Feature: Automotive Design
immune to these effects and display greater reliability with less performance degradation compared to legacy quartz when subjected to temperature changes and vibration. SiTime’s Elite Platform contains silicon Super-TCXOs that
integrate two silicon oscillators fabricated on the same die. One is engineered to be relatively insensitive to temperature. In fact, its temperature response ensures uncompensated frequency stability better than ± 60 ppm, which is two-times better than quartz crystals. The second oscillator, sharing the same substrate and therefore closely thermally coupled, is designed with extremely linear temperature sensitivity. The ratio of their frequencies thus gives a precise and responsive temperature reading, accurate to within 30µK and with a bandwidth of hundreds of hertz. The Super-TCXO’s mixed- signal CMOS IC implements a temperature compensation algorithm that uses his reading to further reduce the frequency shift to less than ±0.1ppm between -40 °C and +105 °C. This thermally coupled design is not possible with quartz
crystal oscillators, which must have a separate temperature sensor that limits the compensation bandwidth to about 10 Hz and prevents tracking rapid temperature changes. Quartz oscillators can thus display abrupt frequency jumps when operated in challenging conditions such as fluctuating airflow and rapid temperature changes. By eliminating frequency jumps, drifts and activity dips,
these devices provide assured system performance at any temperature without the need for expensive incoming TCXO testing. The silicon-based oscillators also integrate an I2C interface, which helps save the system bill of materials (BOM) and simplify circuit design. In addition, high power-supply noise immunity permits further BOM savings by eliminating the dedicated LDO usually needed to provide a separate, low- noise power source for a quartz oscillator.
Important advancements in device
fabrication and packaging now enable silicon-based oscillators to combine
high frequency stability with reliability, ruggedness, miniaturisation and energy effi ciency that are superior to traditional quartz-based devices.
Shock and vibration can permanently damage the crystal or induce jitter that may increase the bit error rate of a high-speed link. The extremely low mass of a MEMS timer mitigates g-force effects, resulting in as much as 30-times better vibration immunity than quartz. With proprietary advanced analogue circuitry also built in, the devices display exceptional dynamic stability, ultra-low phase noise and a broad frequency range.
MEMS oscillators like the SiTime SiT9396/7 display significantly greater stability than quartz alternatives, over a wide range of vibration frequencies.
In addition, MEMS oscillators are more resilient to electromagnetic interference (EMI). This can be shown by injecting electromagnetic energy into a transverse electromagnetic (TEM) cell where the device under test (DUT) is mounted, as specified in IEC 62132-2.
The TempFlat and temperature-sensitive oscillators are separated on the die by less than 100µm.
Shock and Vibration MEMS resonators are more resilient to shock and vibration than quartz crystals due to their smaller size and lower mass.
20 October 2024
www.electronicsworld.co.uk
Conclusion Systems for smart driving need timing that is not only accurate but also robust and reliable. The oscillators needed to coordinate computing and communication within the vehicle have relied on quartz timing since the beginning of the electrification trend. New fabrication and sealing processes that are compatible with ordinary CMOS processes, combined with the convenience and affordability of standard backend packaging, now enable silicon-based oscillators to deliver a smaller, more flexible, robust, reliable and power- saving alternative with equal or better accuracy.
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