MEMS oscillator and quartz clock in the car
For automobiles, a clock generator with temperature resistance and aging resistance is required.
Driver assistance systems in modern vehicles place high demands on the accuracy of the clock signals of their subsystems. Compared to conventional quartz oscillators, MEMS oscillators can better meet these requirements.
Modern vehicles require precise timing signals for the driver assistance system (ADAS-Advanced Driver Assistance System), with cameras, ultrasonic sensors, lidars and radars, as well as for infotainment systems, in-vehicle networks and more. ADAS for autonomous self-propelled vehicles requires a larger oscillator and more modules for timing synchronization.
Classic reasons for semiconductor devices - smaller, faster, cheaper (and / or higher performance, higher reliability) - has been tempting for over 50 years. However, modern digital circuits for automobiles with critical timing requirements are increasingly pushing the previous quartz oscillators to the limit - MEMS oscillators (MEMS) are provided here as an alternative.
Oscillator Technology Overview
The quartz crystal is mechanically cut from the quartz material to define the appropriate frequency, ground to the desired size, then inserted into the housing and sealed. Their thin crystal structure makes them susceptible to vibration damage. In addition, the purity of these blocks is relatively low in manufacturing, which may result in limitations on the quality of the oscillator.
Due to their relatively large size, quartz does not have a special resistance to high shock and vibration loads. In addition, quartz has non-linear temperature characteristics, so they can only meet the required clock frequency over a narrow temperature range.
Unlike MEMS oscillators: their production is carried out in semiconductor factories under high purity conditions of the IC. The reliability of the MEMS oscillator is 20 times that of the quartz oscillator, it can withstand 500 times of the impact load, and the frequency stability is increased by 5 times during vibration.
Due to this design, the MEMS oscillator is very small and sturdy. On the other hand, quartz crystals have a certain size; their price increases as miniaturization increases. In the first particularly narrow automotive application, some cameras must be rebuilt into vehicles due to the size of the quartz.
Table 1. Five temperature levels defined according to AEC-Q100 (0
In this application, MEMS devices are a logical alternative. New automotive applications such as ADAS typically require components with smaller outer casings. Therefore, the size of the MEMS oscillator provides additional parameters to replace the quartz oscillator.
Another advantage of MEMS oscillators is their consistent frequency stability at very high temperatures. Today's MEMS oscillators can meet Class 1 temperature (according to AEC-Q100, ambient temperature is -40 ° C to 125 ° C). The next-generation MEMS oscillators are hotter; therefore, they can also be used in areas of the vehicle that require a Class 0 temperature (-40°C to 150°C) (see Table 1).
More demanding automotive applications are coming soon
In an automotive environment, the ambient temperature of the installation site and/or the desired location of the oscillator on the printed circuit board may result in high operating temperatures. As the performance and processing power of graphics processing units (GPUs) and microprocessors (CPUs) and their associated power ICs continue to increase, traditional quartz oscillators are increasingly reaching their limits:
• More communication in the vehicle requires a higher power IC. This requires more heat loss, so the local ambient temperature of adjacent components is higher.
• The microprocessor in the entertainment system provides a significant amount of heat, although most components are specified inside the vehicle as temperature class 2 (to 105 ° C), the clock generator must correspond to the processor temperature class (up to 125) °C). 1
The crystal oscillator is placed in good stability and is usually close to the relevant IC, which means that the temperature level has been so far 3. This is changing: a powerful processor can easily heat the quartz so that it is needed due to temperature Drift of the frequency range.
To continue using the crystal oscillator here, place it away from the processor. However, this requires more area on the board. Alternatively, a quartz oscillator with higher thermal stability (-50 ° C to 125 ° C) can be used - but this can be three times more expensive.
In contrast, MEMS oscillators have circuits for active temperature compensation. As the temperature changes, the MEMS oscillator circuit can be calibrated up to 30 times per second; for this, it senses the temperature and adjusts the frequency to keep the output frequency constant.
For high temperature applications, this provides excellent temperature stability - down to ±20 ppm. Compared to the cost of high stability quartz oscillators, MEMS oscillators can save costs.
This article is from Allicdata Electronics Limited. Reprinted need to indicate the source.