[time-nuts] PM-to-AM noise conversion (was RE: New Question onHP3048A Phase Noise Test Set)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jan 17 16:46:18 EST 2008

John Miles wrote:
> I see what you mean, Bruce, but why are the two sidebands considered
> incoherent, just because they're noise?  Since the IF is 0 Hz, the other
> sideband appearing at the analyzer input jack is a folded image of the same
> noise spectrum, right?  Given that, shouldn't the SSB correction be -6 dB
> rather than -3 dB?
The phase angle between the USB and LSB noise components is random when
translated to baseband so when averaged over time the resultant
amplitude is the same as if one just added the powers of the 2 components.
When the 2 sidebands are coherent the phase shift between them is fixed
so that their amplitudes add (vectorially).
The phase angle between the LSB and USB components when translated to
baseband depends on the modulation mechanism creating them.
With both LSB and USB components translated to baseband one has no
additional information so that some "reasonable" assumption has to be made.


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