[time-nuts] PM-to-AM noise conversion (was RE:New Question onHP3048A Phase Noise Test Set)
jmiles at pop.net
Thu Jan 17 17:27:03 EST 2008
Right, he wasn't making assumptions about Martyn's case, necessarily, but
addressing my assumptions about sideband coherence.
-- john, KE5FX
> Here I go again. Martyn defined how he was generating the 100 Hz
> beat note.
> So, not much to assume in this case. -
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Bruce Griffiths
> Sent: Thursday, January 17, 2008 4:46 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] PM-to-AM noise conversion (was RE: New Question
> onHP3048A Phase Noise Test Set)
> 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
> > 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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