The need for Oscillators
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 electrical or mechanical input, outputs 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 Rubidium / Atomic 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.
Piezoelectric oscillators have had a wide distribution due to their good short-term stability and low cost compared with the cost of atomic standards.
Atomic oscillators are employed for their remarkable characteristics of both short and long-term stability and accuracy and the most commercially used oscillators are based on Cesium (Cs) and Rubidium (Rb) atomic standards.
It is also important to mention Hydrogen standards (MASERs) as they represent the most precise frequency reference, in spite of their limited range of utilization and very high costs.
Due to the quality/cost ratio, Rubidium oscillators represent a good middle solution between quartz and atomic oscillators of high quality (Cs and H maser) and therefore most often deployed within a large number of technological and scientific applications
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 more costly clocks based on cesium exhibit an inaccuracy of 1 second in 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.
Another fairly important aspect not to be underestimated, is the possibility of synchronizing (or disciplining) any oscillator with a high precision external reference, The oscillator can be disciplined to a satellite system already operative such as GPS, GLONASS and, in the future, the European GALILEO or other such systems. In theory, this strategy allows the use of a medium quality oscillator that is 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 thereby guaranteeing very high performances.
Based on the above there would be no need to design high quality oscillators, however certain negative aspects may also be involved that should be analyzed and accounted for as well. 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 a decision could be made to disrupt the service or lower its quality level.
Another problem is the availability of the signal: This is the case when the reception of the external signal is not possible or when the level of the received signal is so low as to 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 re-establish and re-evaluate the received signal
These considerations point to the necessity for rubidium oscillators, which are most commonly used as a redundant source or as the backup of a complex system of prime-rate quality. In fact, the particular characteristics of the stability of rubidium clocks in the medium-term (approximately one day), allows them to be deployed as free (or self-running) oscillators in situations where they are synchronized from an external source. Examples of this include applications such as 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.