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

[Attila Kinali]

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

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