[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?

[Azelio Boriani]

Given that until now good (maximum stability) OCXO are much less than
100MHz, from the OCXO we exploit its high stability and we impose
accuracy from a coordinated source: the OCXO+EFC method uses the
built-in stability and disciplines the accuracy.
The DDS method virtually can start from any oscillator, apply a
suitable correction function giving the same result, transferring the
hardware characteristic of a BVA (for example) into the driving
function.
Can a DDS be driven with the speed necessary to correct the output so
that it results in the same stability as a BVA, starting from a given
unstable oscillator?
Or, how much unstable can be the 100MHz starting oscillator so that I
can obtain after the DDS+suitable_driving_function  the same final
stability as an ordinary 10MHz OCXO?

On Tue, Dec 8, 2015 at 6:44 PM, Azelio Boriani <azelio.boriani at gmail.com> wrote:
> Something like good_100MHz_OCXO+DDS => same as a BVA?
>
> On Tue, Dec 8, 2015 at 5:32 PM, Attila Kinali <attila at kinali.ch> wrote:
>> Moin,
>>
>> I've been digging through some stuff and stumbled (again) over Rick's
>> paper on high resolution, low noise DDS generation[1] and got confused.
>> The scheme is very simple and looks like to be quite easy and reliably
>> to implement. If I understood it correctly, the critical points are the
>> DDS, its sideband generation and the LO/RF feedthrough in the mixers.
>> Nothing that is not known and nothing that is too difficult to handle
>> (the 10.7MHz filter get rid of most of the feedthrough already and
>> there has been a lot written on how to design DDS for specific applications).
>>
>> What puzzled me is, why this has not been used more often to correct
>> the frequency of OCXOs instead of using some DAC-to-EFC scheme?
>>
>> Given that Archita Hati et al. were getting very low noise numbers on
>> their RF signal generation scheme using dividers [2], I don't think that
>> the noise of the mixers would be the limiting factor here, but rather
>> that the phase noise should be still dominated by the 10MHz oscillator.
>>
>> My guestimate is that something like this would cost approximately 5USD
>> per divider stage, plus 20 USD for the DDS plus initial mixer. The only
>> problem would be to get a narrow band 10.0MHz filter (I couldn't find
>> one within 5 minutes of googling). 5 stages should cost around 50-70USD)
>> and will give a resolution better than 5uHz (100MHz DDS with 24bit)
>> down to 20pHz range (100MHz DDS with 32bit), which are 1:5e-13
>> and 1:2e-15 respectively.
>>
>> Compared to an EFC system that costs somewhere in the range of 10-50USD
>> and gives a resolution of something between 1:5e-12 (0.3ppm tuning range,
>> 16bit DAC) and 1:1e-13 (10^-7 tuning range and 20bit DAC). Especially the
>> 20bit DAC version gives a lot of electrical problems, starting from the
>> stability of the reference, leakage current trough various components and the PCB etc pp, while the DDS scheme, as a "digital" scheme is virtually free
>> of those.
>>
>> So, the DDS scheme is easier to reproduce, more stable over time and
>> costs only slightly more (unless you try to use an LTZ1000 as reference,
>> then the reference alone costs more then the whole DDS scheme).
>>
>> So, what did I miss? Why do people use DAC-EFC control instead of
>> the DDS scheme?
>>
>>                         Attila Kinali
>>
>> [1] "A narrow band high-resolution synthesizer using a direct digital
>> synthesiser followed by repeated dividing and mixing", Richard Karlquist, 1995
>> http://www.karlquist.com/FCS95.pdf
>>
>> [2] "State-of-the-Art RF Signal Generation From Optical Frequency Division".
>> by Hati, Nelson, Barnes, Lirette, Fortier, Quinlan, DeSalvo, Ludlow, Diddams,
>> Howe, 2013
>> http://tf.boulder.nist.gov/general/pdf/2646.pdf
>>
>> --
>> It is upon moral qualities that a society is ultimately founded. All
>> the prosperity and technological sophistication in the world is of no
>> use without that foundation.
>>                  -- Miss Matheson, The Diamond Age, Neil Stephenson
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