SE880 GPSDO
On Wed, Apr 27, 2016 at 7:21 PM, Bruce Griffiths
bruce.griffiths@xtra.co.nz wrote:
Stabilising the GPS receiver antenna temperature is probably a good idea particularly if it has bandpass filter(s).
It's not so clear that temperature stabilization of the antenna is
necessary. There have been reports of tempcos of 0.2 ps to 10 ps/C for
various choke ring antennas so this doesn't seem so bad. I don't know
how a $100 antenna fares in such a test. Temperature stabilization of
the receiver is probably more important. Again though, I don't know of
any data for low-cost receivers, only geodetic timing receivers.
Cheers
Michael
Assuming that you have an amateur radio license, you could use a
well located central station to transmit a CW signal in the 70cm or
23cm band. There should be some effort put into this station
to make it stable (eg by using a good rubidium as frequency source,
or even an ensemble) and low noise.
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN sequence
to get a better SNR and probably get another order of magnitude out of it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
It might be possible to clock the FPGA directly from a suitably massaged CW. Do any clock at 1GHz+???
It would be also possible to do away with LO’s in this case.
Now that all stations have the "same" frequency, one can use the GPS
module to get the time information using long integration times.
Under the assumption that the (sawtooth corrected) PPS is good to +/-10ns
an has a nice, time-invariant distribution, it should be possible to get
below 1ns in precision within 100s. Using common view phase data it
should be possible to get even better than that.
The CW could also carry time, and if it was feasible, local GPS would be unnecessary.
The telescope sites would receive the ticks at different times but the delta could be post processed out provided the positions are known to a few centimeters.
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On Wed, 27 Apr 2016 20:18:10 +0200
Mike Cook michael.cook@sfr.fr wrote:
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN sequence
to get a better SNR and probably get another order of magnitude out of it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
It might be possible to clock the FPGA directly from a suitably massaged CW.
Do any clock at 1GHz+???
It would be also possible to do away with LO’s in this case.
That would make the system more complicated than simpler, because
you need to extract a signal from a noisy environment, pass it through
narrow band filter so you get something that resembles a sinus in order
to use it in the electronics. This kind of works when the reference
signal comes in via cable. With over the air transmission, this wont work.
Using an OCXO together with a PLL basically forms a very narrow band
filter that has a very small tempco, adjustable (and adaptive)
frequency and allows to change the filter coefficiencts quickly
to acquire the signal at start-up.
Very few chips (of any kind, not just FPGA) allow input clocks higher
than a couple 100MHz. Single ended CMOS inputs usually go only up
to 200MHz, often much lower than that. Differential (LVDS and PECL)
ends usually in the 500MHz range.
Also. Running an FPGA at 1GHz is not trivial at all. Most designs
don't do more than 500MHz even on the fastest FPGAs out there.
100-300MHz are common values. And unlike with CPUs, there is often
no need to run an FPGA faster than the data arrives or leaves, as
the functions can be run in parallel and use a pipelined architecture.
The CW could also carry time, and if it was feasible, local GPS would be
unnecessary.
If the CW would carry time, it wouldn't be CW anymore ;-)
Yes, that's the idea with the PRN modulation i mentioned in the other mail.
But it wont lift the necesity of GPS. There is an unknown delay from the
sender to the receiver, through multiple filter and frequency conversion
stages. Some of them can be measured, some of them come from the ambiguity
of the phase in the system. Others, like the path delay, cannot be measured.
One solution would be, to use 2 transmitters with known positions and
known phase relations, then it would be possible to extract time,
given one knows the positions of the receivers exactly.
To get around that requirement, one would need at least 4 transmitters...
Ie. one would be recreating the GPS system... at which point it becomes
simpler to just use GPS and live with the degraded accuracy.
Also keep in mind, that with GPS it is well known where the errors
come from and how big they are. Also lots of techniques are implemented
to counter those. With a DIY-GPS system, one would need to implement
those and measure their performance again, which would be a whole lot of work.
Attila Kinali
--
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
On Wednesday, April 27, 2016 09:40:05 PM Attila Kinali wrote:
On Wed, 27 Apr 2016 20:18:10 +0200
Mike Cook michael.cook@sfr.fr wrote:
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN
sequence
to get a better SNR and probably get another order of magnitude out of
it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
It might be possible to clock the FPGA directly from a suitably massaged
CW. Do any clock at 1GHz+???
It would be also possible to do away with LO’s in this case..
