Il 28/04/2016 23:22, Attila Kinali ha scritto:
On Thu, 28 Apr 2016 21:01:49 +0200
Ilia Platone <[email protected]> wrote:
Thanks Attila, I know how to build a transmitter and a receiver, and now
is more clear the system you designed. But as I will propose this system
to an astro club, and in this astro club there's the possibility that
not all would have a radio license, I need something "free-to-play", if
it concern.
Ok.. that's quite some constraint. This rules out any kind of transmission.
I was wondering if it would be more convenient to lock to a signal from
an AM broadcasting station, if available to a multiple of the OCXO. What
do you think about?
AFAIK most radio and TV transmitters are using some stable reference.
I don't know though what they use these days. It used to be an Rb.
I would guess that using a radio station should in general work.
It should be as close as possible, so that you get as little
reflection as possible and that any multi-path from the troposphere
and ionosphere is minimized. If you still have any AM stations close
by, that would work. But these are more and more switched off and
replaced by digital broadcast systems.
I know that some AM station still exists, a place where to setup the
telescopes will have the local repetitors very close.
I think that airports use AM modulation, but I sincerely don't know if
it's legal even to listen at those frequencies. Their signal should be
strong, however.
The most common radio and TV transmitters these days are DAB and DVB-T.
Both use QPSK or QAM signals. This makes locking to those signals
quite a bit more difficult. What you can do is, use a DAB/DVB-T
tuner chip like the MAX3580 or MAX3541, down convert the signals,
then use the FPGA to track the signals and steer the OCXO's EFC DAC.
Yes, this is a lot more complicated and you need to build quite a bit
of a DVB-T/DAB receiver in the FPGA. Fortunately, this is something
people have already implemented in software using GNURadio. Ie you
can have a look at what they have done, copy the over the parts that
you need. But still, this will be quite some serious effort and will
take you months at best.
Will inform on AM stations.
I also have no idea what the signal stability of the DVB-T and DAB
stations is. Maybe someone else (Magnus?) can comment on that.
I would appreciate his contibution :)
As such... I think using an AM station that is close by would be feasible.
Using DVB-T/DAB stations would be a lot of effort and I would advise
against it in a first step. GPS alone should give you ~1ns when done right.
With more expensive equipment (high qual geodetic or timing antennas with
L1/L2 receivers) you should be able to go below that (see Michael Wouters'
mail).
An alternative approach would be to use an Rb reference instead of an
OCXO at the telescopes. This way you have a frequency stable reference
that you can use like the reference signal I mentioned in the other mail.
You would need one that has low phase noise (that rules out the FE-5680's
that are so cheap on ebay, ie you would need to go for LPRO, PRS10, FRS
or LPFRS). As now you only have a kind of stable reference, but you don't
know how far off it is (and probably not how fast its drifting), some
precision will need to be spend on determining its exact frequency.
But nonetheless it should give you additional precision when doing
the post-processing that you can use to increase the timing solution's
precision.
The problem is not the absolute stability, but the relative error
between the various stations. ie the telescopes clocks must not drift
too much by each other.
Attila Kinali
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
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