[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

Ulrich Bangert df6jb at ulrich-bangert.de
Wed Dec 20 10:25:05 EST 2006


Hello folks,

i like to play the bad boy again: My claim is 

a) that for most of us a GPSD Rb is of little to no use compared to a
good GPSD xtal oscillator

b) that it is a myth that you may use longer loop time constants with Rb
compared to a xtal oscillator

Let us first talk about b). It is necessary to have some basic knowledge
on temperature controllers in order to understand that. Unless the
temperature controller has an active cooling element (i.e. it may heat
AND cool) there is a very simple mechanical model for an temperature
controlled oven like being used in OCXOs: 

Imagine a pot having a small hole through which a fluid can slowly pour
out of the pot. This hole represents the oven's insulation against the
surrounding world and the fluid pouring out represents the energy that
the oven looses to its surrounding due to the fact that the insulation
is not 100%.

Then there is a person with a second pot of fluid. This person can pour
out fluid from the second pot into the first pot. The second pot
represents the oven's heater by which thermal energy may be poured
inside the oven and the person is the temperature controller. The oven's
temperature is represented by the level of the fluid in the first pot.
The controller's task is to always pour just enough fluid from the
second pot into the first pot to keep the fluid level constant despite
the fluid lost through the small hole. One refinement of the model is
that we also consider that the amount of fluid pouring out of the hole
shall not only depend on the hole's size but also on the fluid level
itself inside the pot as well as the surrounding's temperature for which
there is no good counterpart in the model. While this mechanical model
is very easy it resembles everything very well what we need to
understand about oven controllers.

Now that we have this mechanical model, we can think about the
parameters that influence the model's behaviour. One parameter is the
size of the hole. The first idea that we might have is to make the hole
as small as possible = to make the insulation as good as possible. There
are lots of people who pack their HP10811 in big amounts of insulating
material in order to improve it. But is it an improvement? It is surely
an improvement in terms of energy because the fluid (=energy) pouring
out of the hole is lost and we constantly need to put an amount of fluid
(=energy) into the oven to keep the temperature constant. 

Now let us consider what the better insulation does for the control
loop: Unlike in other controller loops we cannot take fluid (=energy)
OUT of the oven. We can only put fluid (=energy) INTO the oven. Once the
controller has generated a overshot (a common effect in controller
loops) we cannot compensate for that overshot because the only way that
fluid (=energy) can leave the process is through the little hole.
Result: If a regulation overshot happens the time constant to get back
to the correct temperature depends on the size of the hole. The smaller
the hole the longer it will take to get back to the right temperature.
Because of that we need to make the loop time constant long enough to
hopefully avoid any overshots at all and near us the right temperature
very slowly from below. However, if there is now a sudden step in the
surrounding's temperature this will have change the amount of fluid
(=energy) leaving the hole per time unit and the long loop time constant
hinders the loop to react as fast as we would like. Most OCXOs are built
that way. Note that the hole (= the not 100% insulation) is the ONLY way
that the temperature inside the oven can be made smaller. People who
addionally insulate their OCXOs make the hole size smaller and that has
the effect that the loop time constant is now too fast for this oven.

In contrast to that one may have the idea to make the hole really big
compared to the situation above. Consider an oven that is not insulated
at all. Instead it has an heating element on one side and a heat sink on
the other side. Clearly, because the oven is thermically good coupled to
its surrounding it looses lots of energy to the surrounding (= big hole
size) so we need to put lots of energy (=fluid) into it permanently. Not
a good idea in terms of efficiency but note the effect on the
controller's loop time constant. Everything can be made fast compared
the small hole size. If there is overshot we can expect the heat sink to
remove the overshot quickly. 

The concept of the big hole size is what we can find with temperature
controllers in Rb oscillators. Note: We are not talking about the
temperature of the xtal oscillator within the frequency contrtol loop.
Instead we talk about the temperature controller for the Rb lamp which
is made the 'big hole size way'. Ever had an FRK-L in your hand and
wondered about the heat sink on its back? Thats the big hole. Ever
wondered why an LPRO shall be mounted on a heat sink of sufficient size:
Thats the big hole.

