GPSDO replacement recommendation
The trick to getting good adev based on a Mosaic-T GPSDO is to simply
bypass the whole sawtooth problem. Dont compare the PPS from the Mosaic-T
with a PPS derived from your steered oscillator, rather run the Mosaic-T
with the steered 10MHz directly, and use the relevant messages from the
Mosaic-T to run the control loop. No external phase-detector necessary, no
PPS sawtooth to struggle with. It is described in the hardware manual for
the mosaic, appendix D.
ADEV of 2e-13@1s on the steered clock (OCXO) has been demonstrated this
way, although with Atomichron real time PPP - but the architecture should
work either way. YMMV.
BR
Ole
On Fri, Apr 19, 2024 at 11:25 AM Zdenek Chaloupka via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi,
it is very difficult to tame Mosaic-T's sawtooth (clock quantisation
error) as the correction data for the sawtooth are reported from Mosaic-T
only after the pulse is out (as oppose to, for example u-blox’s
ZED-F9T/NEO-F10T, or Furuno’s GT-100). Also note that simply cleaning up
sawtooth with a PLL is also not that easy, as the sawtooth is not
uniformly, or normally distributed at times, that means it introduces a
significant bias which needs to be cleaned up by PLL. Therefore you would
need a PLL with 5mHz bandwidth or lower to clean up the sawtooth (and bias)
properly. OCXO’s won’t do, but then a good Rb may be enough to keep PLL
stable at those low bandwidths.
In general I would go with a good low cost receiver (my recommendation is
NEO-F10T, which you can buy cheap here:
https://gnss.store/54-neo-f10t-timing-gnss-modules) and a reasonably good
PLL to clean up the sawtooth. The sawtooth is reported from ubx receivers
before the pulse, so you can correct it directly in the PLL, if it supports
it (supported by Si5383 or AD9545, for example). For some of those PLLs a
reasonably cheap (300$) evaluation boards exist, so you don’t have to
design your own PCB.
Cheers
Zdenek
On 18 Apr 2024, at 15:04, Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
Yup, various folks have indeed done that sort of thing. Some have done
it on multiple Rb’s.
The “filter” in these Rb’s (many of them have that option) is not ideal
for a GPSDO. It does not handle the “corner cases” very well. Unfortunately
90% of the “smarts” in a good GPSDO involves those corner cases. Yes,
that makes the term corner case a bit suspect …. still, it’s the term that
gets commonly used. The “why?” is simply that the feature is primarily
intended for “quick calibration" against a Cs or something similar.
The PRS10 has a bit of a bump in its ADEV curve around 10 seconds. It is
a result of their fancy mag field management approach. That also (at least
to me) makes it a less than perfect choice for this sort of thing. A 10
second gate time on a frequency counter is a pretty common thing …..
On the plus side, you can hook up something like a Mosaic-T and have a
pretty fancy drive into the device. To get the best performance, you would
need to come up with a way to apply the sawtooth correction data to the pps
before it goes over to the Rb. That certainly can be done, it just adds
another layer of excitement to the setup.
Fun !!
Bob
On Apr 17, 2024, at 6:34 PM, Jürgen Appel via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi,
P,erformance wise I imagine a SRs PRS10 rubidium clock connected with
its 1pps input to the 1pps output of a ublox zf9t will be hard to beat for
approx 2. 2kEur with respect to many requirement profiles.
Has anyone here tried that out?
Cheers,
Jürgen
On 14 April 2024 15:53:33 CEST, Bob Camp via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
Cost wise, the surplus UCCM Trimble’s (or the Symmetricom version)
still seem be the one to go with. At the same price, I’d go for the Trimble
version. Both appear to work pretty well.
If cost is no object, there are lots of crazy choices. Each of them
has it's own unique advantages and (usually) unique issues. The SRS FS740
is one example. It does indeed have its quirks. Not exactly cheap, and
still far from the most expensive option.
https://www.thinksrs.com/products/fs740.html
At least two list members have one of them.
In this era of multi-band GNSS as well as anti-spoofing / anti-jam
there are a lot of ways to run up the price :).
How crazy do you want to get?
Bob
On Apr 14, 2024, at 8:24 AM, Attila Kinali via time-nuts <
time-nuts@lists.febo.com> wrote:
Moin,
My house GPSDO (a Trimble UCCM) died recently and I am now looking
for a
replacement. As I have been a bit out of touch with the current state
of affairs regarding GPSDOs, I'd like to ask what the current
recommendation
for getting a GPSDO is, these days.
Thanks in advance
Attila Kinali
--
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
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On Fri, Apr 19, 2024 at 9:21 AM Ole Petter Ronningen via time-nuts
time-nuts@lists.febo.com wrote:
The trick to getting good adev based on a Mosaic-T GPSDO is to simply
bypass the whole sawtooth problem. Dont compare the PPS from the Mosaic-T
with a PPS derived from your steered oscillator, rather run the Mosaic-T
with the steered 10MHz directly, and use the relevant messages from the
Mosaic-T to run the control loop. No external phase-detector necessary, no
PPS sawtooth to struggle with. It is described in the hardware manual for
the mosaic, appendix D.
ADEV of 2e-13@1s on the steered clock (OCXO) has been demonstrated this
way, although with Atomichron real time PPP - but the architecture should
work either way. YMMV.
2e-13@1s was intentional, right? Do you have any further details on
this? The PPS ADEV of GNSS receivers such as the ZED-F9T is almost 5
orders of magnitude worse than 2e-13@1s. So, normally, you'd need this
stability to come from the OCXO. As far as I know, it takes a really
good OCXO to get 2e-13@1s.
