Oscillators are essential components for time synchronisation mechanisms. Indeed, they mark the passage of time and ensure the quality of clocks in a system. The more accurate the oscillator, the less the clock will drift over time. However, there is not one type of oscillator suitable for all applications.
Choosing an oscillator means making a compromise between price, capacity and performance. One of the most important metrics in order to choose the right oscillator is stability. Stability is expressed in ppm (parts per million) when exposed to a change in temperature, or over time. If an oscillator has a nominal frequency of 10MHz, a 1 ppm drift corresponds to a variation of more or less 10Hz.
There is four main types of oscillators: MEMS oscillators: Microelectromechanical Systems; TCXO: Temperature-Compensated Crystal Oscillator; OXCO: Oven-Controlled Crystal Oscillator and rubidium oscillators.
MEMS oscillators
MEMS oscillators (MicroElectroMechanicalSystem) are the easiest and least expensive. Their operating principle is based on micromechanical resonators, often made of silicon. These resonators vibrate at a specific frequency when electrically excited. The advantage of MEMS oscillators is that they are shockproof and they can operate over a wide temperature range.
The best MEMS oscillators can therefore operate in environments ranging from -40 to +150°C. They are small, relatively robust and they consume little power. Unfortunately, these oscillators provide less accuracy than other oscillators and less stability over time. These oscillators are suitable for portable battery-powered devices such as sensors and IoT objects.
TCXO oscillators
TCXO oscillators (Temperature-Compensated Crystal Oscillator) are temperature-compensated quartz oscillators. They use the vibration of a quartz crystal to generate a frequency to measure the passage of time. They compensate for the weaknesses of conventional quartz oscillators when facing temperature changes by having special circuits (analogue or digital) that can modify the output frequency depending on the temperature. This enables them to operate in a temperature range from -40 to +85°C. Besides, they offer a greater level of accuracy and stability than MEMS oscillators. In fact, their frequency stability is generally from 0.1 to 2 ppm/C°. This high level of stability is ideal for many applications such as telecommunications, GPS receivers and industrial sensors.
OCXO oscillators
OCXO oscillators (Oven Controlled Crystal Oscillator) are highly accurate quartz oscillators that are very stable over time. The purpose of these oscillators is not to adjust the output frequency depending on the temperature as it is the case of TCXO oscillators, but to control the temperature of the quartz crystal to guarantee frequency.
This is why they operate inside a small oven that maintains the crystal at an optimum temperature of 70°-90°C, whatever the outside temperature.
By avoiding crystal temperature variations, OCXO oscillators can reach a drift rate of only 0.01 ppm/C°, making them the most accurate quartz oscillators. Besides, their long-term stability is excellent, with a drift of only 0.01ppm per year. These oscillators take up more space than MEMS or TCXO oscillators although there are smaller versions available. They are generally more expensive to produce and consume more power. They are therefore to be privileged for applications requiring excellent stability and for which the cost is a secondary concern. Military applications which cannot tolerate a significant drift often use this type of oscillator.
Rubidium oscillators
Rubidium oscillators are the most stable commercial oscillators. Their operation is not based on the vibration of a crystal but on the atomic resonance of rubidium atoms.
Rubidium oscillators are part of the category of atomic clocks even though they are less accurate than caesium clocks. Their operating mode makes them almost insensitive to temperature variations, which explains why their drift rate is not expressed in ppm/°C but in ppm/day. The latter is of about 10^-11 to 10^-12 ppm/day, which is way ahead of the performance of quartz and MEMS oscillators. Such performance is achieved at a much higher cost and a much larger size. They are mainly used in satellites and nano-satellites or in applications requiring excellent stability that lasts over time (several years).
| Oscillator type | Frequency stability (ppm/°C) | Long-term stability |
|---|---|---|
| MEMS | 1 to 50 ppm/°C | A few ppm/year |
| TCXO | 0.1 to 2 ppm/°C | ≈ 1 ppm/year |
| OCXO | ≤ 0.01 ppm/°C | ≈ 0.1 ppm/year |
| Rubidium | 10 |
≈ 0.001 ppm/year |
Summary table of the features of each oscillator type
Choosing the right oscillator depends on the final application and the associated space/stability/budget constraints. However, technological breakthroughs are constant in this sector and it is important to keep abreast of new developments. For example, we have recently seen the emergence of temperature-compensated MEMS oscillators that achieve a level of stability that only very high-quality quartz oscillators can match.
With over 150 years of expertise in time management and present in more than 140 countries, Bodet Time is a major French leader in time synchronisation and time frequency. Our range of Netsilon time servers is equipped with high-performance TCXO, OCXO and OCXO HQ oscillators.
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