[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 15:27:51 EST 2008

John Miles wrote:
> Thinking about it a bit more, if your spur appears 6 dB too *low*, that
> wouldn't be accounted for by the mixer-as-phase-detector correction factor.
> It's in the wrong direction.  Failure to apply that correction would make
> your spur appear 6 dB too high.
> Out of curiosity, does it matter which spectrum analyzer (3561A versus
> 3585A) you use to verify the spur amplitude?
> -- john, KE5FX

The explanation is very simple.
The 3048 calculates L(f) (a single sideband measure) from the mixer
output where contributions from both sidebands are combined.
For incoherent (noise) components the powers from the 2 sidebands add.
For coherent components the amplitudes add.

Thus to correct the DSB measurements to SSB 3 dB is subtracted from all
incoherent (noise components) and 6dB is subtracted from all coherent
(By definition L(f) is symmetric about the carrier - although this isnt
necessarily true for large offsets).

Since the input to the 3048 includes a single frequency SSB spur,
subtracting 6dB produces a calculate SSB spur amplitude 6dB lower than
that measured by the spectrum analyser.
If equal amplitude (and frequency) spurs symmetric about the carrier
were injected their amplitude would be calculated correctly by the 3048.


More information about the time-nuts mailing list