Re: [time-nuts] Conditioning Rubidium Oscillators

[Jim Harman]

Attila and Bruce and Adrian,

You are right, I stand corrected. Despite the talk in the app note about
aging rates and calculating the average slope of the frequency offset, and
about how m represents the rate of change of the frequency offset, when you
dig into the formulas, they are doing a linear fit to determine the slope
of the phase, which of course corresponds to the frequency offset itself
and not its rate of change.

Sorry for any confusion I may have caused. Attila, good suggestion on the
Kalman filter. I will educate myself about that.


On Tue, Mar 15, 2016 at 8:30 PM, Bruce Griffiths  wrote:

> They actually determine the phase offset and rate of change of phase (i..e.
> frequency offset and not frequency drift as claimed in the paper) from a
> linear
> regression fit to a sequence of phase differences.
> The results from the regression fit are then used in an adaptive PID loop.
>
> Bruce
>
>
> On Tuesday, March 15, 2016 10:28:39 PM Adrian Godwin wrote:
> > My understanding of the article was that although fairly simple control
> > techniques such as PID were used, their innovation was to determine what
> > function the loop was performing, (initial lock, stability, and
> transition
> > from one to the other) and to choose a set of constants for loop control
> > appropriate to each type. The ideal loop characteristics are not the same
> > for all of these.
> >
> > On Tue, Mar 15, 2016 at 7:51 PM, Attila Kinali  wrote:
> > > On Mon, 14 Mar 2016 19:56:42 -0400
> > > Jim Harman  wrote:
> > >
> > >
> > > Disclaimer: Control theory is not my strongest topic. I am pretty sure
> > > that what I have written here is correct. But if anyone finds any
> > > mistakes, please correct me.
> > >
> > > > On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius <
> > >
> > > lars.walen...@hotmail.com>
> > >
> > > > wrote:
> > > > > I read this but couldn´t understand why this is superior to the
> > > > > PI-loop
> > > > > with a pre-filter?
> > >
> > >
> http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Con
> > > trol-%E2%80%93-A-Different-Approach.pdf>
> > > > > Anybody can say why? Even if regression is very useful the
> limitation
> > >
> > > of
> > >
> > > > > the GPS and ionosphere will be the problem?How much better is it
> > >
> > > reasonable
> > >
> > > > > to get??
> > >
> > > A PID loop adapts to a step as input faster than a PI loop.
> > > To give a rule of thumb: the D part is used to predict how fast
> > > the error is moving. If the error is shrinking fast, there will
> > > be an overshoot once the error reaches zero. Thus the D part is
> > > used to make the rate of change slower once the error becomes small..
> > > Nothing more.
> > >
> > > The wikipedia entry on PID controllers explains these things in
> > > some detail and should help understand it.
> > >
> > > > I think the potential benefit of this approach is that it
> continuously
> > > > predicts the long term drift of the oscillator and attempts to
> > > > compensate
> > > > for it.
> > >
> > > Nope, the above appnote does not predict anything. It's not an adaptive
> > > control system at all. All they do is describe an PID controller
> without
> > > using the common language of the control theory people.
> > >
> > > > If the drift is reasonably linear, this means that you can use a
> > > > larger time constant in the control loop and thus be less sensitive
> to
> > > > short term GPS timing variations, while keeping the phase error
> close to
> > > > zero
> > >
> > > If the drift is linear, then the D part of the PID control loop will
> > > actually slightly increase the error compared to a PI controller.
> > >
> > > Proper adaptive control systems can also model higher order (ie changes
> > > that have components with a second or third (or higher) derivative) and
> > > the more fancy stuff even non-linear "drift".
> > >
> > > But the math behind that is not easy, and you need to have a good idea
> > > what the system (including reference, "plant", "sensor" and noise
> sources)
> > > looks like to build a control loop that improves on the PID controller.
> > > It's also quite easy to botch it up and make something that performes
> > > worse
> > > than just a P controller
> > >
> > > > Of course if the oscillator drift is not predictable, this won't help
> > > > and
> > > > might even make things worse.
> > >
> > > A lot of the components of the drift of most oscillators is actually
> > > quite predicatble. If you take a Rb vapor reference, the largest three
> > > drift components will be aging of the cell, temperature and air
> pressure.
> > > All three of them are relatively easy to model and can remove a lot
> > > of uncertainty (how much can be removed depends highly on the make up
> > > of the physics package).
> > >
> > > > I have done some experiments with an OCXO and a controller design
> > > > similar
> > > > to the one Lars posted some time 

