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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Aug 17 12:12:59 UTC 2009


iovane at inwind.it wrote:
> On Mon, 17 Aug 2009 23:18:58 +1200 Bruce Griffiths wrote:
>
>   
>> 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
>>>>>
>>>>>           
>
>   
>>>>>       
>>>>>           
>>>> snip
>>>>         
>
>   
>> 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
>>     
>
> Bruce,
>
> is this what actually time-nuts do? This is unpractical for me, as I have not much 
> time for a circuit building project. Moreover, I would be satisfied even with the 
> analysis at a single frequency, not necessarily the whole phase noise spectrum, 
> and with relative but repeatable measurements.
> So, does it make any sense using a selective voltmeter, such as an HP 3586A or 
> similar? 
>
> Antonio I8IOV
>
>
>   
Some have commercial phase noise measurement systems like Symmetricoms
5115A, 5120A. 5125A.
Some have older HP phase noise measurement systems like the 3048A etc.
The rest of us have to build our own systems based around sound cards or
surplus spectrum analysers together with the necessary mixers, low noise
amplifiers etc.

Unless your sources are very noisy you wont be able to measure their
phase noise with just a selective voltmeter or even an RF spectrum analyser.
If you cant build it, then you need to acquire something like the HP3048
or the NIST phase noise measurement box currently on ebay.(Item:
260423038423)

Bruce




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