|Frequency Tolerance / Stability Over Temperature|
|The Frequency Tolerance
of a crystal or oscillator is defined as the initial deviation of the crystal
or oscillator frequency as compared to the absolute at 25°C. The Frequency
Stability over temp is defined as the frequency deviation compared to the
measured frequency at 25°C OVER the defined operating temperature range
(I.E. 0°C to +70°C). Stability tolerance is sometimes expressed as a percentage
of frequency deviation rather than as Parts Per Million (PPM) The conversion
is as follows:
.01 % = 100 PPM
.005% = 50 PPM
.001 % = 10 PPM etc.
The stability tolerance of a crystal or oscillator needs to be specified, along with the operating temperature range. For instance, a crystal may be specified as having a frequency stability tolerance of ±50 PPM over an operating temperature of -45°C to + 85°C, and having a frequency tolerance of ±50 PPM at +25°C.