return to table of contents
Aging is expressed either as a normalized frequency change per unit time after a specified time period, where the unit of time is usually a day, or as a total normalized frequency change over a specified time period. It is necessary to specify a period because aging can be highly nonlinear, especially when a unit is newly fabricated or is first turned on. Examples of aging specifications are: 1) "5 X 10-10 per day 7 days after turn-on" for an oven controlled crystal oscillator (OCXO), 2) "1 ppm per year" for a temperature compensated crystal oscillator (TCXO), and 3) "1 ppm to end of 20 year life" for a filter. For non-temperature controlled devices, the temperature at which the aging test is to be performed should also be specified. Guidance for specifying aging can be found in MIL-C-49468  and MIL-O-55310 , the "mil-specs" for precision crystal units and crystal oscillators, respectively, and in a paper by Filler .