:25pm Well, apparently the bandwidth of your circuit is much larger than you think. Please remember that the sidebands refer to the frequency of the pss analysis, so if this frequency is rather low, you may need a large number of sidebands to cover the relevant frequency range. |
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Title: Re: Question about SpectrRF time domain noise anal Post by JC on Jul 9th, 2007, 1:37am Frank: The fundamental frequency from PSS is 2GHz. I feel surprised because there is no obvious increment (within 0.5dB) when I sim by PNOISE with noiseType "sources" and sweep "Maxsideband" from 10 to 50. However if by PNOISE with noiseType "timedomain", there is a large increment. I don't know why there is so big difference between "sources" and "timedomain". Is it a usual phenomena? In my opinion "sources" and "timedomain" should show the same trend. -JC |
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Title: Re: Question about SpectrRF time domain noise anal Post by Frank Wiedmann on Jul 9th, 2007, 3:10am You might see this kind of behavior if the circuit is very sensitive to high-frequency noise only during a short part of the period and you are looking at this point in the pnoise timedomain analysis. This might happen e.g. if the signal is saturated most of the time and has fast transitions which you are examining. Pnoise sources analysis averages the noise over the whole period, so the effect may be much less pronounced here because the increase of noise only happens during a small part of the period. |
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Title: Re: Question about SpectrRF time domain noise anal Post by JC on Jul 9th, 2007, 6:58am Frank: Yes. The case you described will cause that behavior. However it seems not the same case in my circuit. I post the circuit Fig. 3 and the simulation results Fig. 1and Fig. 2. I probe the output noise current using dc voltage source with Vdc=0. Fig. 1 is the time domain output noise current of 2GH compnoent for "maxsideband" 10, 20 and 50. Fig. 2 is the output noise current of PNOISE with noiseType "sources" for "maxsideband" 10, 20 and 50. As can seen there is a large increment in time domain noise while the increment is very small of PNOISE sources analysis . By the way I use behavior model for the MOSFET that has no capacitance and no noise effect. So I add a noise current source in parallel with M2. |
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Title: Re: Question about SpectrRF time domain noise anal Post by Frank Wiedmann on Jul 9th, 2007, 11:49pm If you are using behavioral models without capacitance for your MOSFETs, your circuit is not very realistic because it has infinite bandwidth. Still, I have no good explanation for the results you are getting. However, given the infinite bandwidth of your circuit, what surprises me is the fact that the noise power does not increase linearly with the number of sidebands in the pnoise sources analysis. In comparison with the pnoise timedomain analysis, the results of the pnoise sources analysis also seem to be rather low, so maybe there is a problem in your simulation setup. |
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Title: Re: Question about SpectrRF time domain noise anal Post by JC on Jul 26th, 2007, 10:51am Frank, Thanks for your reply. Frank Wiedmann wrote on Jul 9th, 2007, 11:49pm:
This is because the max noise power is limited by the noise source with the PSD 4kTR in the pnoise sources analysis. But what surprises me is that why I can't see that limit in the pnoise tdnoise analysis, in which the power is increasing almost linear with the number of sidebands. Or maybe, as you have said, it is caused by the ideal behavior FET model without capacitive effect. And do you have any ideal about which simulation setting will probably cause this strange phenomenon? -JC |
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Title: Re: Question about SpectrRF time domain noise anal Post by Geoffrey_Coram on Jul 27th, 2007, 7:14am JC wrote on Jul 26th, 2007, 10:51am:
But when you use time domain noise, the noise is sampled at a timepoint, which causes the spectrum to be folded back (repeatedly) onto the range of interest. Since the spectrum is completely white and there's no capacitance to filter the high-frequency components, each copy of the spectrum that is folded back adds the same power, so there's no limit to the total power found in the band of interest. |
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Title: Re: Question about SpectrRF time domain noise anal Post by JC on Jul 30th, 2007, 5:43am Geoffrey_Coram wrote on Jul 27th, 2007, 7:14am:
Thanks. Yes. I agree with you. I think it should be the reason. But now I have another question: What's the relationship between value from time domain pnoise analysis and that from sources pnoise analysis? For example by sources pnoise analysis I can obtain the time-averaged PSD value at frequency 2GHz and by time domain pnoise analysis I can obtain the waveform of frequency component of 2GHz. Since in my case one has limited value and the other unlimited, it seems strange and What does the value from time domain pnoise represent? . -JC |
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