On 3/11/2020 9:39 PM, Michael Tope wrote:
The signal components add coherently at the combiner output yielding a
total signal voltage of 14.14 Volts rms. The noise voltages are
incoherent, so they add as root-sum-square at the output of the
combiner. This yields a total noise voltage of sqrt(0.707^2 + 0.707^2) =
sqrt(1) = 1.0 Vrms. Thus, the combined noise voltage is unchanged, but
the signal voltage goes up by sqrt(2).
This is also why averaging in measurement systems and spectral displays
improves their signal to noise ratio. As the number of averages is
increased, signal to noise increases using the same math as above. I set
averaging on my P3 to the max, the noise averages out, adjust the
display reference level so that the noise is at the bottom of the
display, causing even the weakest carriers (or CW) to be seen above the
noise (and as faint traces in the waterfall).
We used averaging extensively in pro audio measurement systems,
beginning with Time Delay Spectrometry around 1982. Which, by the way,
was invented about ten years earlier by the late Richard Heyser, was was
at JPL at the time. In this AES Paper, I buried a TDS sweep in a music
track at a level that was nearly inaudible and fed it through a popular
broadcast audio processor to study it's dynamic frequency response at
high levels of compression. The sweep was nearly inaudible, the the
system was able to recover it by 64X averaging, combined with TDS's
inherent noise rejection.
http://k9yc.com/AESPaper-TDS.pdf
73, Jim K9YC
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