[time-nuts] Ultra low phase noise floor measurement system for RF devices.

[Dr Bruce Griffiths]

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?
>  
> bye,
> Said
>
>
>
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Said

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.

Bruce