That would make the system more complicated than simpler, because
you need to extract a signal from a noisy environment, pass it through
narrow band filter so you get something that resembles a sinus in order
to use it in the electronics. This kind of works when the reference
signal comes in via cable. With over the air transmission, this wont work.
Using an OCXO together with a PLL basically forms a very narrow band
filter that has a very small tempco, adjustable (and adaptive)
frequency and allows to change the filter coefficiencts quickly
to acquire the signal at start-up.
Very few chips (of any kind, not just FPGA) allow input clocks higher
than a couple 100MHz. Single ended CMOS inputs usually go only up
to 200MHz, often much lower than that. Differential (LVDS and PECL)
ends usually in the 500MHz range.
Also. Running an FPGA at 1GHz is not trivial at all. Most designs
don't do more than 500MHz even on the fastest FPGAs out there.
100-300MHz are common values. And unlike with CPUs, there is often
no need to run an FPGA faster than the data arrives or leaves, as
the functions can be run in parallel and use a pipelined architecture..
The CW could also carry time, and if it was feasible, local GPS would be
unnecessary.
If the CW would carry time, it wouldn't be CW anymore ;-)
Yes, that's the idea with the PRN modulation i mentioned in the other mail.
But it wont lift the necesity of GPS. There is an unknown delay from the
sender to the receiver, through multiple filter and frequency conversion
stages. Some of them can be measured, some of them come from the ambiguity
of the phase in the system. Others, like the path delay, cannot be measured.
One solution would be, to use 2 transmitters with known positions and
known phase relations, then it would be possible to extract time,
given one knows the positions of the receivers exactly.
To get around that requirement, one would need at least 4 transmitters...
Ie. one would be recreating the GPS system... at which point it becomes
simpler to just use GPS and live with the degraded accuracy.
Also keep in mind, that with GPS it is well known where the errors
come from and how big they are. Also lots of techniques are implemented
to counter those. With a DIY-GPS system, one would need to implement
those and measure their performance again, which would be a whole lot of
work.
Attila Kinali
The solution to this conundrum is to use a high speed serial to parallel
converter and proces 4/8/16 timeslots in parallel at 1/4, 1/8 or 1/16 the
serial clock rate as required.
If the high speed serial interface offered by many modern FPGAs could be used
for this then 100ps or finer timestamp quantisation may be feasible.
Bruce
Bruce
Please note that not all the frequencies will be utilizable, here only
433MHz is free-for-all and at low power: only under 50mA transmitting power.
There are some very cheap FSK transmitters that can output at a maximum
rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO
like the 10KHz output of some GPS receivers? The clock being compared to
this would be 10MHz downscaled by some decade counters.
this would be much simpler to implement.
Ilia.
Il 27/04/2016 23:38, Bruce Griffiths ha scritto:
On Wednesday, April 27, 2016 09:40:05 PM Attila Kinali wrote:
On Wed, 27 Apr 2016 20:18:10 +0200
Mike Cook michael.cook@sfr.fr wrote:
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN
sequence
to get a better SNR and probably get another order of magnitude out of
it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
It might be possible to clock the FPGA directly from a suitably massaged
CW. Do any clock at 1GHz+???
It would be also possible to do away with LO’s in this case..
That would make the system more complicated than simpler, because
you need to extract a signal from a noisy environment, pass it through
narrow band filter so you get something that resembles a sinus in order
to use it in the electronics. This kind of works when the reference
signal comes in via cable. With over the air transmission, this wont work.
Using an OCXO together with a PLL basically forms a very narrow band
filter that has a very small tempco, adjustable (and adaptive)
frequency and allows to change the filter coefficiencts quickly
to acquire the signal at start-up.
Very few chips (of any kind, not just FPGA) allow input clocks higher
than a couple 100MHz. Single ended CMOS inputs usually go only up
to 200MHz, often much lower than that. Differential (LVDS and PECL)
ends usually in the 500MHz range.
Also. Running an FPGA at 1GHz is not trivial at all. Most designs
don't do more than 500MHz even on the fastest FPGAs out there.
100-300MHz are common values. And unlike with CPUs, there is often
no need to run an FPGA faster than the data arrives or leaves, as
the functions can be run in parallel and use a pipelined architecture..
The CW could also carry time, and if it was feasible, local GPS would be
unnecessary.
If the CW would carry time, it wouldn't be CW anymore ;-)
Yes, that's the idea with the PRN modulation i mentioned in the other mail.
