Phase Noise

     Phase noise is a small fraction of undesirable frequency near the output frequency, and is usually expressed as the SSB spectral density in dBc/Hz.  Phase noise is dependent mostly on the crystal with the circuitry making up the unit playing a small role.  The measurement is commonly in the 1 Hz bandwidth.  The description of phase noise is "at x Hz offset it is y dBc/Hz".  Low levels of phase noise are achieved through careful circuit design and use of carefully processed high-Q resonators.

Harmonic Distortion

    The non-linear distortion due to un-wanted harmonic spectrum component related with target signal frequency.  Each harmonic component is the ratio of electric power against desired signal output electric power and expressed in terms of dBc.  Harmonic distortion specification is important especially in Sine output when a clean and less distorted signal is required.

Activity Dip

     Activity dips result from mechanical coupling of the principal mode to one or another of a number of interfering modes, which exist but are not electrically excited by the resonator electrodes.  At some temperature, the frequency of the interfering modes coincide with the frequency of the desired mode, energy is lost from the main mode, thereby causing an increase in the resonator equivalent resistance at that temperature.  Accompanying the activity dip is a deviation of the F vs. T characteristic from a smooth curve, but this is often much less pronounced than the resistance increase. In extreme cases, when the oscillator gain is insufficient, the resistance increase stops the oscillation over a range of temperature.  In addition, even when the resistance increase is not large enough to stop the oscillation, the frequency change can cause intermittent failures, e.g., it can cause a loss of lock in phase locked systems.

G-Sensitivity

      It is a measure of the sensitivity to acceleration, also known as Acceleration Sensitivity, which is the frequency shift caused by subjecting the crystal to a constant acceleration.  The most notable test is the Two G Tip-over test.  Here the G-sensitivity is measured by allowing the oscillator to stabilize, and the frequency is measured.  The oscillator is then turned upside down, 180º, and the frequency is measured again.  This test is repeated for each major axis of the oscillator.  The difference in frequency is divided by 2, yielding the static G-sensitivity.  The following table shows the typical g-sensitivity numbers

Vibration Sensitivity

     It is the measure of the oscillator sensitivity to vibration.  It can be viewed in two ways: dynamic and static.  Dynamic sensitivity refers to degradation of phase noise due to vibration while the unit is powered in the target system.  This can be different from the static G-sensitivity number in that the oscillator may posses an internal structural resonance, which will have a higher sensitivity as certain frequencies.  In most case, the dynamic sensitivity is not an issue, since typical oscillators are rack mounted and not subjected to significant vibration levels.  The static sensitivity, also known as sensitivity to transportation, is a more important factor, as it happens in transit from manufacturer to customer and is normally outside of the control of the manufacturer.  The shock and vibration can result in shifts in the calibration frequency, resulting in an offset as the customer.  Packaging and design of the oscillator can help to reduce this effect, so that the part is still within specification on arrival at the customer site.

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