Re: [time-nuts] Conditioning Rubidium Oscillators

[Bruce Griffiths]

They actually determine the phase offset and rate of change of phase (i..e. 
frequency offset and not frequency drift as claimed in the paper) from a linear 
regression fit to a sequence of phase differences.
The results from the regression fit are then used in an adaptive PID loop.

Bruce


On Tuesday, March 15, 2016 10:28:39 PM Adrian Godwin wrote:
> My understanding of the article was that although fairly simple control
> techniques such as PID were used, their innovation was to determine what
> function the loop was performing, (initial lock, stability, and transition
> from one to the other) and to choose a set of constants for loop control
> appropriate to each type. The ideal loop characteristics are not the same
> for all of these.
> 
> On Tue, Mar 15, 2016 at 7:51 PM, Attila Kinali  wrote:
> > On Mon, 14 Mar 2016 19:56:42 -0400
> > Jim Harman  wrote:
> > 
> > 
> > Disclaimer: Control theory is not my strongest topic. I am pretty sure
> > that what I have written here is correct. But if anyone finds any
> > mistakes, please correct me.
> > 
> > > On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius <
> > 
> > lars.walen...@hotmail.com>
> > 
> > > wrote:
> > > > I read this but couldn´t understand why this is superior to the
> > > > PI-loop
> > > > with a pre-filter?
> > 
> > http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Con
> > trol-%E2%80%93-A-Different-Approach.pdf> 
> > > > Anybody can say why? Even if regression is very useful the limitation
> > 
> > of
> > 
> > > > the GPS and ionosphere will be the problem?How much better is it
> > 
> > reasonable
> > 
> > > > to get??
> > 
> > A PID loop adapts to a step as input faster than a PI loop.
> > To give a rule of thumb: the D part is used to predict how fast
> > the error is moving. If the error is shrinking fast, there will
> > be an overshoot once the error reaches zero. Thus the D part is
> > used to make the rate of change slower once the error becomes small..
> > Nothing more.
> > 
> > The wikipedia entry on PID controllers explains these things in
> > some detail and should help understand it.
> > 
> > > I think the potential benefit of this approach is that it continuously
> > > predicts the long term drift of the oscillator and attempts to
> > > compensate
> > > for it.
> > 
> > Nope, the above appnote does not predict anything. It's not an adaptive
> > control system at all. All they do is describe an PID controller without
> > using the common language of the control theory people.
> > 
> > > If the drift is reasonably linear, this means that you can use a
> > > larger time constant in the control loop and thus be less sensitive to
> > > short term GPS timing variations, while keeping the phase error close to
> > > zero
> > 
> > If the drift is linear, then the D part of the PID control loop will
> > actually slightly increase the error compared to a PI controller.
> > 
> > Proper adaptive control systems can also model higher order (ie changes
> > that have components with a second or third (or higher) derivative) and
> > the more fancy stuff even non-linear "drift".
> > 
> > But the math behind that is not easy, and you need to have a good idea
> > what the system (including reference, "plant", "sensor" and noise sources)
> > looks like to build a control loop that improves on the PID controller.
> > It's also quite easy to botch it up and make something that performes
> > worse
> > than just a P controller
> > 
> > > Of course if the oscillator drift is not predictable, this won't help
> > > and
> > > might even make things worse.
> > 
> > A lot of the components of the drift of most oscillators is actually
> > quite predicatble. If you take a Rb vapor reference, the largest three
> > drift components will be aging of the cell, temperature and air pressure.
> > All three of them are relatively easy to model and can remove a lot
> > of uncertainty (how much can be removed depends highly on the make up
> > of the physics package).
> > 
> > > I have done some experiments with an OCXO and a controller design
> > > similar
> > > to the one Lars posted some time ago. I plotted the trend in the 3-hour
> > > average DAC values over many days and used Excel to do a least-squares
> > 
> > fit
> > 
> > > to that data. As long as the oscillator is powered on continuously, this
> > > gives an R^2 of over 90%, so the linearity of the drift is very good. If
> > 
> > I
> > 
> > > use this slope as a correction factor, i.e. adding X DAC counts per day
> > 
> > to
> > 
> > > the output of the PI control algorithm, it significantly reduces the
> > > average TIC error at long time constants
> > 
> > You could use a simple, single variable Kalman filter control loop to
> > improve your PI controller. Just assume linear aging and let the
> > Kalman filter predict it. For a gentle introduction into Kalman filters
> > have a look at [1]. And yes this of course reduces the average TIC error,
> > 

