[time-nuts] PM-to-AM noise conversion (was RE: New Question onHP3048A Phase Noise Test Set)
Mike Feher
mfeher at eozinc.com
Thu Jan 17 15:34:26 EST 2008
So - Bruce - what did you say that I did not? -
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960
-----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 3:28 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:
> 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
>
>
John
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
components.
(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.
Bruce
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