Use MEMS Oscillators for Precise Timing in High-Performance Networks

Technical Overview: Accurate timing and synchronization are essential for position, navigation, and timing (PNT) systems, datacenters, communications networks, and distributed electronic platforms. Oscillators must maintain stability when reference signals are unavailable and when exposed to temperature changes, airflow, vibration, and shock. This article explains clock Stratum levels and compares common oscillator architectures, including temperature-compensated oscillators (TCXOs), oven-controlled oscillators (OCXOs), and digitally controlled OCXOs (DCOCXOs). It then examines SiTime microelectromechanical systems (MEMS) timing devices, highlighting the SiT5711 OCXO and the digitally tunable SiT5721 DCOCXO. Their compact packaging, environmental resilience, frequency stability, holdover performance, and integration simplify precision timing designs for demanding network applications. (Overview courtesy of ChatGPT)

Timing circuits perform critical functions across datacenters, network infrastructure, and advanced communications. They pace processors, set tuning frequencies, enable precision measurement, support data recovery and retiming clocks, and enable high-speed network synchronization. As network speeds and frequencies increase, achieving and maintaining timing-oscillator capabilities, such as initial precision and short and long-term stability, become increasingly challenging. In harsh environments, temperature shifts, shock and vibration, voltage variations, and airflow can compound these effects, shifting the operating frequency beyond stringent limits.

While the classic crystal oscillator has served designers well, advances in microelectromechanical systems (MEMS) technology have made MEMS devices a practical option for high-performance applications.

This article discusses the timing challenges that designers of high-performance systems face. It then introduces MEMS devices from SiTime Corp. that address these challenges with superior performance across multiple parameters.

How timing challenges vary with application

“Timing” and “oscillator” are broad terms with considerable overlap, but they also differ in important ways and priorities. Consider two applications: position, navigation, and timing (PNT) and network synchronization. While both have a clock function at their core, each has different critical parameters.

For PNT systems, precision timing begins with a simple question: what is the time? This synchronization enables systems to operate as coordinated networks rather than as isolated, independent components. Communications systems need this synchronization to maintain encrypted waveforms, frequency-hopping patterns, and low-probability-of-intercept/low-probability-of-detection signals. Radar platforms rely on precise pulse timing and coherent processing to preserve range resolution and target discrimination, while electronic warfare systems synchronize sensing and emissions within tightly controlled timing windows.

Navigation systems also rely on a stable time reference. Inertial navigation systems require precise timing to limit accumulated drift when GNSS updates are unavailable, a situation that is increasingly common due to jamming, spoofing, or sky blockage. Distributed sensor networks must timestamp observations accurately to correlate data across platforms.

When the reference disappears, the effects cascade as communications links lose synchronization, while radar and electronic warfare systems lose coherent processing advantages. Inertial navigation drift grows without correction, sensor timestamps diverge, and operations lose time-based coordination.

Operating without GNSS timing requires a local timing architecture that maintains synchronization during signal loss. Because all oscillators drift over time, the key performance parameter is holdover, which measures how accurately a local oscillator maintains time after losing its reference source.

High-quality oscillators maintain accurate timing long enough to bridge outages or acquire alternative references with acceptable error limits.

For network synchronization, as networks link dispersed nodes, it is essential that these nodes be synchronized with each other’s clocks to achieve optimum performance, an acceptable bit error rate (BER), and data-link recovery. To ensure this, Telcordia (formerly Bell Labs) led the industry in promulgating key network timing specifications and performance levels.

Five Stratum levels are defined by Telcordia GR-1244 Clocks for a synchronized network such as one supporting 5G, defining the intrinsic accuracy of reference clocks needed for certain levels of network. These are (a lower stratum level indicates a more accurate clock):

• Stratum 1
• Stratum 2
• Stratum 3 and 3E
• Stratum 4

Critical parameters for Stratum 3E oscillators include frequency stability, frequency-versus-temperature slope, daily and long-term aging, Allan deviation, activity dips and micro jumps, holdover time, and power consumption.

For example, a Stratum 3 clock has a free-run stability of ±4.6 parts per million (ppm) over 20 years and a holdover requirement of ±0.37 ppm over 24 hours (hrs), both including frequency errors under all conditions. A Stratum 3E clock is a more accurate version of a Stratum 3, with the same ±4.6 ppm free-run stability, but with a holdover spec of ±0.01 ppm over 24 hrs, 37 times tighter than Stratum 3.

Oscillator types match application priorities

Oscillator types vary to meet application demands. Many enhancements build on the basic oscillator (XO), an active device that integrates the resonator and oscillator circuit into a single package. Among the most common variations are:

• Temperature-compensated XO (TCXO): A TCXO incorporates temperature compensation to correct for the resonator's frequency-versus-temperature characteristic.
• Voltage-controlled XO (VCXO): A VCXO includes a control voltage pin that allows the user or the circuit to control (“pull”) the output frequency by a modest amount around the nominal frequency.
• Oven-controlled XO (OCXO): The OCXO provides temperature compensation and uses an oven to maintain nearly constant oscillator temperature even as ambient temperature varies.
• Digitally controlled OCXO (DCOCXO): The DCOCXO is a more recent addition to the family tree.