Re: [time-nuts] Conditioning Rubidium Oscillators

[Adrian Godwin]

My understanding of the article was that although fairly simple control
techniques such as PID were used, their innovation was to determine what
function the loop was performing, (initial lock, stability, and transition
from one to the other) and to choose a set of constants for loop control
appropriate to each type. The ideal loop characteristics are not the same
for all of these.


On Tue, Mar 15, 2016 at 7:51 PM, Attila Kinali  wrote:

> On Mon, 14 Mar 2016 19:56:42 -0400
> Jim Harman  wrote:
>
>
> Disclaimer: Control theory is not my strongest topic. I am pretty sure
> that what I have written here is correct. But if anyone finds any
> mistakes, please correct me.
>
> > On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius <
> lars.walen...@hotmail.com>
> > wrote:
> >
> > > I read this but couldn´t understand why this is superior to the PI-loop
> > > with a pre-filter?
> > >
> > >
> > >
> http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf
> >
> > > Anybody can say why? Even if regression is very useful the limitation
> of
> > > the GPS and ionosphere will be the problem?How much better is it
> reasonable
> > > to get??
>
> A PID loop adapts to a step as input faster than a PI loop.
> To give a rule of thumb: the D part is used to predict how fast
> the error is moving. If the error is shrinking fast, there will
> be an overshoot once the error reaches zero. Thus the D part is
> used to make the rate of change slower once the error becomes small.
> Nothing more.
>
> The wikipedia entry on PID controllers explains these things in
> some detail and should help understand it.
>
>
> > I think the potential benefit of this approach is that it continuously
> > predicts the long term drift of the oscillator and attempts to compensate
> > for it.
>
> Nope, the above appnote does not predict anything. It's not an adaptive
> control system at all. All they do is describe an PID controller without
> using the common language of the control theory people.
>
> > If the drift is reasonably linear, this means that you can use a
> > larger time constant in the control loop and thus be less sensitive to
> > short term GPS timing variations, while keeping the phase error close to
> > zero
>
> If the drift is linear, then the D part of the PID control loop will
> actually slightly increase the error compared to a PI controller.
>
> Proper adaptive control systems can also model higher order (ie changes
> that have components with a second or third (or higher) derivative) and
> the more fancy stuff even non-linear "drift".
>
> But the math behind that is not easy, and you need to have a good idea
> what the system (including reference, "plant", "sensor" and noise sources)
> looks like to build a control loop that improves on the PID controller.
> It's also quite easy to botch it up and make something that performes worse
> than just a P controller
>
> > Of course if the oscillator drift is not predictable, this won't help and
> > might even make things worse.
>
> A lot of the components of the drift of most oscillators is actually
> quite predicatble. If you take a Rb vapor reference, the largest three
> drift components will be aging of the cell, temperature and air pressure.
> All three of them are relatively easy to model and can remove a lot
> of uncertainty (how much can be removed depends highly on the make up
> of the physics package).
>
> > I have done some experiments with an OCXO and a controller design similar
> > to the one Lars posted some time ago. I plotted the trend in the 3-hour
> > average DAC values over many days and used Excel to do a least-squares
> fit
> > to that data. As long as the oscillator is powered on continuously, this
> > gives an R^2 of over 90%, so the linearity of the drift is very good. If
> I
> > use this slope as a correction factor, i.e. adding X DAC counts per day
> to
> > the output of the PI control algorithm, it significantly reduces the
> > average TIC error at long time constants
>
> You could use a simple, single variable Kalman filter control loop to
> improve your PI controller. Just assume linear aging and let the
> Kalman filter predict it. For a gentle introduction into Kalman filters
> have a look at [1]. And yes this of course reduces the average TIC error,
> as you are removing one of the systematics, which has a relatively high
> contribution to the TIC error. The better the model of the system you
> are using and thus the more systematics you compensate for, the smaller
> the error will become and the more it will look like uncorrelated noise.
> But as I have written above, once you start doing more complex models,
> it gets harder to actually improve the output instead of detoriating it.
> If you make a mistake in the model, it will predict the wrong thing
> and thus correct into the wrong direction. And more often than not
> the detoriation will be larger than what 

