Re: [time-nuts] PN on internal/external references and Cross correlation
> 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. A 10-dB deficit may not be too bad, since the performance improvement isn't linear with time. If your instrument takes 8 hours to drop the measurement floor by 20 dB, for instance, it will only take 4 hours to achieve 18.5 dB of improvement, or 2 hours for 17 dB. So if each of the references is 10 dB noisier than the DUT, their effect will be mostly gone after 2 hours. -- john, KE5FX Miles Design LLC ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] PN on internal/external references and Cross correlation
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 1 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] PN on internal/external references and Cross correlation
> 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 1 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] PN on internal/external references and Cross correlation
Hi All, 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 1 cross correlations). In the case of the references have an equal noise contribution compared to the DUT will the results suffer from loss of accuracy? Thanks! Regards, VS ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
