Azelio,
You don't get BVA performance easily out of an oscillator being
significantly less stable than a BVA. Some environmental aspects you can
dampen, some you can compensate, but then as you hit the fundamental
noise processes of the oscillator. Knowing how systematics affects the
oscillator without comparing to a much stabler source, does help you
only to some degree when you loose that source and need to free-wheel
without the correction input. If you have a more stable source, use that
directly. There are oscillators which may put BVAs to the test for noise
and systematics.
Rather than trying to do that, I think the alternative approach should
be viewed as an interesting concept where you overcome several of the
problems with the DAC and EFC approach, such as the DAC resolution (yes,
I've fought that one), the DAC reference stability (as you open the
loop, it becomes an issue), the EFC steering curve (it's not very
linear) and range, the EFC temperature sensitivity (which becomes
uncompensated as you go open-loop, but also shows up as subtle
state-changes in closed-loop).
Another benefit of not steering the oscillator directly is when you
build an ensamble of them, then you can more precisely predict their
futute behavior when you don't steer them, as you reduce those unknown
non-linear errors.
So, in short, I think you would fool yourself in believing you can
significantly alter the noise process performance of your oscillator,
see it in an improvement in how you can linearly compensate the offset
of your oscillator, and with that possibly focus more on trimming some
of the systematic corrections better. The noise of the core oscillator
will keep dominate, it's just the reducing of the syntesis chain we keep
down.
Cheers,
Magnus
On 12/08/2015 07:36 PM, Azelio Boriani wrote:
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
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