Re: [time-nuts] Conditioning Rubidium Oscillators

[Attila Kinali]

On Mon, 14 Mar 2016 19:56:42 -0400
Jim Harman  wrote:


Disclaimer: Control theory is not my strongest topic. I am pretty sure
that what I have written here is correct. But if anyone finds any
mistakes, please correct me.

> On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius 
> wrote:
> 
> > I read this but couldn´t understand why this is superior to the PI-loop
> > with a pre-filter?
> >
> >
> > http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf
>
> > Anybody can say why? Even if regression is very useful the limitation of
> > the GPS and ionosphere will be the problem?How much better is it reasonable
> > to get??

A PID loop adapts to a step as input faster than a PI loop.
To give a rule of thumb: the D part is used to predict how fast
the error is moving. If the error is shrinking fast, there will
be an overshoot once the error reaches zero. Thus the D part is
used to make the rate of change slower once the error becomes small.
Nothing more.

The wikipedia entry on PID controllers explains these things in
some detail and should help understand it.


> I think the potential benefit of this approach is that it continuously
> predicts the long term drift of the oscillator and attempts to compensate
> for it.

Nope, the above appnote does not predict anything. It's not an adaptive
control system at all. All they do is describe an PID controller without
using the common language of the control theory people.

> If the drift is reasonably linear, this means that you can use a
> larger time constant in the control loop and thus be less sensitive to
> short term GPS timing variations, while keeping the phase error close to
> zero

If the drift is linear, then the D part of the PID control loop will
actually slightly increase the error compared to a PI controller.

Proper adaptive control systems can also model higher order (ie changes
that have components with a second or third (or higher) derivative) and
the more fancy stuff even non-linear "drift".

But the math behind that is not easy, and you need to have a good idea
what the system (including reference, "plant", "sensor" and noise sources)
looks like to build a control loop that improves on the PID controller.
It's also quite easy to botch it up and make something that performes worse
than just a P controller

> Of course if the oscillator drift is not predictable, this won't help and
> might even make things worse.

A lot of the components of the drift of most oscillators is actually
quite predicatble. If you take a Rb vapor reference, the largest three
drift components will be aging of the cell, temperature and air pressure.
All three of them are relatively easy to model and can remove a lot
of uncertainty (how much can be removed depends highly on the make up
of the physics package).
 
> I have done some experiments with an OCXO and a controller design similar
> to the one Lars posted some time ago. I plotted the trend in the 3-hour
> average DAC values over many days and used Excel to do a least-squares fit
> to that data. As long as the oscillator is powered on continuously, this
> gives an R^2 of over 90%, so the linearity of the drift is very good. If I
> use this slope as a correction factor, i.e. adding X DAC counts per day to
> the output of the PI control algorithm, it significantly reduces the
> average TIC error at long time constants

You could use a simple, single variable Kalman filter control loop to
improve your PI controller. Just assume linear aging and let the
Kalman filter predict it. For a gentle introduction into Kalman filters
have a look at [1]. And yes this of course reduces the average TIC error,
as you are removing one of the systematics, which has a relatively high
contribution to the TIC error. The better the model of the system you
are using and thus the more systematics you compensate for, the smaller
the error will become and the more it will look like uncorrelated noise.
But as I have written above, once you start doing more complex models,
it gets harder to actually improve the output instead of detoriating it.
If you make a mistake in the model, it will predict the wrong thing
and thus correct into the wrong direction. And more often than not
the detoriation will be larger than what the total gain of a simpler
model would have been.

