The needs Many engineering applications require a precise time and/or frequency reference signal generally obtained from oscillators or clocks. An oscillator is a device that, if submitted to an electric or mechanic input, produces in output a periodic signal, with various degrees of “cleanness’’ and with periodicity T , that is to say with frequency f=1/T An Accurate and stable oscillator is intended to be used where timing is critical however the host device may need to perform autonomously. For example: A satellite requires an internal clock which is synchronized with clocks on earth. Since the satellite could be out of touch with ground stations from time-to-time, when contact is re-established, both the internal and earth clocks must be in perfect synchronization to maintain communication. The most commercially used accurate and stable oscillators are the cesium and rubidium atomic standards and quartz standards. Piezoelectric oscillators have had a wide distribution for their good short-term stability This mainly thanks to their cost which generally is quite low in comparison with the cost of atomic standards. Atomic oscillators are employed for their remarkable characteristics of both short and long-term stability and accuracy.. The most commercially used oscillators are the cesium and rubidium atomic standards and quartz standards. It is also important to mention the hydrogen standards (MASERs) as they represent the most precise frequency reference, in spite of their limited range of utilization and very high costs.   When are Rubidium oscillators required? Rubidium oscillators represent a middle solution between quartz and atomic oscillators of high quality (Cs and H maser) when considering the quality/cost ratio. They maintain satisfactory performances within a large number of technological and scientific applications Unlike cesium, which offers the best characteristics appertaining to accuracy, the rubidium is more industrially widespread, mainly thanks to its better quality/price ratio. I tOne major drawback is the cost of a cesium oscillator. A rubidium oscillator provides a very high accurate and stable frequency output with stabilities of parts per 1E-11. This is equivalent to a time accuracy of 1 second in 1,000 years. The best crystal oscillators exhibit an inaccuracy of 1 second in 10 years, whereas the more costly clocks based on the atomics element cesium exhibit an inaccuracy of 1 second 1 million years. A rubidium-GPS Clock combines a rubidium oscillator with a GPS receiver. The rubidium frequency is locked to the GPS signal, thus combining the excellent Short Term-Stability of the Rubidium Standard with the superb Long-Term-Stability of the GPS signal. The stability and accuracy of the GPS signal is derived from a system of 24 satellites each carrying on board an ensemble of atomic clocks. These are tracked and maintained traceable to UTC/USNO within 100ns. The trend today is towards smaller and smaller instruments, and simultaneously guaranteeing high quality performances, have not only scientific-technological reasons, but may also be economically justified. In fact, the last generation of rubidium clocks has a more extended range of application, thus enabling an increase in production (Currently this increase is estimated at hundreds of units per year).A consequent decrease of the costs is related to production, marketing and selling. Another fairly important aspect not to be underestimated, is the possibility of synchronizing (or disciplining) any oscillator with a high precision external reference, The aforementioned could take place thanks to the satellite systems already operative as is GPS, GLONASS and, in the future, the European GALILEO. In theory, this strategy allows the use of a medium quality oscillator, periodically re-calibrated through a circuit that receives the signal from an external reference point. Both the accuracy and the drift level of this signal have the same order of magnitude of the cesium oscillators facilitating very high performances. (these, by the way, are the employed devices in the above-mentioned satellite systems), that allows obtaining very high performances. should a negative aspects not be involved there would be no need to design high quality oscillators Therefore, it is necessary to analyze these aspects. Firstly it is important to remember that the diffusion of the navigation and timing signals is under the direct control of government-military structures, At any moment these could decide to disrupt the service or on the other hand, lower its quality level of quality. Another problem is the availability of the signal: This is the case when with the reception of the external signal is not possible or when the level of the received signal is so low as become completely unusable. In this case, we face the so called holdover state, i.e. that there is no external link and, consequently, the performances lower to the same level of those of the free, local oscillator, at least until the reference link is resumed. This operation requires considerable time since it is necessary to elaborate the received signal These considerations point to the use of the rubidium oscillators. They are employed as a redundant source or as the backup of a complex system of prime-rate quality. In fact, the particular characteristics of the rubidium clocks stability on the medium-term (approximately one day) allow to employ them as free oscillators (or self running oscillators) in case they are synchronized from the extern (for example in the nodes of an inferior hierarchic level telecommunication network, denoted as mutually synchronized conditions, or with GPS receivers of good quality, which are able to make up for a possible loss of the reference signal).

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