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https://designers-guide.org/forum/YaBB.pl Simulators >> RF Simulators >> Pnoise warning: "Relative residual of linear ..." https://designers-guide.org/forum/YaBB.pl?num=1171557018 Message started by Thomas on Feb 15th, 2007, 8:30am |
Title: Pnoise warning: "Relative residual of linear ..." Post by Thomas on Feb 15th, 2007, 8:30am Hi, I'm simulating a chopper amplifier. It works quite well, but when I include a sensor as a Verilog-A model a warning appears during the Pnoise simulation. It says “Relative residual of linear system is significantly above tolerance. Results may be inaccurate.” The simulation finishes and the results seem reasonable, but I'm not sure how much I can trust them. Does somebody have any idea how to solve this problem? I already increased maxacfreq to a high value and varied reltol, vabstol and iabstol. I have set the solver to std instead to turbo. The only thing what avoids this warning is to reduce the tolerance from 1e-9 (default) to 1e-7. But that doesn't really solve the problem, it just doesn't forces spectre to spit out some warnings. Other question. How many sidebands should I use in the Pnoise. I thought that I increase the number of sidebands until there are no significantly changes. So I simulated up to 1000 sidebands, but it doesn't really look like its going to converge. Does somebody have some experience values which make sense to use? Thank you so much. Cheers Thomas |
Title: Re: Pnoise warning: "Relative residual of linear . Post by tawna on Apr 11th, 2007, 3:29pm The warning message about residual means that the simulator is not able to fully converge the circuit at that point and that the data may not be not reliable. You can get this message as well if you have chosen to perform a periodic small signal analysis that is very close to the fundamental. This warning indicates that the periodic small signal analysis being run needs a more accurate PSS solution. One way to get a more accurate solution is to make the PSS take more time points. You might need to set the PSS parameter "maxacfreq" to a higher value. This will force spectre to take more timepoints and therefore improve the accuracy of the Periodic small signal solution. Why does this happen? SpectreRF injects the small signal tone in the time domain then demodulates out the PSS harmonics to leave a signal that is just harmonics of the Periodic small signal analysis tones. SpectreRF then takes a fourier analysis of this solution to give the resultant small signal harmonics (sidebands). If the small signal tone is close to a PSS harmonic the algorithm might not be able to resolve the small signal tone accurately enough. To get more accuracy, you can set maxacfreq (PSS option) to a value greater than 40*fundamental (default value). Another tip is to set the solver method in your Periodic small signal analysis option to "std" (standard). The default is "turbo" (recycled Krylov Sub-space technique). The standard method is useful for frequency points close to PSS harmonics (10-100Hz) in any of the small signal analyses. |
Title: Re: Pnoise warning: "Relative residual of linear . Post by Geoffrey_Coram on Apr 18th, 2007, 4:17am Very nice explanation, Tawna. However, it seemed to me that Thomas was saying he *doesn't* get the residual warning when the Verilog-A sensor is omitted. Did I read that right, Thomas? In which case, you might need to take a close look at the Verilog-A code and see if you're doing something funny there -- eg, if the sensor output is the wrong order of magnitude for the tolerances, like you have an output voltage that is 1e-9 but you have a voltage tolerance of 1e-6. |
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