What Affects Frequency Accuracy and How is it Determined?

The frequency accuracy of the signal generator is affected by the stability of the internal reference oscillator and the amount of time that has passed since the signal generator was last calibrated.

The three different parameters that affect stability are: aging rate, temperature, and line voltage. A stable reference oscillator will ensure that the frequency output of the signal generator remains accurate in between calibrations. Over time, the stress placed on the oscillator's quartz crystal will affect the oscillation frequency. Temperature fluctuations cause changes in the crystal structure, which affect the oscillation frequency. The piezoelectric nature of quartz is also affected by the electric fields created inside the source by the line voltage. The aging rate indicates how far the reference oscillator will drift (either up or down) from its specified value over time.

How is frequency accuracy improved?
To improve the performance of the quartz oscillator, temperature compensation circuitry is used to limit the variations in output frequency that result from variations in the operating temperature. Crystal oscillators with such compensation are referred to as temperature compensated crystal oscillators (TCXO). The standard internal reference oscillator is a TCXO. Option 1E5, the high-stability time base reference oscillator, has been placed in an oven controlled environment. This environment maintains a constant temperature and provides shielding from the affects of line voltage.

Examples of frequency accuracy calculations
Below are example calculations showing how the stability of the reference oscillator affects frequency accuracy.

Equations:
Frequency accuracy = ^Aaging rate + ^Atemp + ^Aline voltage
^Aaging rate = ^fcw^*taging^{* t}cal
^Atemp = ^fcw^*ttemp
^Aline voltage = ^fcw^*tline voltage

Where ^fcw = CW frequency, ^taging = aging rate, ^tcal = time since last calibrated, ^tTemp = Temperature affects, ^tLine voltage = Line voltage affects

Example for standard internal reference oscillator: ^fcw = 2.4 GHz

^taging < ±1ppm/yr
^tcal = 1 year
^Aaging rate = (2.4 x 10⁹)(± 1 x 10^-6)(1) = ± 2400 Hz
^Atemp = (2.4 x 10⁹)(±1 x 10^-6) = ± 2400 Hz
^Aline voltage = (2.4 x 10⁹)(± 0.1 x 10^-6) = ± 240 Hz
± 5040 Hz

Example for Option 1E5 internal reference oscillator (high-stability timebase): ^fcw = 2.4 GHz

^taging < ±0.1ppm/yr
^tcal = 1 year
^Aaging rate = (2.4 x 10⁹)(± 0.1 x 10^-6)(1) = ± 240 Hz
^Atemp = (2.4 x 10⁹)(± 0.05 x 10^-6) = ± 12 Hz
^Aline voltage = (2.4 x 10⁹)(± 0.002 x 10^-6) = ± 4.8 Hz
± 256.8 Hz