Distortions in crystal oscillators are not fully described; many causes of short-term instabilities have been known. Temperature fluctuations can cause short-term instabilities through thermal-transient effects, and via activity dips at the oven set point in OCXOs. Other causes include Johnson noise in the crystal unit, random vibration, noise in the oscillator circuitry due to the active and passive components, and fluctuations at various interfaces on the resonator.
When low noise is required in the higher frequency range, Surface Acoustic Wave oscillators and Dielectric Resonator Oscillators (DROs) are sometimes used. When compared with multiplied-up (bulk-acoustic-wave) quartz oscillators, these oscillators can provide lower noise far from the carrier at the expense of poorer noise close to the carrier, poorer aging, and poorer temperature stability. SAW oscillators and DROs can offer lower noise far from the carrier because these devices can be operated at higher drive levels, thereby providing higher signal-to-noise ratios, and because the devices operate at higher frequencies, thereby reducing the "20 log N" losses due to frequency multiplication by L(f) = -180 dBc/Hz noise floors have been achieved with state-of-the-art SAW oscillators. For high-frequency bulk-wave oscillators, such noise floors are realizable only in environments that are free of vibrations at the offset frequencies of interest.
Sometimes the compatibility of oscillators for an application is limited by deterministic phenomena. In other instances, random processes establish the performance limitations. Except for vibration, the short-term instabilities almost always result from noise. Long-term performance of quartz and rubidium standards is limited primarily by the temperature sensitivity and the aging, but the long-term performance of cesium and some hydrogen standards is bound primarily by random processes.
Noise can have frequent undesirable effects on system performance. Some of these effects are the following:
(1) It limits the ability to determine the current state and the predictability of precision oscillators. (2) It limits synchronization and synchronization accuracies.
(3) It can limit a receiver's useful dynamic range, channel spacing, and selectivity.
(4) It can cause bit errors in digital communications systems.
(5) It can limit radar performance, particularly Doppler radar.
(6) It can cause loss of lock, and limit acquisition and reacquisition capability in phase-locked-loop systems.
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