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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