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

[Bob Camp]

Hi

Let’s see:

EFC uses reference out of the OCXO. 
EFC comes on the OCXO at no added cost.
16 bit DAC costs ~$2 to $5

Total cost for EFC setup $2 to $5. Net result is a system with 
spurs that are how ever far down you wish them to be. (It’s all
about grounding in this case). 

Bob

> On Dec 8, 2015, at 11:32 AM, 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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