Anderson Electronics, Inc. - Tutorial (Specifying Frequency Stability)

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Specifying Frequency Stability

Frequency stability or temperature coefficient, by its own definition, has two elements, frequency change (deviation) and temperature range. Frequency change is expressed as positive and negative deviation from the nominal design frequency in Hz., percent or PPM. The temperature range, also known as the Operating Temperature Range - OTR. is delineated by stating the upper and lower limits. Three examples of the same stability specification for a crystal are as follows:

+/- 100 Hz. -40/+85 deg. C*
+/- .001% -40/+85 deg. C*
+/- 10 PPM -40/+85 deg. C*

*As referenced to the frequency at 25 deg. C. Calibration tolerance, aging and temperature stability are the components of ABSOLUTE STABILITY and are additive. It is important to understand each component's limitation and the effect it has on manufacturability and cost/price.

FREQUENCY DEVIATION:

VERY HIGH PRECISION: +/- 2 PPM (cost driver)
HIGH PRECISION: +/- 5 PPM (cost driver)
PRECISION: +/- 10 PPM (moderate cost driver)
SEMI-PRECISION: +/- 20 PPM
LOW PRECISION: +/- 30 PPM
NON-PRECISION: +/- 100 PPM

GENERALIZED TEMPERATURE RANGES:

FULL MILITARY: -55/+125 deg. C (cost driver)
MILITARY: -55/+105 deg. C
SECONDARY MILITARY: -40/+ 90 deg. C
INDUSTRIAL: -40/+ 85 deg. C
COMMERCIAL: -20/+ 70 deg. C
OCXO (typical) +75/+ 85 deg. C

Note that deviation and temperature range can be mutually exclusive. There are limitations as a result of the Laws of Physics. Narrow frequency deviations are typically not possible over very wide temperature changes. Reducing the frequency deviation and/or widening the operating temperature range are major cost drivers in quartz crystal resonators.

Even if there appears to be a Cut-Angle that will produce a desired temperature coefficient, the limited tolerance associated with the Cut-Angle will make manufacture of the resonator impractical due to the low yield of temperature stability compliant resonators.

Frequency-Temperature vs. Angle-of-Cut, AT-cut

Other Effects on Stability

Electric field - affects doubly-rotated resonators; e.g., a voltage on the electrodes of a 5 MHz fundamental mode SC-cut resonator results in a Δf/f = 7 x 10^-9 per volt. The voltage can also cause sweeping, which can affect the frequency (of all cuts), even at normal operating temperatures.
Magnetic field - quartz is diamagnetic, however, magnetic fields can induce Eddy currents, and will affect magnetic materials in the resonator package and the oscillator circuitry. Induced ac voltages can affect varactors, AGC circuits and power supplies. Typical frequency charge of a "good" quartz oscillator is <<10^-10 per gauss.
Ambient pressure (altitude) - deformation of resonator and oscillator packages, and change in heat transfer conditions affect the frequency.
Humidity - can affect the oscillator circuitry, and the oscillator's thermal properties, e.g., moisture absorbed by organics can affect dielectric constants.
Power supply voltage, and load impedance - affect the oscillator circuitry, and indirectly, the resonator's drive level and load reactance. A change in load impedance changes the amplitude or phase of the signal reflected into the oscillator loop, which changes the phase (and frequency) of the oscillation. The effects can be minimized by using a (low noise) voltage regulator and buffer amplifier.
Gas permeation - stability can be affected by excessive levels of atmospheric hydrogen and helium diffusing into "hermetically sealed" metal and glass enclosures (e.g., hydrogen diffusion through nickel resonator enclosures, and helium diffusion through glass Rb standard bulbs).

Zero Temperature Coefficient Quartz Cuts

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