[time-nuts] PN on internal/external references and Cross correlation

Vasco Soares vesoares at deea.isel.ipl.pt
Thu Sep 28 19:46:11 EDT 2017

Thanks for clarifying this issue! I may use references with close-in PN 
(1Hz-1kHz) 10 dB noisier than the DUT, however as you said it may 
require an overnight run for 1 Hz offset, which isn't the nice part... I 
assume then that speeding up the measurement process can only be 
obtained using references sources at least quieter than the DUT.

Em 2017-09-28 23:29, John Miles escreveu:
>> In a PN measurement system with a dual channel cross correlation is 
>> there a
>> simple rule of thumb for how low should be the PN of the (external)
>> references compared to the DUT? (even with a 20 dB noise floor
>> improvement with 10000 cross correlations)
> If you have two references, then it's OK for them to be somewhat
> noisier than the DUT.  Their contribution will average out of the
> cross spectrum over time just like the rest of the instrument noise.
> There is no penalty in accuracy, only in measurement time.
> Given a choice, you're better off using references with good close-in
> noise performance rather than good broadband performance, since it
> takes longer for those FFT segments to converge.  You can achieve 20
> dB of noise floor improvement at offsets >10 kHz within a few minutes,
> but a 20-dB improvement at 1 Hz might require running overnight or
> even longer.
>> In the case of the references
>> have an equal noise contribution compared to the DUT will the results 
>> suffer
>> from loss of accuracy?
> With two references it's not a problem.  If you have only one
> reference source -- or if your measurement setup doesn't do cross
> correlation at all -- then the reference needs to be at least 10 dB
> quieter than the expected DUT performance to keep its contribution
> below 0.5 dB.
> -- john, KE5FX
> Miles Design LLC
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