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positive phase noise

Andy Liu
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.

  • 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 simulation
    and 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 to
    be 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 disregard
    data 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.

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