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