[time-nuts] Spin exchange tuning

[cdelect at juno.com]

Ole,

Spin exchange tuning for a classic (non-autotuned) Maser is performed to
put the cavity to the frequency that corresponds to the point that
eliminates the frequency shift caused by changes in the Hydrogen flux.
This enhances the short term stability at short time scales. This tunes
the cavity OFF of resonance by a small amount!
The procedure is to plot the Maser frequency at a few different cavity
tuning voltages at one pressure and repeat it for another pressure.
Plotting these points against frequency gives you two lines that
intersect at the spin exchange tuning point. You then set the cavity
tuning voltage to correspond to that spot on the graph.
There is a major problem in this however. You need a 2nd Maser as the
reference source as you need 1X10-14th resolution at 100 Seconds! On the
classic Maser you would repeat this at some interval, Months to years to
maintain your tuning point. (some applications can get by without ever
doing the Spin exchange
tuning!) On an autotuned Maser you still need to determine the point and
a corresponding offset to be applied to the servo circuit,  once
established the servo can be turned on to maintain the tuning at the
correct point. (Just servoing the cavity to the peak of the response will
NOT put you at the spin exchange tuning point! )
On the EFOS2 Maser that lives here I have never completed the spin
exchange tuning. (No 2nd Maser!!!)
However for the stuff I do, measuring AD of precision standards, the
Maser has performed admirably for 9 years now. I have been planning to
try a different method to do the spin exchange tuning that would not
require the 2nd Maser. If it works out I will let you know.

Cheers,

Corby