For further reading I recommend to have a look at [2] to get the basics
and google for "adaptive control" and "system identification".

Attila Kinali


[1] "Kalman Filtering", by Dan Simon, 2001
http://academic.csuohio.edu/simond/courses/eec644/kalman.pdf

[2] "Feedback control of dynamic systems" by Franklin et al.

-- 
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

Re: [time-nuts] Conditioning Rubidium Oscillators

[Björn]

I have seen no national timelab using cheap L1 stuff for high quality time 
transfer.

Btw... its been discussed here multiple times over the years. Ashtech Z12-T and 
fast forward over the past 20years. Look at a geodetic receiver with optional 
external freq input. They are available from most of the big names.

A very interesting question why someone spend $$$ on masers and not a much 
smaller amount on good antennas and multi frequency gnss receivers.

Btw the local vlbi-site have lots of high end gnss rx.

--
    Björn

 Originalmeddelande Från: Hal Murray 
<hmur...@megapathdsl.net> Datum:2016-03-15  04:18  (GMT+01:00) 
Till: Discussion of precise time and frequency measurement 
<time-nuts@febo.com> Kopia: hmur...@megapathdsl.net 
Rubrik: Re: [time-nuts] Conditioning Rubidium Oscillators 

elfchief-timen...@lupine.org said:
> Are there currently any decent (intended (or at least usable) for
> timekeeping) GPSs that support using L2/L5 for this purpose? Google is
> turning up lots of nothing, for me... 

I don't know of any.  I'm somewhat sure that they don't exist or maybe they 
are hidden in the military sector.

Tom Clark and Rick Hambly work on timing for VLBI.  They run a yearly 
workshop on timing.  Their slides are available on the web.  They use low 
cost L1 gear.  I'm sure they would use something better if it were available 
commercially.  They are starting with Hydrogen Masers so cost cutting on the 
GPS receiver would not be a problem.

-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Conditioning Rubidium Oscillators

[Tom Van Baak]

Hal,

One point being missed here is that when you get to high-end, dual-frequency 
(L1/L2), carrier-phase GPS there's often less need for real-time results. The 
receivers are run more in a data logging mode. Many sites collect data 
continuously and then apply GPS corrections hours to days later, as they are 
computed and refined by world-wide networks, such as http://www.igs.org/ .

In other words, the goal is not so much to steer your clock as is done with a 
GPSDO, but simply to know how far off your clock is; even if that means waiting 
a few days for all the calculations to settle. They are run in a mode of "what 
time was it" rather than "what time is it".

For really good clocks this is a better way to do timing. A GPSDO can only 
steer based on recent past and present data, using the rough broadcast values 
of SV clock & position. But when you post-process with high-end receivers you 
get to use past, present, and future data. The SV clock and orbit data you use 
are significantly more precise, because it's calculated from hundreds or 
thousands of GPS sites world-wide. Plus there's no DAC or EFC or h/w noise or 
tempco; "paper clocks" avoid all of this.

For applications like what Tom & Rick (VLBI) are involved in, there's no 
particular need for the H-maser to have the "right" time or be at the "right" 
frequency. All that's needed is a record of what the time/frequency offsets 
are. Those kind of clocks tend to be left alone.

/tvb

- Original Message - 
From: "Hal Murray" <hmur...@megapathdsl.net>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Cc: <hmur...@megapathdsl.net>
Sent: Monday, March 14, 2016 8:18 PM
Subject: Re: [time-nuts] Conditioning Rubidium Oscillators


> 
> elfchief-timen...@lupine.org said:
>> Are there currently any decent (intended (or at least usable) for
>> timekeeping) GPSs that support using L2/L5 for this purpose? Google is
>> turning up lots of nothing, for me... 
> 
> I don't know of any.  I'm somewhat sure that they don't exist or maybe they 
> are hidden in the military sector.
> 
> Tom Clark and Rick Hambly work on timing for VLBI.  They run a yearly 
> workshop on timing.  Their slides are available on the web.  They use low 
> cost L1 gear.  I'm sure they would use something better if it were available 
> commercially.  They are starting with Hydrogen Masers so cost cutting on the 
> GPS receiver would not be a problem.
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Poul-Henning Kamp]

In message <20160315031836.0f5b3406...@ip-64-139-1-69.sjc.megapath.net>, Hal Mu
rray writes:

>Tom Clark and Rick Hambly work on timing for VLBI.  [...]  They use low 
>cost L1 gear.

