QUESTION:
Any ideas on the future possibility/potential of strontium and hydrogen maser technologies for TSG application?
ANSWER:
You have made a very relevant observation. Atomic clock technologies have the promise of providing extremely good stability, making atomic standards the best choice for holdover clocks in equipment such as a TSG (or BITS/SSU/SASE). Of the various atomic standards developed so far, only the Cesium beam tube and Rubidium lamp technologies have made it in a "mass market" manner. Active Hydrogen Masers provide exceptionally good stability, of the order of 1 part in 10E14, but these devices are about the size of a small refrigerator and cost about a quarter of a million dollars (U.S.). I am not aware of any commercially available standards using Strontium, though there are several Physics Labs that are continually investigating the properties of different atoms and ions for use in a "physics package" for constructing an atomic standard.
Atomic clocks based on a Cesium beam tube can serve as primary reference sources; their free-running accuracy is of the order of 1 part in 10E12 or better. They can be used in a "slave" mode but rarely used in this manner in a telecommunications scenario. The best choice for a holdover clock in a TSG is rubidium. It has exceptional stability (drift rates of less than 1 part in 10E11 per week are not atypical) and allowing a TSG in holdover to retain near stratum-1 accuracy for weeks! Rubidium also has the nice property of acting as a "firewall". Unlike quartz, it has a "hard stop" and cannot be steered beyond a certain limit. This permits a TSG with a rubidium oscillator to disqualify rogue references autonomously. A very nice safety feature. Furthermore, continual improvements in the design and manufacturing technology surrounding rubidium are bringing the cost of rubidium down to where it is commensurate with (ovenized) quartz oscillators of inferior performance.
So, while strontium and hydrogen are possibilities, they are "futures". They have a long way to go before they can achieve the price-performance of rubidium.
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