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# difference between the two phase noise functions in Direct Plot Form?

Last post Fri, Jun 11 2010 11:42 AM by Tawna. 5 replies.
 Started by SonerYaldiz 08 May 2010 11:29 AM. Topic has 5 replies and 3391 views
• #### Sat, May 8 2010 11:29 AM

difference between the two phase noise functions in Direct Plot Form?
 Hi,I am trying to simulate the phase noise of a 25GHz LC oscillator using pss (flexible balance) and pnoise analysis. In the Direct Plot Form, there are two different Phase Noise options:1) Analysis: pnoise  Function:Phase Noise2) Analysis: pnoise jitter Function: Phase NoiseThe two above give different results (#1 is approximately ~2.3dBc lower than #2 @ 10MHz offset). What is the difference between these two functions? I would like to note that #1 provides much more numerically stable results than #2.
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• Post Points: 20
• #### Wed, May 26 2010 5:32 PM

• Tawna
• Joined on Thu, Jul 10 2008
• Snohomish, WA
• Posts 165
• Points 4,515
Re: difference between the two phase noise functions in Direct Plot Form?
• Post Points: 20
• #### Fri, Jun 11 2010 9:04 AM

Re: difference between the two phase noise functions in Direct Plot Form?
 Hi Tawna,Thanks for pointing out the solutions. I have a further question: What is the numerical noise floor for phase noise if we use harmonic balance? SpectreRF Theory states that the noise floor is -120 dB but I was not sure whether this applies to both shooting method and the harmonic balance.Regards,Soner Yaldiz
• Post Points: 20
• #### Fri, Jun 11 2010 9:44 AM

Re: difference between the two phase noise functions in Direct Plot Form?
 Soner,The numerical noise floor is not a fixed number for either engine. It will typically be lower with harmonic balance than shooting, but even saying that shooting has a numerical noise floor of some fixed number (e.g. -120dB) is a bit cavalier! However, I can pretty safely say that it will be something above about -300dB since that's the (approximate) limit of double precision floating point numbers ;-)The numerical noise floor will depend upon the reltol and abstol (primarily) used, but there's a complex (and circuit dependent) relationship.Regards,Andrew.
• Post Points: 20
• #### Fri, Jun 11 2010 10:58 AM

Re: difference between the two phase noise functions in Direct Plot Form?
 Hi Andrew,I am using reltol 1e-7, vabstol 1e-9 and iabstol 1e-12 and conservative in harmonic balance. I have an LC VCO with amplitude control (where I can change a bias voltage and increase the amplitude of oscillation). At two different bias voltages, (everything else the same) I observe phase noise of -117.7 and -125.7 dBc/Hz at 10MHz respectively. When I sweep tstab, I observe < 5mdBc/Hz numerical noise on -117 but I observe ~ 200mdBc/Hz numerical noise on -125.7 dBc/Hz. Based on your reply, 0.2 dBc noise is too much, isn't it? What other simulation parameters would you recommend me to adjust? Thanks for your fast response and regards,Soner
• Post Points: 20
• #### Fri, Jun 11 2010 11:42 AM

• Tawna
• Joined on Thu, Jul 10 2008
• Snohomish, WA
• Posts 165
• Points 4,515
Re: difference between the two phase noise functions in Direct Plot Form?
 Hi Soner, It is difficult to give exact advice without seeing your circuit and at the very least your options and analysis statements in your netlist.I recommend that you contact Cadence Customer Support (support.cadence.com) and work with an AE on this.  A reltol of 1e-7 and vabstol of 1e-9 is very very tight (way moreso than typically necessary). Other things will affect accuracy, such as the number of harmonics and sidebands specified.  Are you using augmented pnoise? How have you set oversample?   You may want to increase oversample from 1 to 2 when more accuracy is required (for example, you are running harmonic balance as the solver and seeing phase noise vary - particularly when varying tstab).You also have not specified if you have any warning messages - they can be very helpful in diagnosing problems. best regards,Tawna Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
• Post Points: 5