Environmental Influences on Oscillator Stability
In addition to temperature effects, other changes in an oscillator’s environment can have a detrimental effect on the stability. The importance of various other influences depends upon the level of precision achieved by the oscillator. An inexpensive non-compensated oscillator will drift so much with slight temperature changes that subtle influences will be insignificant but the best precision oscillators may be significantly effected by changes in relative humidity or even ambient light! The best oscillators offer temperature stabilities of a few tens of parts per trillion over a fairly wide range so even tiny influences become significant.
Power Supply Noise
A common source of externally induced noise is the power supply. High quality oscillators have internal voltage regulators but good line rejection is not automatically achieved. Designs should be tested for the full spectrum of expected line-related noise.
Related Article: Finesse Regulator Noise
Magnetic fields can modulate the frequency of an oscillator as many designers have discovered after mounting an oscillator next to a power transformer. Magnetic shielding materials may be added to the oscillator but an ounce of prevention is worth a pound of Mu-metal. Trouble occurs when external fields interact with any magnetic materials in the crystal mounts or iron-core inductors and transformers. These fields are typically alternating at the line frequency and produce modulation sidebands.
Humidity, ambient light or any other influence which can change the temperature or thermal properties of the oscillator case can cause small changes in the oscillator frequency but these effects are usually trivial. The highest precision oscillators are sufficiently stable for these effect to become significant. Oscillator tuning hole seal screws should always be replaced after adjustment to keep out moisture.
Sound and Vibration
Acoustical noise can induce phase noise but it can be quite difficult to characterize and test, often earning a “no bid” from oscillator vendors. Mechanical vibration is more readily tested with an ordinary shake table and many vendors have the capability.
Related Article: Vibration-Induced Phase Noise