Oops, I should have mentioned: The unit was a Symmetricom MHM-2010 (I hope I got that right), except that it was built before the company was bought by Symmetricom (which was later bought by Micro-Semi in turn).
This is an active maser, meaning a self-sustaining oscillator whose gain medium was a volume of hydrogen atoms (at low pressure) maintained in a population- inverted state by squirting a thin stream of state-selected H atoms into a glass bulb, and simultaneously pumping on the bulb to maintain a low pressure. The inside of the bulb was treated so that the collisions with the surface did not usually cause a quantum state change of the H atom involved. I've read that the average excited atom typically "survived" a large number of such wall collisions before being "consumed" by contributing a quantum of energy to the oscillating mode; this has always amazed me. So the primary frequency-determining mechanism is the collision-broadened line width of the gain mechanism. However, the cavity resonance exhibits a noticeable frequency-pulling effect, and our maser has a feedback loop that strives to keep the cavity tuned to the center of the medium's gain profile. But I think this loop is not a tight loop, ergo not completely successful. Anyway, a small amount of RF power (a fraction of a pW as I understand it) is extracted from the cavity as the useful output. This drives a frequency synthesizer to make a useful standard frequency output. The "divide ratio" of that synthesizer is adjustable in fine steps, with one step being a fractional frequency change of about 7E-17. I've long wondered what causes the slow frequency drift, typically amounting to about 3E-14 over a time span of several months. Dana On Tue, Nov 21, 2017 at 1:39 AM, Mike Cook <michael.c...@sfr.fr> wrote: > > > Le 20 nov. 2017 à 20:53, Dana Whitlow <k8yumdoo...@gmail.com> a écrit : > > > > > > > > In my pre-retirement job I rode herd on an active Hydrogen maser > system,and even > > that has a clear drift tendency. Generally a couple or three times per > > year I had to make a frequency adjustment in the neighborhood of 3E-14. > And still being > > privy to its performance, I was amused to note that its drift tendency > was > > interrupted by the hurricane Maria. On the day of eye passage over the > site the frequencysuddenly > > decreased by a few parts in 10^14, held about constant for roughly a > week,then > > resumed almost its original value and drift rate thereafter. If anybody > inthis group > > can explain* that* behavior (that is, held for a week before resuming old > > habits), I’d love to learn about it. > > You don’t mention the make of the instrument, but I suspect the same > basic technology is used by all. > To quote from the Oscilloquartz page on their CH1-76A product: > « The quantum device is used as a frequency discriminator in an automatic > frequency tuning system of a crystal oscillator. » > They don’t however quote stability relative to air pressure. However….. > It is known that atmospheric pressure changes can induce OCXO frequency > changes due to deformation of the crystal envelope causing stray > capacitance changes. > As the eye of a hurricane has greatly reduced air pressure than normal, by > as much as 15%, it could be related. > > > > > > Dana > > > > > > On Mon, Nov 20, 2017 at 1:40 PM, Bob kb8tq <kb...@n1k.org> wrote: > > > >> Hi > >> > >> There is no direct relation for an Rb to 10 MYz. Cs beam tubes are what > >> have a direct relation. > >> Even then, the qualifier is “under standard conditions”. They are > >> sensitive to magnetic field. Rb’s > >> also are sensitive to magnetic field. Both can be tuned by varying the > >> field. In the case of an Rb > >> that also takes care of a multitude of other issues. > >> > >> In the case of Rb, there is a distribution of cells coming out of the > >> manufacturing process. Some > >> are pretty close to the “right” frequency. Others are way off (as in > 100’s > >> of KHz or more). All of them > >> are capable of meeting the required specs. DDS techniques allow those > >> cells to be used in a > >> production part. That increases the yield and thus drops the production > >> cost. > >> > >> Since you now magically have a DDS in the Rb, you can do all sorts of > >> interesting things. If you > >> suddenly need a 9.99900 MHz standard …. here it is … If you need to do > >> temperature compensation > >> via a lookup table … it just takes a bit of testing and some code to > make > >> it happen. Indeed, the DDS > >> does also give you some issues. Without some sort of cleanup oscillator, > >> you will have spurs and > >> phase noise on the output. > >> > >> Lots of fun …. > >> > >> Bob > >> > >> > >>> On Nov 20, 2017, at 1:34 PM, Jerry Hancock <je...@hanler.com> wrote: > >>> > >>> I know this is going to sound dumb as I know many GPSDOs had rubidium > >> oscillators in them. I can see why, in that during holdover, they would > >> tend to be more stable vs others, but given that there is a direct > >> mathematical relationship between the rubidium frequency and potentially > >> the 10Mhz desired output frequency, why do they have to be disciplined > or > >> better yet, what advantage does it bring? Also, I can see how you > >> discipline a DOCXO with the external voltage, how do you discipline a > >> rubidium? Pulse stretching? > >>> > >>> I guess I don’t understand how the technology works, but it seems like > >> an RF signal is swept that would be used to detect a dip at a pretty > well > >> defined frequency. This dip can be used to discipline the oscillator to > >> something like 9Ghz or a factor of what, 900+ times better than 10Mhz. > So > >> wouldn’t that be able to get your desired 10Mhz to 10,000,000.001 or > pretty > >> much my level of measurement? Or does is the dip not quite that > precise? > >> If you can point me to a write-up on this I’ll go away. > >>> > >>> Thanks to Gilbert for providing me with at least one rubidium > oscillator > >> that is working out of 5 though 2 others seems to stay locked for a few > >> hours during my testing. > >>> > >>> Jerry > >>> _______________________________________________ > >>> time-nuts mailing list -- time-nuts@febo.com > >>> To unsubscribe, go to https://www.febo.com/cgi-bin/ > >> mailman/listinfo/time-nuts > >>> and follow the instructions there. > >> > >> _______________________________________________ > >> time-nuts mailing list -- time-nuts@febo.com > >> To unsubscribe, go to https://www.febo.com/cgi-bin/ > >> mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > > "The power of accurate observation is commonly called cynicism by those > who have not got it. » > George Bernard Shaw > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.