Something like good_100MHz_OCXO+DDS => same as a BVA?
On Tue, Dec 8, 2015 at 5:32 PM, Attila Kinali <[email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
