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

Brooke Clarke brooke at pacific.net
Sun Apr 17 19:11:50 EDT 2005

Hi David:

The diagram supposes that you can come up with a 1 kHz oscillator whose 
phase noise is much smaller than what you are trying to measure.
Is that the case?


David Kirkby wrote:

> 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 digitally).
> 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.
> http://www.southminster-branch-line.org.uk/tmp/pn.jpg
> (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
> http://www.thinksrs.com/downloads/PDFs/Manuals/SR830m.pdf
> 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.

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