MEMS Oscillators Cut Size and Power

March 25, 2016
These MEMS-based Super-TCXOs provide standard and custom LVCMOS output frequencies as high as 1 MHz, drawing microcurrents of energy and in miniature CSP housings.

Power consumption is a concern for any electronic device with a battery. For consumer and commercial mobile/portable products in the near future, the majority will require energy-efficient circuits and components to extend battery life. Time-keeping is essential to saving power in many circuits—for example, starting and ending “sleep modes” to conserve energy when a circuit is not in use.

Fortunately, microelectromechanical-systems (MEMS) and analog technology from SiTime enabled the development of low-power timing components: temperature-compensated crystal oscillators (TCXOs) with a wide range of uses, from sleep clocks to timing devices for Internet-of-Things (IoT) sensors. The firm recently unveiled three products from its Super-TCXO family, with ±5 ppm stability at 32.768 kHz and at factory-programmable frequencies from 1 Hz to 1 MHz, all contained within tiny chip-scale packages (CSPs).

For applications where even microamperes of current per active component can make a difference in the power consumption of a systems design, MEMS is an effective means for generating frequencies and saving power. By leveraging the low-power operation of silicon CMOS circuitry, the new Super-TCXOs provide LVCMOS outputs with excellent stability over wide operating temperature ranges. They achieve clock frequencies as high as 1 MHz, and at a fixed frequency (32.768 kHz) which has become popular for numerous timing applications both in consumer and industrial electronic products.

SiTime has built a strong reputation for its MEMS oscillators, developing miniature, cost-effective alternatives to crystal resonators and oscillators and other oscillator technologies for audio through microwave frequencies. In addition to combining electrical and mechanical devices in its MEMS technology, the firm has also developed sophisticated analog circuitry to maintain frequency stability over time and temperature. This circuitry also pairs with the Super-TCXOs to manage power consumption.

Although each Super-TCXO is supplied in a miniature CSP, it is actually a MEMS resonator with programmable analog circuitry within that package. Analog circuitry includes temperature sensing and regulation, a programmable divider, a low-power fractional-N phase-lock loop (PLL), voltage regulators, sustaining amplifier, and a programmable driver. In effect, SiTime has succeeded in bringing a technology once considered exotic to the masses.

The MEMS oscillators are fabricated with the firm’s unique MEMS First process, which includes forming a protective seal around the MEMS resonator. This is achieved by the firm’s EpiSeal process, during which the MEMS resonator is annealed at temperatures exceeding +1000°C. This creates a strong vacuum chamber around the resonator for protection, ease of packaging, improved performance, and long-term reliability.

The EpiSeal process grows a polysilicon cap on top of the resonator cavity, eliminating the need for additional cap wafers or custom packaging. As a result, the Super-TCXOs can be treated like any other packaged CMOS device, not as some “exotic” MEMS device. In addition, it allows the integration of additional components and circuitry within the MEMS oscillator package for improved performance and ease of use, plus the elimination of such discrete circuits as temperature compensation outside of the oscillator package.   

As an example, model SiT1566 is a low-jitter Super-TCXO with 32.768-kHz output and ±5 ppm total frequency stability under all conditions, all inclusive, without overmolding. Its current draw is a mere 4.5 μA for a supply voltage of 1.8 V ±10%. The integrated peak-to-peak phase jitter is typically 1.8 ns RMS and maximum of 2.5 ns RMS, while the RMS period jitter is typically 2.5 ns and worst-case performance of 4.0 ms for 10,000 measurement samples. The peak-to-peak period jitter is typically 20 ns, also for 10,000 samples.

The SiT1566’s temperature coefficient is calibrated and corrected over temperature by a portion of the programmable analog circuitry included within each package: an active temperature correction circuit. The miniature MEMS TCXO achieves dynamic temperature frequency response of ±0.5 ppm/s for a temperature ramp of as much as 1.5°C/s. SiT1566 models are available for operating temperature ranges of -20 to +70°C and -40 to +85°C.

Amazingly, the MEMS resonator and the analog circuitry are housed in a CSP measuring 1.5 × 0.8 × 0.6 mm (see figure) and consuming just 1.2 mm2 of circuit-board area. These MEMS Super-TCXOs are designed to provide significant power savings in electronic products—and in communication networks that rely on precision timing devices such as this—to send different parts of the network into “sleep” modes to conserve power. The SiT1566 becomes active quickly to conserve power, with a power-on start-up time of 300 ms to reach 90% of the final rated output voltage.

Model SiT1568 is a 32.768-kHz Super-TCXO with in-system auto-calibration. It can compensate for assembly-related frequency errors by connection to a 10-MHz external reference oscillator and running the auto-calibration routine during final system test. The frequency stability of a model SiT1568 can be up to ±25 ppm stability following overmolding and prior to autocalibration, and then reduced to less than ±5 ppm all-inclusive stability after auto-calibration. It has current draw of just 4.5 μA from a supply at 1.8 V ±10%. It is also supplied in the same miniature CSP housing as the SiT1566, with temperature-related specifications similar to those of the SiT1566.

The third member of the Super-TCXO family is model SiT1576, with factory-programmable LVCMOS output frequency from 1 Hz to 1 MHz. It is also specified for ±5 ppm total frequency stability under all conditions. Its current draw is a function of frequency for a supply voltage of 1.8 V ±10%, only 2 μA for a 1-Hz output, 4.5 μA for a 33-kHz output, 8 μA for a 100-kHz output, and 20 μA for a 1-MHz output.

As with the 32.768-kHz oscillators, the integrated peak-to-peak phase jitter is typically 1.8 ns RMS and maximum of 2.5 ns RMS, while the RMS period jitter is typically 2.5 ns and worst-case performance of 4.0 ms. The peak-to-peak period jitter is typically 20 ns.

These LVCMOS clocks are a fraction of the size of traditional quartz crystal oscillators, with extremely low power requirements for wearable and portable electronic products (including for home, industrial, and medical IoT sensors). The short turn-on times and high frequency accuracy can result in accurate setting of network sleep times and reliable saving of power at system levels.

In addition, the small size of the package simplifies placement in most circuit and system designs. The MEMS oscillators are capable of driving multiple loads, such as the wakeup clock for Bluetooth circuitry and the timing device for a microprocessor in a battery powered design. In doing so, it can save design time, components, cost, and design size. All three Super-TCXOs products are lead-free and both RoHS- and REACH-compliant. Versions available for use over the two operating temperature ranges: -20 to +70°C and -40 to +85°C.

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About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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