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

[Magnus Danielson]

God kväll Attila,

On 12/08/2015 05:32 PM, Attila Kinali 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?

The main reason I would say is habbits, people have habbits and stick to 
them. Many follow the design patterns of others, often as found in 
books, as tought in university, as inherited within a company, as found 
on Internet, as design by fellow hams, whatever.

If you would setup essentially a micro-stepper design, such as those 
being used for cesium and hydrogen masers, but maybe adapted to a 
hobbyist needs and with straight-forward way of building and tune-up, 
then we could alter the design pattern. The phase-noise and long term 
stability issues is clear.

Doing control loop using a phase-stepper is a little bit different, and 
has a few minor design-challenges, but once mastered is essentially the 
same. EFC or C-field control then becomes more an initial setup.

An alternative approach divider wise is to use re-generative dividers.
For Rick's approach there would be a number of these at the same 
frequency (nominally), so the same design-pattern would apply. However, 
that would only be meaningful if you need really need to keep the noise 
down.

Cheers,
Magnus