[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 17:46:48 EST 2008

John Miles wrote:
> Ah, I didn't realize that the two sidebands weren't mirror images.  That
> could make sense.
> It does seem to make a big differene where the spur came from.  My earlier
> test used the second FM lobe from the 8640B's internal 1-kHz oscillator (the
> first lobe was driven offscreen).  I just repeated the test with an external
> 30-kHz source, with the amplitude turned way down to allow me to look at the
> first lobe.  In that test (green/red traces) the 11729C and 8566
> measurements were only a couple dB apart:
> http://www.thegleam.com/ke5fx/11_MHz.gif
> I then used a crystal filter to lop off the lower sideband from the FM'ed
> signal generator.  As expected this had no effect on the amplitude measured
> by the spectrum analyzer directly, but the spur levels on the 11729C with
> the SSB filter in place are now about 8 dB too low, instead of 6 dB too high
> as the DSB measurement was before.
> 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

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.


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