But it wont lift the necesity of GPS. There is an unknown delay from the
sender to the receiver, through multiple filter and frequency conversion
stages. Some of them can be measured, some of them come from the ambiguity
of the phase in the system. Others, like the path delay, cannot be measured.
One solution would be, to use 2 transmitters with known positions and
known phase relations, then it would be possible to extract time,
given one knows the positions of the receivers exactly.
To get around that requirement, one would need at least 4 transmitters...
Ie. one would be recreating the GPS system... at which point it becomes
simpler to just use GPS and live with the degraded accuracy.
Also keep in mind, that with GPS it is well known where the errors
come from and how big they are. Also lots of techniques are implemented
to counter those. With a DIY-GPS system, one would need to implement
those and measure their performance again, which would be a whole lot of
work.
Attila Kinali
The solution to this conundrum is to use a high speed serial to parallel
converter and proces 4/8/16 timeslots in parallel at 1/4, 1/8 or 1/16 the
serial clock rate as required.
If the high speed serial interface offered by many modern FPGAs could be used
for this then 100ps or finer timestamp quantisation may be feasible.
Bruce
Bruce
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.
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
Unlikely to be useful.In particular using an off the shelf FSK transmitter is far from optimum.The other issue is the low power and the possibility of interference from others on the same frequency.It would probably be more effective to use GPS and accept a lower time resolution.
Bruce
On Thursday, 28 April 2016 9:00 PM, Ilia Platone <info@iliaplatone.com> wrote:
Please note that not all the frequencies will be utilizable, here only
433MHz is free-for-all and at low power: only under 50mA transmitting power..
There are some very cheap FSK transmitters that can output at a maximum
rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO
like the 10KHz output of some GPS receivers? The clock being compared to
this would be 10MHz downscaled by some decade counters.
this would be much simpler to implement.
Ilia.
Il 27/04/2016 23:38, Bruce Griffiths ha scritto:
On Wednesday, April 27, 2016 09:40:05 PM Attila Kinali wrote:
On Wed, 27 Apr 2016 20:18:10 +0200
Mike Cook michael.cook@sfr.fr wrote:
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN
sequence
to get a better SNR and probably get another order of magnitude out of
it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
It might be possible to clock the FPGA directly from a suitably massaged
CW. Do any clock at 1GHz+???
It would be also possible to do away with LO’s in this case..
That would make the system more complicated than simpler, because
you need to extract a signal from a noisy environment, pass it through
narrow band filter so you get something that resembles a sinus in order
to use it in the electronics. This kind of works when the reference
signal comes in via cable. With over the air transmission, this wont work.
Using an OCXO together with a PLL basically forms a very narrow band
filter that has a very small tempco, adjustable (and adaptive)
frequency and allows to change the filter coefficiencts quickly
to acquire the signal at start-up.
Very few chips (of any kind, not just FPGA) allow input clocks higher
than a couple 100MHz. Single ended CMOS inputs usually go only up
to 200MHz, often much lower than that. Differential (LVDS and PECL)
ends usually in the 500MHz range.
Also. Running an FPGA at 1GHz is not trivial at all. Most designs
don't do more than 500MHz even on the fastest FPGAs out there.
100-300MHz are common values. And unlike with CPUs, there is often
no need to run an FPGA faster than the data arrives or leaves, as
the functions can be run in parallel and use a pipelined architecture..
The CW could also carry time, and if it was feasible, local GPS would be
unnecessary.
If the CW would carry time, it wouldn't be CW anymore ;-)
Yes, that's the idea with the PRN modulation i mentioned in the other mail.
But it wont lift the necesity of GPS. There is an unknown delay from the
sender to the receiver, through multiple filter and frequency conversion
stages. Some of them can be measured, some of them come from the ambiguity
of the phase in the system. Others, like the path delay, cannot be measured.
One solution would be, to use 2 transmitters with known positions and
known phase relations, then it would be possible to extract time,
given one knows the positions of the receivers exactly.
To get around that requirement, one would need at least 4 transmitters....
Ie. one would be recreating the GPS system... at which point it becomes
simpler to just use GPS and live with the degraded accuracy.
Also keep in mind, that with GPS it is well known where the errors
come from and how big they are. Also lots of techniques are implemented
to counter those. With a DIY-GPS system, one would need to implement
those and measure their performance again, which would be a whole lot of
work.
Attila Kinali
The solution to this conundrum is to use a high speed serial to parallel
converter and proces 4/8/16 timeslots in parallel at 1/4, 1/8 or 1/16 the
serial clock rate as required.
