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

David Kirkby david.kirkby at onetel.net
Mon Apr 18 03:14:55 EDT 2005

Brooke Clarke wrote:
> 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?
> Brooke

Yes - I don't think my method avoids that problem, which I realise is 
always the difficult one.

Commercial lock-ins do have "low" phase noise oscillators built-in, or 
could be fed from an external oscillator. In the case of the lock-in I 
mentioned, the oscillator phase noise is said to be less than 0.0001 deg 
RMS at 1kHz.

I'm not actually convinced the oscillator shown needs to be fed to the 
adder, but ultimately for the lock-in to work, it needs a reference 
oscillator, so that can not be avoided.

dave k

> 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.

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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