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

BriMDavis at aol.com BriMDavis at aol.com
Fri Jan 18 22:50:55 EST 2008

Martyn wrote:
>I did do a plot selecting Phase Modulation Spectral Density (dB/Hz) 
>on the graph instead of dBc/Hz.  Here the spurs were 3 dB lower!!!
 To make sure I understand your results, here's how I would 
summarize your measurements, inserting some made-up numbers 
for observed spur and noise floor levels:
 RF analyzer directly measuring 10 MHz signal :
   - spur at 10.000100 Hz is @ -50 dBc

3048A L(f) measuring coherently demodulated phase:
   - spur at 100 hz is @ -56 dBc
   - noise floor is -160 dBc/Hz at 5 MHz offset

3048A Sphi(f) measuring coherently demodulated phase:
   - spur at 100 hz is @ -53 dB
   - noise floor is -157 dB/Hz at 5 MHz offset

If this is what you are seeing for relative changes in level
for the different measurements, I think the 3048A is working 
just fine.
Here's another experiment to try [1]:
 - change spur to an offset of 500 kHz @ -50 dBm
this moves the spur into broadband analyzer territory
 - re-run your L(f) and S(f) plots at this offset 
I would expect the same 6 dB, 3dB spur results for L(f), S(f)
 - change the measurement setup BW to zero Hz instead of 1 Hz,
and measure Sphi(f) again
      This forces the 3048A to NOT apply bandwidth corrections
to the collected data, so it will display a segmented-looking 
trace with increasing floor offset across each band break
[1] It's been about 7 years since I've actually pushed the buttons 
on a 3048A, so my recollection of setup and S/W behavior might 
be a bit fuzzy

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