[time-nuts] SE880 GPSDO

Ilia Platone info at iliaplatone.com
Thu Apr 28 18:35:58 EDT 2016


Il 28/04/2016 23:22, Attila Kinali ha scritto:
> On Thu, 28 Apr 2016 21:01:49 +0200
> Ilia Platone <info at iliaplatone.com> 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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