[time-nuts] Phase noise at 1-100Hz

[Bruce Griffiths]

John Miles wrote:
>    
>> Javier Herrero wrote:
>>      
>>> Hello,
>>>
>>> I will need to measure the phase noise of a 35MHz oscillator in the
>>> range of 1 to 100Hz (well... and also at a higher range but this is no
>>> problem), and I would like to know about the different alternatives. I
>>> would like not to have to mess too much with mixers and soundcard
>>> sampling, if possible :)
>>>        
> You probably need to find someone with an HP 3048A or similar outfit.  A TSC
> 5125A will also do the job.  The requirement to go below 100 Hz takes a lot
> of the 'easy' solutions like the 11729C out of contention.
>
>    
>>> I think that the HP 5372A with opt 040 is able to do that, is this
>>> right and worthy? If so, opt 040 is a pure software option or requires
>>> also additional hardware? Would be possible also with a 5370A and
>>> software processing of the raw data? Any other alternatives?
>>>
>>> Thanks! Regards,
>>>
>>> Javier
>>>
>>>        
>> The 5372A or 5370A will only suffice if the phase noise is sufficiently
>> high.
>> They are likely to be too noisy by several orders of magnitude for a
>> good crystal oscillator.
>>      
> That was my first thought as well, but the 5370 might be OK for measuring
> TCXOs and the like.  An HP paper from the mid-1970s suggests that it's
> possible to reach ~-150 dBc/Hz at 1 Hz with contemporary hardware
> (5345A-based, 2-ns resolution):
>
> http://www.hparchive.com/seminar_notes/a-114.pdf
>
> They mix down to a low IF before measurement, which they appear to vary in
> order to get a lower floor at close-in offsets.
>
> If nothing else, it should be possible to back the PN spectrum out of an
> ADEV graph plotted with data from a counter, taking the noise slope into
> consideration.  It would take a lot of hacking (not to mention the effort
> needed to get hundreds of measurements per second out of the counter, which
> I also haven't looked into.)
>
> There's a more user-friendly app note here:
> http://www.ko4bb.com/Manuals/HP_Agilent/1%29_HP_App_Notes/HP_AN225_Measuring
> _Phase_Noise_with_HP_5390A.pdf
>
> Could be a reasonable way to measure PN at offsets below 10 or 100 Hz, at
> least until the higher-order noise slopes invalidate some of the assumptions
> baked into the software.  It wouldn't scale very well to broadband offsets
> in any event.
>
> -- john, KE5FX
>
>
>
>    
To take full advantage of the resolution of a 5370A it may be necessary 
to use an external zero crossing detector to reduce the trigger noise to 
a sufficiently low level with the relatively low beat frequency signal.

Bruce