The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> strange "noise" riding at frequency measurement https://designers-guide.org/forum/YaBB.pl?num=1439370622 Message started by hiSkill_11 on Aug 12th, 2015, 2:10am

Title: strange "noise" riding at frequency measurement Post by hiSkill_11 on Aug 12th, 2015, 2:10am When I simulated a RC oscillator with digital controller, and use calculator freq funtion to plot out clock freqency, I found unrelated noise riding at frequency result? Is it realistic or just tool accuracy issue? I use spectre with maxmium step 10ps and other default setting. The target frequency is about 72Mhz. Thanks, any comment?

Title: Re: strange "noise" riding at frequency measurement Post by sheldon on Aug 13th, 2015, 10:26pm hiSkill,    Do you mean the pulse or the small variations in frequency? For the small variations, the accuracy is 10ps. Using my handy paper napkin  to help with the calculations. Adding ad subtracting 10ps for the period at 72MHz, the expected variation is frequency is about 100kHz[+/- 50kHz variation from the center frequency] so the low level noise seems to be consistent with the numerical accuracy control. Not sure what is causing the step.                                                                     Sheldon

Title: Re: strange "noise" riding at frequency measurement Post by hiSkill_11 on Aug 14th, 2015, 2:38am Hi Sheldon, Thanks for your good calculation which guiding me well understood the phenomenon. The step is due to digital trimming algortith which drives a nonaccurate clock to a target one. So the frequency jittering is due to accuracy setting, right? If i set maximum step 1ps, then the expected varaition is 10kHz, right? Could you also please explain a little why the maximum step impact on the frequency solution of circuit? Is there any ways to kill the "jitter" without affecting simulation speed? Thanks very much. sheldon wrote on Aug 13th, 2015, 10:26pm: hiSkill,    Do you mean the pulse or the small variations in frequency? For the small variations, the accuracy is 10ps. Using my handy paper napkin  to help with the calculations. Adding ad subtracting 10ps for the period at 72MHz, the expected variation is frequency is about 100kHz[+/- 50kHz variation from the center frequency] so the low level noise seems to be consistent with the numerical accuracy control. Not sure what is causing the step.                                                                     Sheldon

Title: Re: strange "noise" riding at frequency measurement Post by Ken Kundert on Aug 14th, 2015, 9:17am You would be better served if you stopped adjusting maxstep (return it to its default value) and instead moved the frequency measurement to inside the simulator. Using the calculator to measure frequency  is innately low accuracy because it occurs outside the simulator and employs linear interpolation. A better approach is to use Verilog-A to measure the frequency. An example how to is given in the period measurement example on the Verilog-AMS page. In general, adjusting maxstep is a bad way of controlling simulation error. Instead you should adjust reltol. In addition, it is better to perform timing measurements in verilog-a instead of the calculator. If you do these two things you will find that your simulations are both much faster and much more accurate. -Ken

Title: Re: strange "noise" riding at frequency measurement Post by hiSkill_11 on Aug 19th, 2015, 2:39am Many thanks to you ! Ken Kundert wrote on Aug 14th, 2015, 9:17am: You would be better served if you stopped adjusting maxstep (return it to its default value) and instead moved the frequency measurement to inside the simulator. Using the calculator to measure frequency  is innately low accuracy because it occurs outside the simulator and employs linear interpolation. A better approach is to use Verilog-A to measure the frequency. An example how to is given in the period measurement example on the Verilog-AMS page. In general, adjusting maxstep is a bad way of controlling simulation error. Instead you should adjust reltol. In addition, it is better to perform timing measurements in verilog-a instead of the calculator. If you do these two things you will find that your simulations are both much faster and much more accurate. -Ken

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