[time-nuts] How do time-nuts measure phase noise?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Aug 17 11:18:58 UTC 2009


Steve Rooke wrote:
> 2009/8/17 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
>   
>> iovane at inwind.it wrote:
>>     
>>> Please, how do time-nuts measure phase noise? What may be a minimal setup whit recording capability?
>>> Thanks,
>>> Antonio I8IOV
>>>
>>>
>>>
>>>       
>> A minimal setup for the classical method requires 2 frequency sources at
>> the same frequency one of which can be phase locked in quadrature (with
>> low loop bandwidth) with the other.
>> The output spectrum of the phase detector outside the PLL loop bandwidth
>> is then proportional to the relative phase noise spectrum of the 2
>> sources and is measured using a spectrum analyser.
>> A double balanced diode mixer is used as a phase detector (most other
>> types of phase detectors are far too noisy).
>> A sound card with a suitable low noise preamp can be used as an FFT
>> based spectrum analyser in the ~10Hz to ~20kHz range, the actual
>> frequency limits depend on the sound card and its sampling rate.
>>     
>
> So how do you discriminate against the phase noise of the source you
> are using as a standard frequency? Do you have to use a known low
> phase noise standard to mix with your unknown source?
>
> Steve
>
>   
>> Bruce
>>
>>     
1) Use 3 sources, S1, S2 and X.

2) Phase lock S1 in quadrature to X using a diode double balanced mixer
using a low bandwidth PLL.

3) Phase lock S2 in quadrature to X using a diode double balanced mixer
using a low bandwidth PLL.

Then for frequencies outside the PLL bandwidth the crosspower spectrum
of the 2 mixer outputs should be predominantly due to the phase noise X.

In practice its best if the phase noise of the 2 standards S1, S2 isn't
significantly worse than that of X.

It is also desirable that both S1 and X and S2 and X be locked at their
respective mixers quadrature point (not necessarily the same as that for
zero dc output)

Bruce






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