[time-nuts] PM-to-AM noise conversion

John Miles jmiles at pop.net
Thu Jan 17 19:17:22 EST 2008


> > Really scratching my head now.  I understand why the SSB filter makes a
> > difference, because the mirror image of the FM spur is
> coherent.  I don't
> > understand why the 11729C's view of this DSB measurement (11.5
> MHz, 30 kHz,
> > first lobe) is closer to 'correct', while the earlier one (20
> MHz, 1 kHz,
> > second lobe) was ~6 dB too high...
> >
> > -- john, KE5FX
> >
> >
> >
> John
>
> In the case of coherent FM the phases of the LSB and USB first lobe
> components when translated to baseband are the same.
> Consequently their amplitudes add.
> The phases of the USB and LSB second harmonic (of the modulation
> frequency) lobes differ by 180 degrees when translated to baseband so
> that the 2nd harmonic of the
> Thus their amplitudes subtract.

Hmm, I didn't know that.  Wouldn't that effect have suppressed the 2nd lobe
almost entirely on the baseband measurement, though, rather than making it
appear 6 dB too high?  I should probably repeat that measurement to make
sure of what I'm doing.

Much appreciate the advice, by the way!

-- john, KE5FX




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