If the high speed serial interface offered by many modern FPGAs could be used
for this then 100ps or finer timestamp quantisation may be feasible.
Bruce
Bruce
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.
--
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 10:07:56 +0200
Ilia Platone info@iliaplatone.com wrote:
Please note that not all the frequencies will be utilizable, here only
433MHz is free-for-all and at low power: only under 50mA transmitting power.
Does this mean you don't have an amateur radio license?
Then it would be a good time to get one :-)
BTW: you also have the 868MHz SRD and the 2.4GHz range available,
with similar power constraints though.
There are some very cheap FSK transmitters that can output at a maximum
rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO
like the 10KHz output of some GPS receivers? The clock being compared to
this would be 10MHz downscaled by some decade counters.
this would be much simpler to implement.
I don't get exactly what you mean, but these FSK transmitters for sub-GHz
radios will not work. Their only purpose is low power radio transmissions
over a couple of 10m. Yes, you can use them to send data. Yes, you can
lock to that. But you will not get the control over phase/frequency you
need to implement a good frequency transfer or time transfer system.
Building your own 70cm is not difficult. There are plenty of amateur
radio books from the 70s and 80s around that explain how to build
one with minimal effort. The QRP community has also quite a few designs
that are very simple to build (and a bit more modern).
All you actually need for a CW transmitter is some oscillator, ie a VCO,
that you can either build yourself (L-C tank with a varactor)
or buy as a chip. Use some simple PLL to lock it to your reference
(ADF4002 or similar are a decent choice). Add a simple amplifier
to get the signal to a decent level and a piece of wire as antenna.
The receiver is a bit more involved. There you need to downmix
the received signal to a frequency that is not higher than your
OCXO's frequency, feed that to an amplifier and from there to
a PLL that steers the OCXO. The downmixing LO needs to be derived
from the OCXO as well.
The PLL of the receiver can be an analog design as with the transmitter
or a digital design where you digitze the signal and process it in
an FPGA that produces an output for a DAC that then steers the OCXO.
But all this depends on quite a bit of knowledge on how to design
analog circuits. If you have never done that, it would be a good
idea to find a companion where you live that helps you with the
project.
Attila Kinali
--
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
Il 28/04/2016 12:28, Attila Kinali ha scritto:
On Thu, 28 Apr 2016 10:07:56 +0200
Ilia Platone info@iliaplatone.com wrote:
Please note that not all the frequencies will be utilizable, here only
433MHz is free-for-all and at low power: only under 50mA transmitting power.
Does this mean you don't have an amateur radio license?
Then it would be a good time to get one :-)
:) yes, maybe...
BTW: you also have the 868MHz SRD and the 2.4GHz range available,
with similar power constraints though.
There are some very cheap FSK transmitters that can output at a maximum
rate of 9600bps: a 1KHz quad signal on these carriers, can drive a GPSDO
like the 10KHz output of some GPS receivers? The clock being compared to
this would be 10MHz downscaled by some decade counters.
this would be much simpler to implement.
I don't get exactly what you mean, but these FSK transmitters for sub-GHz
radios will not work. Their only purpose is low power radio transmissions
over a couple of 10m. Yes, you can use them to send data. Yes, you can
lock to that. But you will not get the control over phase/frequency you
need to implement a good frequency transfer or time transfer system.
Building your own 70cm is not difficult. There are plenty of amateur
radio books from the 70s and 80s around that explain how to build
one with minimal effort. The QRP community has also quite a few designs
that are very simple to build (and a bit more modern).
All you actually need for a CW transmitter is some oscillator, ie a VCO,
that you can either build yourself (L-C tank with a varactor)
or buy as a chip. Use some simple PLL to lock it to your reference
(ADF4002 or similar are a decent choice). Add a simple amplifier
to get the signal to a decent level and a piece of wire as antenna.
The receiver is a bit more involved. There you need to downmix
the received signal to a frequency that is not higher than your
OCXO's frequency, feed that to an amplifier and from there to
a PLL that steers the OCXO. The downmixing LO needs to be derived
from the OCXO as well.
The PLL of the receiver can be an analog design as with the transmitter
or a digital design where you digitze the signal and process it in
an FPGA that produces an output for a DAC that then steers the OCXO.
But all this depends on quite a bit of knowledge on how to design
analog circuits. If you have never done that, it would be a good
idea to find a companion where you live that helps you with the
project.
Attila Kinali
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.
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?
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
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
--
Reading can seriously damage your ignorance.
-- unknown
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
--
Reading can seriously damage your ignorance.
-- unknown
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