[time-nuts] Ultra low phase noise floor measurement system for RF devices.
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Mar 31 20:06:47 EDT 2007
SAIDJACK at aol.com wrote:
> In a message dated 3/30/2007 23:47:12 Pacific Daylight Time,
> bruce.griffiths at xtra.co.nz writes:
> Thermal noise and other noise in each channel are statistically
> independent and their product averages to zero.
> The actual residual decreases as the number of spectra averaged increases.
> Hi Bruce,
> hmmm. Not sure I understand. I thought that if you average Gaussian noise
> sources the noise would only be reduced by a factor of root(n), n being the
> number of channels; and not totally cancel out? Is the difference caused by
> creating a product of the two channels rather than a summation?
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Multiplying the 2 signals is essential as the non correlated components
of the 2 signals will then average to zero (for infinite measurement
time) whilst the correlated components of the 2 signals will not.
A cross spectral analysis is somewhat analogous to using an array of
single frequency correlators. The residual noise decreases as the
integration time increases. When multiple power spectra are averaged the
noise floor will decrease as 1/n where n is the number spectra averaged.
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