The crux of this seems to be that getting better time information out
of multiple bands *in real time* is not a solved problem.

If/when you solve it, you would still have the problem of how to
electrically instantiate that time information in the picosecond
domain.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Hal Murray]

elfchief-timen...@lupine.org said:
> Are there currently any decent (intended (or at least usable) for
> timekeeping) GPSs that support using L2/L5 for this purpose? Google is
> turning up lots of nothing, for me... 

I don't know of any.  I'm somewhat sure that they don't exist or maybe they 
are hidden in the military sector.

Tom Clark and Rick Hambly work on timing for VLBI.  They run a yearly 
workshop on timing.  Their slides are available on the web.  They use low 
cost L1 gear.  I'm sure they would use something better if it were available 
commercially.  They are starting with Hydrogen Masers so cost cutting on the 
GPS receiver would not be a problem.

-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Conditioning Rubidium Oscillators

[Bob Camp]

Hi

What you are looking for are survey receivers with time tag inputs and outputs. 
The Novatel boards are
one example of this. For about $5K you can get a nice new one with all the 
needed options. On the surplus 
market the Trimble NetRS shows up over a 10:1 price range, it may or may not do 
the job since you are looking 
for L5 and it’s L1/L2 only. For L1/L2/L5 you are looking at gear that came out 
in the last couple of years. That will be
expensive. L1/L2 is about the only way to go surplus. 

There are a long line of Ashtech receivers that do or do not have timing, it 
all depends on cables you can only see 
once you have it torn apart in your hot little hand. There are also Trimble’s 
successors to the 15 year old NetRS at 
various price points. 

Bob




> On Mar 14, 2016, at 6:21 PM, Jay Grizzard  
> wrote:
> 
> Are there currently any decent (intended (or at least usable) for timekeeping)
> GPSs that support using L2/L5 for this purpose? Google is turning up lots
> of nothing, for me...
> 
> -j
> 
>> Since it is a ???space weather??? determined thing, with L1 only, all 
>> you can do is guess at confidence levels. The real answer is to go to 
>> L1/L2 or L1/L2/L5 and at least reduce the level of the problem.
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Jim Harman]

On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius 
wrote:

> I read this but couldn´t understand why this is superior to the PI-loop
> with a pre-filter?
>
>
> http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf
>
> Anybody can say why? Even if regression is very useful the limitation of
> the GPS and ionosphere will be the problem?How much better is it reasonable
> to get??
>

I think the potential benefit of this approach is that it continuously
predicts the long term drift of the oscillator and attempts to compensate
for it. If the drift is reasonably linear, this means that you can use a
larger time constant in the control loop and thus be less sensitive to
short term GPS timing variations, while keeping the phase error close to
zero

Of course if the oscillator drift is not predictable, this won't help and
might even make things worse.

I have done some experiments with an OCXO and a controller design similar
to the one Lars posted some time ago. I plotted the trend in the 3-hour
average DAC values over many days and used Excel to do a least-squares fit
to that data. As long as the oscillator is powered on continuously, this
gives an R^2 of over 90%, so the linearity of the drift is very good. If I
use this slope as a correction factor, i.e. adding X DAC counts per day to
the output of the PI control algorithm, it significantly reduces the
average TIC error at long time constants


-- 

--Jim Harman
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Jay Grizzard]

Are there currently any decent (intended (or at least usable) for timekeeping)
GPSs that support using L2/L5 for this purpose? Google is turning up lots
of nothing, for me...

