[time-nuts] active vs passive hydrogen maser long term stability

[Attila Kinali]

Hi,

I just read [1] and stumbled over a small detail in figure 1, which
compares the stabilities of various frequency standards:

The stability of active hydrogen masers is higher than the passive
ones for short and medium term taus. But for long term, the active
maser has a higher variation than the passive.

I thought that the long term drift of the hydrogen maser was mostly
due to changes in the wall coating (thus changing the wall shift) and
temperature dependent variations of the cavity. These two variations
should be the same for both passive and active masers. Yet, their long
term stability shows a dramatically different behaviour.

Can someone explain where this difference comes from or can point me
to documents/papers/books that might explain it?

Thanks in advance

			Attila Kinali




[1] "An Introduction to Frequency Standards", 
by Lewis, 1991
http://www.nist.gov/calibrations/upload/ie79-7.pdf
http://ieeexplore.ieee.org/iel1/5/2781/00084969.pdf‎ (better quality, but paywall)

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin