[time-nuts] GPS noise reduction

John Ackermann N8UR jra at febo.com
Sun Apr 6 12:47:16 EDT 2008

That also leads to a thought I've had -- if you have a pair of roughly
comparable oscillators at say 5 MHz, what about combining their outputs
in a mixer and using the 10 MHz sum output which (apart from noise added
by the mixer) ought to be sqrt-2 better than either unit alone?

Tom Van Baak said the following on 04/06/2008 12:04 PM:
>> Actually, in this application will a bit of interlocking not 
>> hurt, as the two oscillators should longterm have the same 
>> frequency and interlocking will pull them together to their 
>> average frequency. If you further aid this interlocking by 
>> externally couple them together for higher frequencies (up to 
>> say 10-100 Hz) then you can sum their outputs and get a 
>> reduced noise response, a 3 dB improvement. The noise 
>> processes internal to the oscillators will be uncorrelated 
>> where as the locking causes the signals to be in phase.
> This is very true. I saw that HP did this in their cal lab with a
> half-dozen(?) hand-picked 10811 in order to get a combined
> virtual OCXO with better phase noise than any one physical
> OCXO. Rick, maybe you remember some details of this cool
> 10811 ensemble?

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