-j

> Since it is a ???space weather??? determined thing, with L1 only, all 
> you can do is guess at confidence levels. The real answer is to go to 
> L1/L2 or L1/L2/L5 and at least reduce the level of the problem.
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Bob Camp]

Hi

Yes indeed, the ionosphere is *not* predictable day to day so no matter what 
you saw yesterday, today will be different. Tomorrow likely will be different 
as well. Two bumps a day (at least) and even during the day they aren’t certain 
to be the same. 

Since it is a “space weather” determined thing, with L1 only, all you can do is 
guess at confidence levels. The real answer is to go to L1/L2 or L1/L2/L5 and 
at least reduce the level of the problem. You can also do some coordinated 
things at L1, but then you no longer have a stand alone device. You are part of 
a network and reliant on a data flow from it.  You are also dependent on that 
network’s data being more accurate than the item you are trying to discipline. 

Bob

> On Mar 14, 2016, at 2:37 PM, Lars Walenius <lars.walen...@hotmail.com> wrote:
> 
> I read this but couldn´t understand why this is superior to the PI-loop with 
> a pre-filter?
> 
> http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf
> 
> Anybody can say why? Even if regression is very useful the limitation of the 
> GPS and ionosphere will be the problem?How much better is it reasonable to 
> get??
> 
> Lars
> 
> 
> Från: GandalfG8--- via time-nuts<mailto:time-nuts@febo.com>
> Skickat: den 3 mars 2016 22:00
> Till: time-nuts@febo.com<mailto:time-nuts@febo.com>
> Ämne: [time-nuts] Conditioning Rubidium Oscillators
> 
> I'm sure I won't be the only one receiving newsletters from Precise  Time
> and Frequency Inc, but I thought it worth mentioning their latest  white
> paper as it offers some thoughts on the conditioning of  rubidium standards 
> via
> RS232 control.
> 
> "Rubidium Control - A Different Approach" can be found here
> 
> http://ptfinc.com/resources/
> 
> A very basic registration is required but I've always found their mailings
> to be interesting and informative whilst never being intrusive, and
> there's always an unsubscribe option anyway.
> 
> Please not that I have no affiliation whatsoever, just passing on something
> I found interesting.
> 
> Regards
> 
> Nigel
> GM8PZR
> 
> 
> 
> 
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Re: [time-nuts] Conditioning Rubidium Oscillators

[Lars Walenius]

I read this but couldn´t understand why this is superior to the PI-loop with a 
pre-filter?

http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf

Anybody can say why? Even if regression is very useful the limitation of the 
GPS and ionosphere will be the problem?How much better is it reasonable to get??

Lars


Från: GandalfG8--- via time-nuts<mailto:time-nuts@febo.com>
Skickat: den 3 mars 2016 22:00
Till: time-nuts@febo.com<mailto:time-nuts@febo.com>
Ämne: [time-nuts] Conditioning Rubidium Oscillators

I'm sure I won't be the only one receiving newsletters from Precise  Time
and Frequency Inc, but I thought it worth mentioning their latest  white
paper as it offers some thoughts on the conditioning of  rubidium standards via
RS232 control.

"Rubidium Control - A Different Approach" can be found here

http://ptfinc.com/resources/

A very basic registration is required but I've always found their mailings
to be interesting and informative whilst never being intrusive, and
there's always an unsubscribe option anyway.

Please not that I have no affiliation whatsoever, just passing on something
 I found interesting.

Regards

Nigel
GM8PZR




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[time-nuts] Conditioning Rubidium Oscillators

[GandalfG8--- via time-nuts]

I'm sure I won't be the only one receiving newsletters from Precise  Time 
and Frequency Inc, but I thought it worth mentioning their latest  white 
paper as it offers some thoughts on the conditioning of  rubidium standards via 
RS232 control.
 
"Rubidium Control - A Different Approach" can be found here
 
http://ptfinc.com/resources/
 
A very basic registration is required but I've always found their mailings  
to be interesting and informative whilst never being intrusive, and  
there's always an unsubscribe option anyway.
 
Please not that I have no affiliation whatsoever, just passing on something 
 I found interesting.
 
Regards
 
Nigel
GM8PZR
 
 
 
 
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