SE880 GPSDO
Il 28/04/2016 23:22, Attila Kinali ha scritto:
On Thu, 28 Apr 2016 21:01:49 +0200
Ilia Platone info@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
Rimini 999KHz RAI broadcast AM
Taranto 1008 era kerkyra corfu (greece) very strong even during daytime.
I'm informing, on mid-waves there's a nice signal even for long
distances usually.
Ilia.
Il 29/04/2016 00:21, Bruce Griffiths ha scritto:
Multipath effects due to local terrain (mountains etc) may be a significant issue.Relying on AM broadcasts is fraught with issues. Whilst you may find one in Italy, what about Greece (another potential site)?
Bruce
On Friday, 29 April 2016 9:22 AM, Attila Kinali <attila@kinali.ch> wrote:
On Thu, 28 Apr 2016 21:01:49 +0200
Ilia Platone info@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.
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.
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.
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.
Attila Kinali
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
On Fri, 29 Apr 2016 00:35:58 +0200
Ilia Platone info@iliaplatone.com wrote:
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.
Unless you want to do online data analysis, this is not really a problem.
The idea is to record all kind of information of the reference oscillator
and the GPS system. Under the assumption that the reference oscillator
frequency is stable enough, you can measure the frequency using GPS
very precisely. You can even factor in some frequency drift (which you
will need unless you let the Rb's run for a couple of hours/days before use).
Or to put it more generally, the more data you have, the more you can
compensate for non-idealities during post-processing.
Yes, you want to achieve absolute timing accuracy, but for that you will
need to start with some stable frequency source, either physical or
virtual, in order to be able to determine time accurately.
At the moment I am just throwing ideas on how to build the system at you.
There are many ways how to build such a system. I somewhat filter on what
I think is a reasonable approach with moderate cost/effort required.
At some point you will need to decide on one or two approaches and calculate
what kind of performance you need from which part to achieve your goal.
Then you will see whether the system can actually work and how much effort
you need to put into the detailed design to get there.
--
Reading can seriously damage your ignorance.
-- unknown
Here in Italy the DVB-T practice is to use GPSDOs to lock the
transmitters' carriers. This is needed to exploit the Single Frequency
Network (SFN) system. The frequency and time stability are stated in
the ETSI TR 101 190, 8.3.1 and 8.3.2 on page 41, in short they say 1us
is enough for the time and about the frequency it is 1/1000th of the
DVB-T carriers' spacing that is, at worst, 1116Hz so 1.1Hz of
stability.
http://www.etsi.org/deliver/etsi_tr/101100_101199/101190/01.03.02_60/tr_101190v010302p.pdf
On Fri, Apr 29, 2016 at 12:35 AM, Ilia Platone info@iliaplatone.com wrote:
Il 28/04/2016 23:22, Attila Kinali ha scritto:
On Thu, 28 Apr 2016 21:01:49 +0200
Ilia Platone info@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On Thu, 28 Apr 2016 23:22:24 +0200
Attila Kinali attila@kinali.ch 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.
Here another crazy idea:
If you can ensure line of sight between stations, you can use a
laser link between them. Modulate the laser with an RF signal in the
10-100MHz range. Use this on the receiver side to lock the OCXO.
Proceed as before...
This approach should be fairly simple to build, but needs some care
to ensure that you are not endangering anyone with the laser beam.
Other than that, you don't need a license for running such a system.
There have been hobbyist laser communication links around for a couple
of years, though i would advise to check the designs carfully as some
of them have EMI and other problems that the original designers and users
didn't care about.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
If you add a small beam expander, then there should be few safety issues.In this case a laser beam power of a few (1??) mW may suffice.Similar collection optics at the receiver will also be required. One can use small telescopes for this purpose. I've used an eyepiece with a 12" (305mm) dobsonian to produce a 300mm diameter beam from a green laser pointer. You shouldn't need to go quite that far though.
Bruce
On Friday, 29 April 2016 12:01 PM, Attila Kinali <attila@kinali.ch> wrote:
On Thu, 28 Apr 2016 23:22:24 +0200
Attila Kinali attila@kinali.ch 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.
Here another crazy idea:
If you can ensure line of sight between stations, you can use a
laser link between them. Modulate the laser with an RF signal in the
10-100MHz range. Use this on the receiver side to lock the OCXO.