Whats the whole story good for and what impact does it have on claim b)?
A thermically good insulated OCXO like an FTS1200 may show a 6 hr(!)
delay until its frequency reacts on a step change in surrounding
temperature. In contrast to that Rb reacts almost IMMEDIATLY to any
change in surrounding temperature. Big surprise! 

Neverteless it would not be honest to state this without discussing HOW
MUCH the OCXO and the Rb reacts because their tempco is different. In my
flat i measured the following tempcos

HP10811: 1.4E-11/K @ 25 °C
FTS1200: 7.7E-12/K @ 25 °C
LPRO   :-6.9E-13/K @ 25 °C

With an LPRO and some daily degrees temperature change in your flat you
may expect frequency variations of some parts in 1E-12 which follow the
temperature immediatly, much more then the 1E-12 aging that we would
have expected from the Rb. Note that this is what i measured on a
relaive modern design like the LPRO. Older constructions like the FRK-L
may exhibit a bigger tempco. The FRK-L's specs suggest a typical tempco
of 5E-12/K giving raise to frequency changes of some parts in 1E-11
during a typical day.

As a result you may set the loop time constant

a) to a BIG value which will NOT compensate for the temperature driven
effects, then your standard's output frequency will be supperimposed by
the temperature driven effects

b) to a smaller value which will compensate the temperature driven
effects but will be no improvement in terms of loop time constant
against a xtal oscillator.

Of course the whole discussion does not apply if your flat is
temperature controlled to within a degree or better.

If a Rb cannot be used with a big time constant it is no improvement
against a xtal oscillator, the main difference being that it  needs more
energy. 

Concerning a): Every Rb contains a xtal 'flywheel' oscillator. While
being a OCXO this oscillator is NOT of the same class and quality as a
OCXO as the FTS1200. This being due to the fact that it serves only as
the 'flywheel' for the atomic processes and the atomic processes
determine the long time behaviour of the complete Rb oscillator. That is
why a good OCXO may exhibit an ADEV being an order of magnitude smaller
that the output of a Rb at observation times tau=1-100s. 

For that reason the best choice in simplicity, ADEV, money and energy is
to combine a good GPS receiver with a good OCXO. If simplicity, money
and energy do not count an perhaps better design would use:

a) a Rb locked to GPS with a relative short loop time constant. That
would guarantee that the overall task of keeping the loop locked is
easier to fulfill with Rb less prone to environmental changes

and 

b) a very good OCXO which is phase locked to the standard's output with
a loop time constant of say 50 s to improve the ADEV for small
observation times.

Regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Brooke Clarke
> Gesendet: Dienstag, 19. Dezember 2006 20:05
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS 
> locking circuit
> 
> 
> Hi Brendan:
> 
> It's my take that there are two aspects of the Brooks (no relation) 
> design that need to be addressed for optimum operation:
> 1. the filter time constants of the stock design are not 
> correct for a 
> Rb oscillator, and that can be fixed by getting a custom PIC 
> from Brooks. 2. the TIC although suitable for use with the 
> older Motorola GPS 
> receivers is not optimum for use with the newer M12+T 
> receivers.  There 
> is not a fix for this now, but maybe in 2007.
> 
> Keep in mind that this design does work and that the above 
> items relate 
> to optimization not bug fixes.
> 
> Have Fun,
> 
> Brooke Clarke
> 
> w/Java http://www.PRC68.com
> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
> http://www.precisionclock.com
> 
> 
> 
> Brendan Minish wrote:
> 
> >My original Question has sparked off quite an interesting discussion 
> >and I learnt a lot
> >
> >Since it seems that the Brooks Shera Project is not the 
> optimum way of 
> >GPS disciplining a Rubidium Oscillator can anyone here point 
> me in the 
> >direction of other DIY projects (or even ideas) that might yield a 
> >better result.
> >
> >73
> >Brendan EI6IZ
> >
> >
> >
> >
> >_______________________________________________
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> >time-nuts at febo.com 
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> >
> >  
> >
> 
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