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 <[email protected]> wrote: > 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.
