Hi The problem with a variable speed DC fan close to the rubidium is that you change the magnetic field when you change the fan speed. Since the rubidium is sensitive to the magnetic field, you tune it.
The gotcha with a simple controller is that you may control a point to a specific temperature, but in this case you want to control the entire device. Gradients and the change in gradients matters quite a bit. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of WarrenS Sent: Wednesday, December 23, 2009 9:39 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Cheap Rubidium OR you could use a small PC type IC cooler fan blowing at the heat sink from a short distance away, driven from a Lady Heater type variable speed fan active PID Temp controller. That has NO trouble holding 0.01 Deg C at where ever you place the Temp sensor, over normal day / night room temperature variations, even with it all out on an open table without a box, insulation, OR swimming pool. ws ***** Try an insulated water cooled metal block for the baseplate heatsink The water can be recirculated and heated/cooled as required. With a recirculating system and say 25W dissipation with no temperature control you will only need about 5 cubic meters (5 tons) of water to maintain a temperature rise of less than 0.01C for 3 hours. Assuming that the 25W rubidium dissipation is the only significant source of heating for the water. That means you need a well insulated swimming pool in your basement. Controlling the temperature of a smaller amount of recirculating water is probably a simpler proposition. Bob Camp wrote: > Hi > > Water might work. It would take quite a bit of it. > > Here's my "wild guess" level math: > > 1) The basement moves 0.1 to 1 C short term / over a day. > > 2) I want to get to< 0.01 > > That takes the time constant out to>= 10X the time I'm interested in. > > 3) The time period of interest is 3 to 30 hours. > > That gets to a time constant of at least 10 days. > > At the same time you have>10 watts coming out of the gizmo. You can't put the thermal mass inside a vacuum bottle. > > I suspect that some combination of thermal mass and active stabilization will be needed. > > So much fun .... > > Bob > > > On Dec 23, 2009, at 2:18 AM, Don Latham wrote: > > >> sheesh! How about a right-sized water jug? >> Don >> >> ----- Original Message ----- From: "Bob Camp"<lists at cq.nu> >> To: "Discussion of precise time and frequency measurement"<time-nuts at febo.com> >> Sent: Tuesday, December 22, 2009 8:23 PM >> Subject: Re: [time-nuts] Cheap Rubidium >> >> >> >>> Hi >>> >>> I agree that if you simply bolt the rubidium to an old engine block and toss a blanket over it, you might get some pretty good thermal stability in the "hour to couple hours" time period. That's certainly a better approach than putting some kind of DC heater (and it's varying magnetic field) near the rubidium. >>> >>> I'm still wondering if they do indeed hit 1x10-13 (as in almost 1x10-14) or not. I suspect not. I'm sure that they do indeed get into the 1x10-13's, just not sure they get to the bottom of that region. >>> >>> Bob >>> >>> >>> On Dec 22, 2009, at 8:26 PM, Bruce Griffiths wrote: >>> >>> >>>> Bob Camp wrote: >>>> >>>>> Hi >>>>> >>>>> If I randomly pick up a FE 5680A data sheet, I find that it's short term stability is 1.4/sqrt(Tau) x 10-11. Since I never doubt anything I see on a data sheet, this immediately tells me I should get 1.4x10-12 at 100 seconds, and I only have to wait for 10,000 seconds to get to 1.4x10-13. >>>>> >>>>> Since the temperature performance is at the 1x10-12 / C level, I would need a room that's stable to *much* better than 0.1 C over a 3 hour period to get there. I suspect that 0.01C might not be good enough ... >>>>> >>>>> So here's the question: >>>>> >>>>> Has anybody run any of the cheap rubidiums (FE or Efratom) in a *very* stable temperature environment to see how close they get / what the floor is? I've run through a lot of data on the web, but I haven't really found what I'm looking for. >>>>> >>>>> Thanks! >>>>> >>>>> Bob >>>>> >>>>> >>>>> >>>> Figure 7 on the FE5680 page (also on the data sheet) indicates that you may need somewhat less than 3hours to achieve ADEV ~1E-13. >>>> 0.01C stability should be adequate.however its not necessary to control the room temperature to this stability if the FE5680 is in an enclosure with a sufficiently high time constant whilst having a sufficiently low thermal resistance so as to avoid overheating the FE5680. >>>> >>>> Bruce >>>> >>>> >>>> _______________________________________________ >>>> _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