Proceed as before...
This approach should be fairly simple to build, but needs some care
to ensure that you are not endangering anyone with the laser beam.
Other than that, you don't need a license for running such a system.
There have been hobbyist laser communication links around for a couple
of years, though i would advise to check the designs carfully as some
of them have EMI and other problems that the original designers and users
didn't care about.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
One advantage of a free space optical link for this application, is that most of the factors that produce severe attenuation of the optical signal also preclude observation of the stellar sources as well. Thus the link only needs to work well under near ideal conditions.Increasing the aperture of the transmit and receive optics reduces the required transmitter power and the associated safety hazards of the transmitted optical beam.
Bruce
On Friday, 29 April 2016 1:02 PM, Bruce Griffiths <bruce.griffiths@xtra..co.nz> wrote:
If you add a small beam expander, then there should be few safety issues.In this case a laser beam power of a few (1??) mW may suffice.Similar collection optics at the receiver will also be required. One can use small telescopes for this purpose. I've used an eyepiece with a 12" (305mm) dobsonian to produce a 300mm diameter beam from a green laser pointer. You shouldn't need to go quite that far though.
Bruce
On Friday, 29 April 2016 12:01 PM, Attila Kinali attila@kinali.ch wrote:
On Thu, 28 Apr 2016 23:22:24 +0200
Attila Kinali attila@kinali.ch 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.
Here another crazy idea:
If you can ensure line of sight between stations, you can use a
laser link between them. Modulate the laser with an RF signal in the
10-100MHz range. Use this on the receiver side to lock the OCXO.
Proceed as before...
This approach should be fairly simple to build, but needs some care
to ensure that you are not endangering anyone with the laser beam.
Other than that, you don't need a license for running such a system.
There have been hobbyist laser communication links around for a couple
of years, though i would advise to check the designs carfully as some
of them have EMI and other problems that the original designers and users
didn't care about.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
This thread is now closed. For questions beyond SE880, please start a new thread.
/tvb
According to this,
http://www.nist.gov/manuscript-publication-search.cfm?pub_id=912449
there are many practical challenges with a one way free-space optical link.
They were, however, aiming for much higher stability than is needed here.
There are a lot of ideas being tossed around here. It would probably
help to focus the discussion a bit if we knew:
(1) The budget (presumably small)
(2) Site constraints - is there line of sight, is it really impossible
to run a fibre (I know trenching costs are high but a fibre strung
along a fence with some UV protection might be acceptable in this
application)
(3) What expertise there is that can be drawn upon
Cheers
Michael
On Fri, Apr 29, 2016 at 11:11 AM, Bruce Griffiths
bruce.griffiths@xtra.co.nz wrote:
One advantage of a free space optical link for this application, is that most of the factors that produce severe attenuation of the optical signal also preclude observation of the stellar sources as well. Thus the link only needs to work well under near ideal conditions.Increasing the aperture of the transmit and receive optics reduces the required transmitter power and the associated safety hazards of the transmitted optical beam.
Bruce
On Friday, 29 April 2016 1:02 PM, Bruce Griffiths <bruce.griffiths@xtra..co.nz> wrote:
If you add a small beam expander, then there should be few safety issues.In this case a laser beam power of a few (1??) mW may suffice.Similar collection optics at the receiver will also be required. One can use small telescopes for this purpose. I've used an eyepiece with a 12" (305mm) dobsonian to produce a 300mm diameter beam from a green laser pointer. You shouldn't need to go quite that far though.
Bruce
On Friday, 29 April 2016 12:01 PM, Attila Kinali <attila@kinali.ch> wrote:
On Thu, 28 Apr 2016 23:22:24 +0200
Attila Kinali attila@kinali.ch 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.
Here another crazy idea:
If you can ensure line of sight between stations, you can use a
laser link between them. Modulate the laser with an RF signal in the
10-100MHz range. Use this on the receiver side to lock the OCXO.
Proceed as before...
This approach should be fairly simple to build, but needs some care
to ensure that you are not endangering anyone with the laser beam.
Other than that, you don't need a license for running such a system.
There have been hobbyist laser communication links around for a couple
of years, though i would advise to check the designs carfully as some
of them have EMI and other problems that the original designers and users
didn't care about.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.