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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Im Auftrag von Brooke Clarke > Gesendet: Dienstag, 19. Dezember 2006 20:05 > An: [email protected] > 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 > > > > > > > > > >_______________________________________________ > >time-nuts mailing list > >[email protected] > >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts > _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
