Hi Far more than you *ever* wanted to know about state selectors:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820012652.pdf Bob > On Jun 17, 2016, at 3:33 PM, Ole Petter Ronningen <opronnin...@gmail.com> > wrote: > > Hi. Apologies for a long post. > > I'm trying to read up on the "care and feeding of hydrogen masers". While > they are conceptually simple from a distance, there's quite a bit going on > in the quantum mechanics department when looked at up close. Somewhat > frustratingly, I am not mentally equipped to really grasp the finer (or > even coarser) points of that particular department. The topic of this post > is the concept of spin exchange, and it's relation to cavity (auto) tuning. > I've read papers on the subject, but I am having difficulties building a > "workable intuition", so I turn to the group. > > Here's what I think I understand, and I respectfully ask for corrections if > I am way off base here.. > > Spin-exchange in a hydrogen maser happens when two atoms collide, and > exchange spin, as it were.. (Hazy on the details here..) The number of > spin-exchange collisions is directly proportional to the density of atoms > in the cavity. These collisions *will* happen, but is a problem in hydrogen > masers for two reasons: 1) it takes away energy from the cavity, resulting > in lower signal output power, which degrades stability, and, 2) more > significantly, it results in a frequency shift. > > The frequency shift, as far as I can gather, is directly related to the > cavity resonant frequency - there is no way to *stop* spin exchange taking > place (apart from reducing the hydrogen density to a level where collisions > are rare, in which case the density will be too low for oscillation to take > place), but it is possible to reduce the impact the spin exchange has on > the output frequency. > > While the resonant frequency obviously influences the output power of the > maser cavity, the "mistuning" of the cavity also increases the effect spin > exchange has. In other words, in a perfectly tuned cavity, spin exchange > does not result in a frequency shift. In a badly tuned cavity, increasing > or decreasing the hydrogen flux (thereby increasing or decreasing the > number of collisions taking place) results in a corresponding > increase/decrease of the output frequency. Since the cavity ages, and the > cavity resonant frequency follows that aging, the long term stability of > the maser is degraded unless the aging can be compensated for. Which is > what cavity auto-tuning is all about. > > From my understanding, there are a few ways to implement cavity auto-tuning: > 1. From the above, it follows that a modulation of the hydrogen flux into a > mis-tuned cavity will result in a frequency shift following the modulation > frequency. Using a stable reference, this shift can be measured, and > corrections can be made to the cavity varactor voltage. Once the output > frequency no longer shifts in response to the changes in hydrogen flux, the > cavity is correctly tuned. > > 2. It is also possible to modulate the cavity varactor voltage. By > measuring the output power of the cavity, an error signal can be obtained > and used to correct the average varactor voltage. A square wave of i.e. > 100hz, centered on the approximate correct varactor voltate is put in the > varactor, and cavity output power is measured. If the output power measured > on the "low" of the square wave is lower than the signal measured when the > "high", lower the offset by some mV, and vice versa. Suitable filtering > would of course be required. > > The idea is that this method should not result in appreciable degradation > of the short/medium term stability of the maser, because the frequency of > the atoms interacting with the electromagnetic field in the maser cavity > takes time to respond to the changes in the resonant frequency, but the > output power responds "instantly". (Hazy on those details as well..) By > modulating the cavity varactor voltage (much) faster that the time constant > of the maser cavity, the modulation is effectively filtered out. > > I am very interested in this method, as it seems to me that it would be > easy (feasible) to retrofit this to older masers never equipped with cavity > auto tuning. > > There is at least one more way, which involves injecting a signal into the > maser cavity through a second coupling loop. At least one vendor I know of > does this in their newest design. I do not understand even the basics of > this method. > > Any insights and/or corrections of my understanding is most welcome. > > Thanks, > Ole > _______________________________________________ > 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.