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

[Dr Bruce Griffiths]

> Mike
>
>  It actually measures the additive phase noise of  components 
> (amplifiers, splitters, transformers etc.).
> Since it uses a cross correlation technique it can easily achieve a 
> noise floor below the thermal noise.
> Cancellation of the carrier in the interferometer/bridge allows use of a 
> source with a phase noise well above that of the measurement system.
>
> In is possible to build lower phase noise oscillators using very low 
> phase noise sustaining amplifiers.
> Such amplifiers can be implemented using feedforward correction techniques.
> An RF bridge/interferometer is used to null the carrier at the input to 
> the low noise figure correction amplifier, so that only the phase noise 
> and other error components produced by the main amplifier are seen by 
> the correction amplifier the output of which is combined with (correct 
> delay and gain) the output of the main amplifier to reduce the composite 
> amplifier phase noise. Phase noise is absent when a carrier is not 
> present, so the phase noise contribution of the correction amplifier 
> will be very small if the interferometer/bridge is well balanced. This 
> technique has been used to reduce the phase noise of microwave 
> oscillators. In principle there's no reason it cannot be used at lower 
> frequencies.
>
> Bruce
>   
Addendum/Correction

Use of the cross correlation technique and cancellation of the carrier in the interferometer/bridge allows use of a 
source with a phase noise well above that of the measurement system.

The test signal phase noise is common to both correlator inputs and is 
rejected by the cross correlation technique.

Bruce