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# phase noise analysis - VCO

Last post Thu, Mar 29 2012 10:43 AM by Andrew Beckett. 2 replies.
 Started by rsrk 01 Mar 2012 11:31 AM. Topic has 2 replies and 4055 views
• #### Thu, Mar 1 2012 11:31 AM

• rsrk
• Joined on Thu, Feb 10 2011
• Posts 9
• Points 150
phase noise analysis - VCO
 Hi,  I have a query regarding calculation of phase noise of VCO by pss and pnoise analysis. I can plot pnoise and hence can find phase noise at an offset frequency. General method      At the same time, from same analysis I can calculate/estimate output noise, at an offset frequency, due to all components of the VCO - from virtuoso itself. Carrier power can be calculated from transient analysis - Vout_meansquared                By basic definition : Phase Noise = 10*log*((Noise@f_offset, 1HZ) / Pcarrier).My confusion is is it appropriate to expect the phase noises evaluated by above two approaches to be same? I am getting different values from these two methods. If its not appropriate, then can someone shed light why is it so ? For Clarification : From attached file VCO_Pnoise.pdf  phase noise plot : phase noise at 7MHz offset can be understood to be -104.3 dBC/Hz   Output noise summary at 7MHz offset : output noise at 7MHz offset is 1.322e-17 V^2 / Hz  Transient analysis : Vout peak = 714mV                   = > P_carrier = 0.25 V^2 (rough calculation)From above two values and the second definition of phase noise : Phase Noise = 10*log*(1.322e-17 / 0.25) = -162.767 dBc/HzCan someone shed light why the Value of phase noise calculated by above two methods is not matching with each other ? Thanks for your help and support.RegardsSiva Rama Krishna  VCO_Pnoise.pdf
• Post Points: 20
• #### Thu, Mar 29 2012 7:53 AM

• dwight6
• Joined on Wed, Mar 28 2012
• Posts 4
• Points 80
Re: phase noise analysis - VCO
 Hello,From the phase noise plot it seems to me that is the PN of the 5th harmonic of the signal. is it right? If so, it should be the power of the 5th harmonic and the output noise at 7mhz of the same harmonic. regards
• Post Points: 20
• #### Thu, Mar 29 2012 10:43 AM

Re: phase noise analysis - VCO
 I looked at this a couple of weeks ago, and thought I'd responded - but it seems my reply didn't happen (there were some issues at the time - so maybe that's why).Anyway, looking back through my notes, my example works fine. All that it does is look at the output noise using:noise=value(getData("out" ?result 'pnoise)) ; "out" means output referred noisesig=harmonic(v("/vout" ?result 'pss_fd) 1) ; the 1 depends on the relative harmonic you chose when doing pnoise, the /vout depends on where you said the noise was to be measuredand then the phase noise (which is really output noise in dBc) is:db20(noise/(sig/sqrt(2.0)))That's it. Nothing complicated. You're comparing the transient results - you should take a look at the pss results and ensure that the signals are as big as you think they should be. The numbers you're reporting sound very small, so something odd is going on here.I would suggest you contact customer support if you're not getting anywhere.Regards,Andrew.
• Post Points: 5