[time-nuts] Re: Phase noise with a lock-in amplifier.

David Kirkby david.kirkby at onetel.net
Sun Apr 17 19:00:17 EDT 2005

Mike Feher wrote:
> Guys, unless I am grossly misunderstanding something, the mixer method has
> been the long standing method by which phase noise is measured. There are
> countless papers on the subject including several by Dave Allen and Fred
> Walls of NIST. We were using this method over 20 years ago when working on
> the Milstar program whose phase noise on the output carrier at 44 GHz was
> impossible to meet. NIST even developed a phase noise test set for the
> purpose which HP went on to produce. Just think how low the close in phase
> noise of a 10 MHz source had to be if every multiplication (N) of that
> source to get to 44 GHz degraded it by 20*logN. I digress, but, we used the
> basic mixer as a phase detector, which I believe is what this discussion is
> about. Regards - Mike    
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960

OK, it seems then I am not mistaken about mixing to get the phase noise 
  down to a low frequency.

The point of others about the delay line needing to be huge to get 
uncorrelated noise close in is possibly valid.

However, there is nothing stopping that mixer being two A/D's, with 
their outputs fed to a CPU that does a simple multiplication. Although I 
suggested an analogue RF mixer, it could be done in the digital domain. 
(Conincedently, DSP based lock-in amplifiers implement the multipliers 

If we can accept that we can transfer/mix or whatever you want to call 
it the phase noise on the oscillator down to a low frequency, can the 
next bit of my suggestion work - use a commercial dual phase lock-in 
amplifier to measure that noise?

Take a look at this - which is a variation of what I was originally 
suggesting, as it has this adder.


(sorry about the poor drawing)

The phase noise is mixed to a low frequency.

Assume you want to make a noise measurement at 1kHz offset from the 
carrier. You add (not multiply) a 1kHz sine wave to the filtered output 
from the mixer. The lock-in amp will easily measure the 1kHz signal now. 
But the better lock-in's can measure noise on that too. Page 3-25 of the 
manual for the SR830 DSP lock-in


describes noise measurements. The measurement of noise is made at the 
reference frequency.

This might not be the simplist/best way of doing it. I am just wondering 
it if is a way to do it.

The good thing about lock-ins is their abiltiy to dig out small signals, 
ignoring everything except those frequencies very close to the reference.

Dr. David Kirkby,

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/

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