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Phase Noise Plot to Equivalent C/N ratio conversion?

Discussion in 'Electronic Design' started by Paul, May 1, 2009.

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  1. Paul

    Paul Guest

    I'd like to convert a phase noise plot (in dBc/Hz @ offsets) to
    an
    equivalent Carrier-to-Noise ratio (C/N), over a specific processing
    bandwidth.

    Yes, these NGs are a long shot for getting info on this sort of
    thing,
    but it can't hurt to ask.
     
  2. If I got it right : you have a ( phase_noise vs frequency )
    and you want to convert this into a ( dBc vs frequency ) ?

    What's "dBc" against mW ( and/or mV ) rms ?
     
  3. Guest

    Hi Paul,

    First undo the "dB" operation, so you get "Noise power density (W/
    Hz)" over "carrier power" ratios (W) versus frequency (don’t convert
    to voltage ratios).

    Integrate versus frequency over the required frequency span, and you
    have "noise power" over "Carrier power" ratio. This equals 1/(C/N)
    ratio. Make a good judgment on the skirts adjacent to the carrier to
    avoid that carrier power is assumed noise power.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
    PM is the one shown without a, b and c.
     
  4. Paul

    Paul Guest


    Ok, this is exactly what i did. However, it's debated where
    exactly, is the cutoff frequency between the carrier and the noise.
     
  5. Paul

    Paul Guest


    How do you define Delta phi? The change in phase?

    How would you get this from a phase noise plot?

    Sounds like you are talking out of your ass!
     
  6. Guest

    Hello Paul,

    I assumed you did measurements with a spectrum analyzer (just scalar
    measurements, no vector based stuff). For determining "delta phi" you
    need a vector analyzer (or at least a quadrature down conversion
    setup).

    You might change the RBW setting and look how the spectrum changes.
    Below a certain RBW setting, it will not change. In that case the
    skirts can be because of the signal's phase/frequency noise, or phase/
    frequency noise from the analyzer.

    When you have a known stable source, you might be able the measure the
    frequency/phase noise contribution from the analyzer. An indication of
    the analyzer's noise is the response at zero Hz.

    I don't know your application, but noise very close to the carrier
    might be suppressed by your application (digital frequency tracking
    loop or just block length of the digital demodulation process?). Is
    it possible for you to derive the CNR from the output at some stage
    in you digital processing? Of course in that case the receiver front-
    end must be good.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
    The real address is the one without a, b and c.
     
  7. Paul

    Paul Guest

    We measured this with a dedicated Agilent phase noise
    analyzer. It contributes very little to the phase noise in most
    cases.

    Ok, you were on the right track by first converting the dBc/
    Hz
    to a power ratio, and then integrating by adding these powers
    together. I have a basic MATLAB script that does this.

    The real question is: where does the Carrier end, and the
    Noise begin, when looking at a phase noise plot? I'm sure it's
    dependant on your DSP processing, as some people have said
    that it's the phase noise outside of your tightest FIR filter that
    should be integrated, to get the total Noise.
     
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