Comparative analysis of oscillators: MEMS vs TCXO vs OCXO vs RUBIDIUM

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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).

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.

A crystal oscillator is an electronic oscillator circuit that uses the mechanical reverberation of a vibrating crystal (typically a quartz crystal) to create an electrical signal with precise frequency. It is often used for stabilization applications, to help keep track of time, and to provide a stable clock signal for digital integrated circuits.

Environmental factors such as humidity, pressure, vibration and in particular, temperature, affect the precision of the crystal oscillator&#;s frequency. To limit their level of influence, modified crystal oscillators are offered. Two popular versions are the temperature-compensated crystal oscillator (TCXO &#; &#;XO&#; is the old acronym for &#;crystal oscillator&#;) and oven-controlled crystal oscillator (OCXO). Both offer excellent short-term stability, with limitations coming mostly from the electronic components that are included in the oscillator circuits, and affects brought on by aging of the crystal.

TCXO: A smart, affordable solution

TCXOs are used to provide a higher level of temperature stability than the standard crystal oscillator (XO). Power consumption is greater with the TCXO and the cost is more, too, but these matters are inconsequential when you consider the fact that it effectively addresses the aforementioned environmental factors, and delivers a much more reliable signal than a normal XO.

Due to their higher level of performance, you are likely to find TCXOs in many of today&#;s smart phones, GPS devices and other base station applications.

Design

TCXOs use an element known as a voltage-controlled crystal oscillator (VCXO). Included is a temperature sensor that &#; when the temperature changes &#; will apply a small correct voltage to a varactor, which then produces a frequency that is equal and opposite to the change in frequency produced by the temperature. This counterbalance-based circuitry allows the TCXO to continue to provide a stable frequency without being influenced by environmental factors.

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Power consumption

TCXOs require a short warm-up period (normally 100ms, or sometimes longer depending on the unit&#;s design). The warm-up allows all of the components to reach thermal equilibrium and ensures that the device delivers a precise signal. Those worried about power consumption will be pleased to hear that power used during this stage is for the most part, nominal.

Calibration

To remove the affects of an aging crystal, TCXOs come with an external adjustment that enables the frequency to be reset periodically. Time between these calibration adjustments varies based upon the level of accuracy required, but average recommendations are every six months to a year. Shorter periods can be used if higher levels of accuracy are necessary.

It should be noted that making any other form of adjustment to the mechanical tuning of the TCXO (other than that which is approved by the manufacturer) can change the electrical tuning sensitivity in the TCXO&#;s design which, in turn, will cause it to under- or over-correct. Applying temperature compensation can be problematic, too, because the temperature coefficient of the crystal changes with temperature. These changes won&#;t be linear and will only result in complicating the design of the total compensation network.

OCXO: Higher price and bigger footprint, but a much better performance

OCXOs work on the theory that if you heat the crystal beyond a temperature which it would normally encounter in the working environment, then the temperature of the crystal can be maintained at a constant level (normally around 158&#;176°F, or 70&#;80°C), thus resulting in a far greater degree of frequency stability. This version of crystal oscillator is typically used with tasks that require precise frequency, such as controlling radio transmitter frequency, cellular base stations and military communications equipment.

Design

To ensure that that it is optimized for a higher internal operating temperature, the crystal used in an OCXO is typically made from a special cut (AT- or SC-cut). The &#;oven&#; that is used is a thermally-insulated enclosure that contains the crystal as well as the oscillator assembly, buffering circuitry and supply voltage regulation. A thermistor temperature sensor is used to control the power of the heater and ensure that a precise temperature within the oven is maintained constantly.

While having a precisely regulated, temperature controlled environment all but guarantees top tier performance of the crystal, one should keep in mind that having everything together in an enclosed oven-like unit means that an OCXO is physically larger than a TCXO. As such, it cannot be used in many of the TCXO-appropriate miniature applications.

Power consumption

A longer warm-up period is required when using an OCXO because the oven will only begin to operate once it hits a precise temperature (some OCXO heaters actually require about an Amp of power during this time). Additionally, the on-going heat supply that the &#;oven&#; is maintained at requires a considerable amount of power, too. Such a high level of power consumption means that an OCXO cannot be run solely on batteries.

Calibration

As with the TCXO, periodic calibrations are required every six months to a year. This is largely dependent upon the device&#;s design and requirements of the application for which it&#;s being used.

Bottom line

A summary of figures that both devices have to offer:

*Figures courtesy of "Tutorial Precision Frequency Generation Utilizing OCXO and Rubidium Atomic Standards with Applications for Commercial, Space, Military, and Challenging Environments IEEE Long Island Chapter March 18, ."

TCXOs are smaller and more economical than OCXOs. They are a terrific solution for portable units that necessitate a reasonably accurate source.

OCXOs, on the other hand, might be more expensive and come with a larger footprint, but the precision of their frequency signal can&#;t be beat. &#;

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