Matt
Hi
If you are running a control loop, reporting before vs after is simply a wrinkle in how you write the code. There are a number of folks who do their GNSS modules this way. If you are trying to do real time correction, you do indeed get a “delayed correct” pulse.
Bob
On Apr 19, 2024, at 3:01 AM, Zdenek Chaloupka z.c.nl@ieee.org wrote:
Hi,
it is very difficult to tame Mosaic-T's sawtooth (clock quantisation error) as the correction data for the sawtooth are reported from Mosaic-T only after the pulse is out (as oppose to, for example u-blox’s ZED-F9T/NEO-F10T, or Furuno’s GT-100). Also note that simply cleaning up sawtooth with a PLL is also not that easy, as the sawtooth is not uniformly, or normally distributed at times, that means it introduces a significant bias which needs to be cleaned up by PLL. Therefore you would need a PLL with 5mHz bandwidth or lower to clean up the sawtooth (and bias) properly. OCXO’s won’t do, but then a good Rb may be enough to keep PLL stable at those low bandwidths.
In general I would go with a good low cost receiver (my recommendation is NEO-F10T, which you can buy cheap here: https://gnss.store/54-neo-f10t-timing-gnss-modules) and a reasonably good PLL to clean up the sawtooth. The sawtooth is reported from ubx receivers before the pulse, so you can correct it directly in the PLL, if it supports it (supported by Si5383 or AD9545, for example). For some of those PLLs a reasonably cheap (300$) evaluation boards exist, so you don’t have to design your own PCB.
Cheers
Zdenek
On 18 Apr 2024, at 15:04, Bob Camp via time-nuts time-nuts@lists.febo.com wrote:
Hi
Yup, various folks have indeed done that sort of thing. Some have done it on multiple Rb’s.
The “filter” in these Rb’s (many of them have that option) is not ideal for a GPSDO. It does not handle the “corner cases” very well. Unfortunately > 90% of the “smarts” in a good GPSDO involves those corner cases. Yes, that makes the term corner case a bit suspect …. still, it’s the term that gets commonly used. The “why?” is simply that the feature is primarily intended for “quick calibration" against a Cs or something similar.
The PRS10 has a bit of a bump in its ADEV curve around 10 seconds. It is a result of their fancy mag field management approach. That also (at least to me) makes it a less than perfect choice for this sort of thing. A 10 second gate time on a frequency counter is a pretty common thing …..
On the plus side, you can hook up something like a Mosaic-T and have a pretty fancy drive into the device. To get the best performance, you would need to come up with a way to apply the sawtooth correction data to the pps before it goes over to the Rb. That certainly can be done, it just adds another layer of excitement to the setup.
Fun !!
Bob
On Apr 17, 2024, at 6:34 PM, Jürgen Appel via time-nuts time-nuts@lists.febo.com wrote:
Hi,
P,erformance wise I imagine a SRs PRS10 rubidium clock connected with its 1pps input to the 1pps output of a ublox zf9t will be hard to beat for approx 2. 2kEur with respect to many requirement profiles.
Has anyone here tried that out?
Cheers,
Jürgen
On 14 April 2024 15:53:33 CEST, Bob Camp via time-nuts time-nuts@lists.febo.com wrote:
Hi
Cost wise, the surplus UCCM Trimble’s (or the Symmetricom version) still seem be the one to go with. At the same price, I’d go for the Trimble version. Both appear to work pretty well.
If cost is no object, there are lots of crazy choices. Each of them has it's own unique advantages and (usually) unique issues. The SRS FS740 is one example. It does indeed have its quirks. Not exactly cheap, and still far from the most expensive option.
https://www.thinksrs.com/products/fs740.html
At least two list members have one of them.
In this era of multi-band GNSS as well as anti-spoofing / anti-jam there are a lot of ways to run up the price :).
How crazy do you want to get?
Bob
On Apr 14, 2024, at 8:24 AM, Attila Kinali via time-nuts time-nuts@lists.febo.com wrote:
Moin,
My house GPSDO (a Trimble UCCM) died recently and I am now looking for a
replacement. As I have been a bit out of touch with the current state
of affairs regarding GPSDOs, I'd like to ask what the current recommendation
for getting a GPSDO is, these days.
Thanks in advance
Attila Kinali
--
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
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Yes, the OCXO dominates on short tau, so to get 2e-13 you need a good one.
I used an Oscilloquartz 8601. But it is nicely scalable - if you have an
OCXO "only" capable of 1e-12, tune the steering loop so that is what you
get until the ADEV "turns downwards".
BR
Ole
On Sat, Apr 20, 2024 at 2:24 AM Matt Huszagh via time-nuts <
time-nuts@lists.febo.com> wrote:
On Fri, Apr 19, 2024 at 9:21 AM Ole Petter Ronningen via time-nuts
time-nuts@lists.febo.com wrote:
The trick to getting good adev based on a Mosaic-T GPSDO is to simply
bypass the whole sawtooth problem. Dont compare the PPS from the Mosaic-T
with a PPS derived from your steered oscillator, rather run the Mosaic-T
with the steered 10MHz directly, and use the relevant messages from the
Mosaic-T to run the control loop. No external phase-detector necessary,
no
PPS sawtooth to struggle with. It is described in the hardware manual for
the mosaic, appendix D.
ADEV of 2e-13@1s on the steered clock (OCXO) has been demonstrated this
way, although with Atomichron real time PPP - but the architecture should
work either way. YMMV.
2e-13@1s was intentional, right? Do you have any further details on
this? The PPS ADEV of GNSS receivers such as the ZED-F9T is almost 5
orders of magnitude worse than 2e-13@1s. So, normally, you'd need this
stability to come from the OCXO. As far as I know, it takes a really
good OCXO to get 2e-13@1s.
Matt
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