s an acronym for microwave amplification by stimulated emission of radiation (it is the microwave equivalent of a laser). For short-term measurement, hydrogen maser clocks are more accurate than cesium clocks, but as time increases, clocks based on hydrogen masers are not as accurate because they suffer a signific- ant frequency shift through a process known as aging. Of the several factors that affect almost all oscillators, aging is one of the most important to consider. Aging is simply the amount by which the natural frequency of the oscillator changes over a defined period. Characterizing this can be difficult as it has many dependence; for example, aging can be acceler- ated by elevating the temperature. For quartz, the rate of aging can start out quite high upon initial manufacture but decrease markedly over the weeks following being placed into service. For this reason, some oscillators are intentionally "aged" before being placed in a device. Aging is expressed as a value per year (or some other time frame), SO that an aging value of 1 X 10-9 / day indicates that the oscillator will change frequency by up to 1 ppb per day. Table 3-2 show the most common oscillator types and their characteristics. Stability in this table shows just one of the ways to measure it-over quite a short measurement interval. In metrology, the observation time is represented by the lowercase Greek letter tau (T). Table 3-2 Oscillator Types Value Rubidium Cesium Beam Hydrogen Maser Technology Quartz Quartz Rubidium Cesium Hydrogen Stability(t = 11x10-8 +1 x 10-11 to 15 x 10-12 to +1 x 10-11 1 sec) +1 x 10-9 +1 x 10-12 Aging / year +5 X 10 15 x 10-10 This table contains typical short-duration values at a constant temperature based on data from data- sheets from several vendors. Depending on the criteria and method of testing, and specific fabrica- tion process, some of the values may vary significantly for any technology. The table is included merely to indicate the relative values between classes of oscillators using a short-durati