[time-nuts] New Question on HP3048A Phase Noise Test Set

BriMDavis at aol.com BriMDavis at aol.com
Thu Jan 17 23:00:55 EST 2008

Martyn first wrote:
>To test the 3048A accuracy I inject a spur at a known level using a 
>directional coupler (similar to the single-sided spur calibration method).
>I confirm the spur level on the spectrum analyzer.  However the 3048A always 
>says the spur is 6 dB lower than it actually is (6 dB plus/minus 0.5 dB).
and later added:
>So who is right?  The spectrum analyser or the HP3048A.
Perhaps they are both right:
 - The spectrum analyzer is displaying the power spectrum 
of the RF signal
 - The 3048A is displaying the power spectrum of the 
demodulated phase of the RF signal

Question : 
 Are you asking the 3048A to plot L(f) or S(f) ?
 As best I recall, when you ask it to plot L(f), it takes the actual
measurement data, which is S(f), and then subtracts 3dB for the
noise floor portions of the plot, using the small angle assumption
L(f) = S(f) / 2
 I don't remember how it treats the spurs between plot modes, give it 
a whirl and see how the displayed noise floor and spurious change when 
you switch between plots of L(f) and S(f)

have fun,
I shall not attempt to add to the coherent/non-coherent sideband
discussion, but IIRC hard limiting a signal having a small SSB spur
(stripping the AM component and leaving just PM) will create a 
pair of PM sidebands that are 6 dB down from the original spur level
as viewed on a spectrum analyzer.

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