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# positive phase noise

Last post Thu, Jul 16 2009 2:52 PM by Tawna. 2 replies.
 Started by Andy Liu 11 Jun 2009 10:34 AM. Topic has 2 replies and 5700 views
• #### Thu, Jun 11 2009 10:34 AM

• Andy Liu
• Joined on Thu, Apr 30 2009
• Posts 4
• Points 110
positive phase noise
 I simulated the phase noise of an LC VCO. The phase noise at very low offset is positive. For example, the phase noise is +13.2 dBc/Hz at 100 Hz offset whilst the phase noise at 1 MHz offset is -100 dBc/Hz. The carrier frequency is 5 GHz. Should phase noise be negative? If the component at 5.0000001 GHz is 13.2 dB larger than that at 5.0000000 GHz, should be the carrier frequency be 5.0000001 GHz instead? If the phase noise at low offset is not simulated correctly, what is the limit of the offset? Thanks.
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• Post Points: 35
• #### Fri, Jun 12 2009 12:07 AM

 Frank Wiedmann Joined on Thu, Jul 17 2008 Muenchen, Bavaria Posts 75 Points 1,160 Re: positive phase noise Post Points: 5
• #### Thu, Jul 16 2009 2:52 PM

• Tawna
• Joined on Thu, Jul 10 2008
• Snohomish, WA
• Posts 193
• Points 5,190
Re: positive phase noise
 You may want to take a look at http://sourcelink.cadence com solution 1832229 (SLK) Why Is There A Positive dBc/Hz Phase Number Displayed When Plotting Pnoise? ADE figures the "phase noise" by taking the amplitude of the oscillation from the noise free pss simulationand dividing it into the noise levels calculated from a pnoise simulation. The "output noise" plot is just  the raw, unscaled, noise levels at the oscillator output, that is the raw results of the pnoise simulation.If you plot both on a log plot, then they should differ by a constant, and that constant should be the level of the oscillator output.ADE produces the "phase noise" output, which is normalized by the carrier amplitude.  However, that normalization does not generally give 0dBc at 1Hz.  For that to happen the noise level would have tobe exactly the same as the oscillation level at 1Hz, which need not be the case.Phase noise levels greater than 0dBc do not make sense, since the noise has more power than the actual oscillation, so they decided to disregarddata for values less than the cross-over frequencies.The cross over frequency is simply the frequency where the ADE output is 0dBc.  In other words, the frequency where the power in the carrier is the same as the calculated noise power.The bottomline:  disregard/discard phase noise values greater than 0dBc, as they are non-physical. Best regards, Tawna Wilsey Staff Support AE, Global Customer Support Cadence Design Systems, Inc.
• Post Points: 5