Quartz oscillators

Quartz oscillators use the piezoelectricity principle discovered by Pierre Curie at the end of the 19th century: an acoustic wave propagating in a piezoelectric crystal creates a stress in the crystal which induces a voltage proportional to this stress. A resonant acoustic vibration therefore creates at the terminals of a piezoelectric resonator a periodic electrical signal of a frequency equal to that of the mechanical resonance. Even if there are other piezoelectric materials than quartz, of SiO2 formula, the latter is very mainly used to make oscillators, which explains the generic name of “quartz oscillators”.

A quartz oscillator includes a quartz resonator that will set the oscillation frequency and amplified feedback to maintain the resonant oscillation. The electrical signal frequency delivered by the oscillator is therefore mainly fixed by the resonator mechanical resonance frequency. Depending on the type of crystal, its cut and its geometry, different propagation and resonance modes can be considered: surface waves (SAW – Surface Acoustic Waves), bulk waves (BAW – Bulk Acoustic Waves). The oscillator final performance depends directly on the material quality and the resonator manufacturing processes, as well as the resonator integration and the electronics associated in the oscillator.

BVA resonator
SAW resonator

A resonator important characteristic is the quality factor which is equal to the ratio of the oscillation frequency to the resonance frequency width. The higher the quality factor, the more the crystal oscillator will have good performance in phase noise and frequency stability.

The quality factors of the best crystal oscillators today are around 109, corresponding to short-term frequency stabilities better than 10-13 in relative value. This level of stability is necessary in high-performance atomic clocks, in radar systems or in precise satellite orbitography techniques using Doppler velocity measurements, such as the DORIS system developed by CNES.

The ability to mass-produce quartz oscillators offering excellent compromises in terms of performance / power consumption / volume / reliability / cost has led to the current situation where almost all electrical consumer or electronic devices include a quartz oscillator: watches, clocks, cell phones, computers, televisions, remote controls, digital cameras, etc. Quartz oscillators are used there as frequency or time references, and also to produce very high acuity filters thanks to their high-quality factor. The quartz oscillators world market is considerable since several billion units are manufactured each year, in various versions, the most economical of which consume only a few microwatts.

The quartz oscillators main defect is their frequency high sensitivity with the environment: thermal, vibrational, magnetic, radiative. This high sensitivity, on the contrary, is used in piezoelectric sensors delivering a frequency whose value is directly linked to temperature, force, acceleration, stress, etc. The quartz oscillators sensitivity to radiation can cause problems for specific applications, in particular space.

In order to minimize the thermal sensitivity, crystal oscillators are designed to operate near a temperature corresponding to a reversal point where the thermal sensitivity is zero to the first order. To further reduce the influence of the temperature variations on the oscillator frequency, different control levels are implemented in very high-performance devices: TCXO-Temperature Compensated crystal Oscillator, OCXO-Oven Controlled crystal Oscillator. For example, a crystal oscillator with a thermal sensitivity of +/- 10 ppm (ppm: parts per million) for operation in a temperature range [-50°C, +50°C] will have its sensitivity reduced to +/- 1 ppm (resp. +/- 0.01 ppm) in a TCXO (resp. OCXO) device.

Frequency variation as a function of temperature for an OCXO

Current research aims at further improving the quartz oscillators performance while seeking to reduce power consumption and volume as much as possible for integration into miniaturized devices. The use of other piezoelectric materials with better properties than quartz – higher piezoelectric coupling, reduced acoustic speed and attenuation, better mechanical characteristics, … – is a promising avenue explored with materials such as aluminum orthophosphate (AlPO4) or Gallium (GaPO4), langasite (La3Ga5SiO14), langanite (La3Ga5.5Nb0.5 O14) or langatate (La3Ga5.5Ta0.5O14).

In addition, new silicon oscillators types based on MEMS technologies are increasingly replacing quartz oscillators in applications with low performance requirements, but are demanding in terms of size, power consumption and manufacturing cost.

Resonator of a 5 MHz silicon MEMS oscillator (source SiTime)
Silicon oscillator in a cell phone