MEMS for higher-performance oscillators

Because of their long history, many designers automatically associate “oscillator” with a quartz crystal core. In the basic design, a pure piece of quartz crystal (often denoted as XTAL) is “stimulated” to oscillate at its natural mechanical resonant frequency using the piezoelectric effect and a circuit operating in a self-reinforcing feedback loop. The XO (crystal plus support circuit) can provide frequencies from tens of kilohertz (kHz) up to hundreds of megahertz (MHz). However, current performance and functional demands now exceed what these units can meet without added cost and complexity. MEMS technology and devices offer a viable and attractive alternative to quartz-based designs.

Basic MEMS technology first appeared commercially in the 1990s as accelerometers to trigger airbag inflation. Innovators have extended this silicon-based solid-state technology to create many new sensor types as well as oscillators, making it mature and field-proven, with decades of design, production, and field experience.

SiTime’s oscillators use a MEMS resonator coupled with an analog CMOS circuit (Figure 1, top) that integrates multiple sophisticated functions for configuration, compensation, and optimized performance (Figure 1, bottom). All support circuitry, including the oscillator circuit, various compensation types, and drivers for output loads, is integrated into the silicon design.

Figure 1: SiTime oscillators use a design that combines a MEMS resonator with an analog CMOS circuit (top); the circuit integrates multiple sophisticated functions for configuration, compensation, and optimized performance (bottom). (Image source: SiTime)

MEMS are an attractive alternative to quartz designs in high-performance, tight-tolerance applications because they can provide high initial accuracy and stability, especially against temperature-induced drift.

MEMS also let designers add features to the silicon assembly to provide unique functions that compensate for unavoidable shortcomings in basic materials. Their small resonator mass and silicon construction make MEMS oscillators highly resistant to shock, vibration, circuit-board bending, and fatigue compared with quartz devices. Hermetic sealing also helps prevent contamination that could degrade long-term performance.

The associated analog CMOS die features:

• High-precision temperature sensing
• High-performance temperature compensation
• Inclusion of advanced mixed-signal circuitry

Examples of MEMS timing devices

Two SiTime oscillator examples showcase the capabilities and versatility of MEMS timing devices. The SIT5711AI-KW-33E-10.000000F is a 10 MHz, 3.3 V “ovenized” OCXO in a 10-lead, 9.00 mm × 7.00 mm × 6.28 mm high package. It integrates temperature and voltage-compensation circuitry within the device package. It also maintains temperature within a narrow band via a closed-loop arrangement to further minimize the detrimental impact of temperature changes, yielding exceptional dynamic stability despite airflow or rapid temperature ramping.

Specifically, its frequency stability is ±5 parts per billion (ppb) (Figure 2, left), and its frequency slope versus temperature (ΔF/ΔT) is ±50 parts per trillion (ppt)/°C (Figure 2, right).

Figure 2: The SiT5711 device's high performance is shown by two key parameters: frequency stability (left) and frequency slope versus temperature (right). (Image source: SiTime)

Other key features include:

• A 2 × 10^-11 Allan deviation at 10 seconds (s) averaging time, under airflow
• 0.1 ppb/g vibration, suitable for outdoor pole-mounted equipment
• Resistance to microphonic and/or board bending effects
• On-chip regulators for power-supply noise filtering
• ±0.25 ppb daily aging and ±500 ppb 20-year aging
• Excellent holdover through a wide range of conditions
• An LVCMOS output with user-controlled enable/disable; other SiT5711 configurations offer clipped-sinewave output
• No activity dips or micro jumps
• GR-1244 Stratum 3E compliance

While the SiT5711 comes in a compact package, it is also available in larger packages as a drop-in replacement for legacy crystal units (Figure 3), allowing an upgrade without redesigning the circuit board.

Figure 3: Although the SiT5711 is offered in an ultra-miniature 9 × 7 mm package, it is also available in larger packages as a drop-in replacement for legacy devices. (Image source: SiTime)

The second example is the SIT5721AI-KW333-T-10.000000F (Figure 4), a highly integrated 10 MHz DCOCXO that enables output frequency tuning via an I²C interface (SCL, SDA). It is suitable for specialized applications that require slightly shifting the output frequency to adapt to channel issues. The device offers 5 ppt frequency-pull resolution over a ±6.25 ppm range.

Figure 4: The SiT5721 application schematic highlights digital “pullability” of the nominal frequency via an I²C interface. (Image source: SiTime)

The SiT5721’s design leverages SiTime’s unique DualMEMS and TurboCompensation temperature-sensing technologies to deliver robust dynamic performance and highly stable timing (Figure 5), even under impinging airflow, temperature perturbations, vibration, shock, and electromagnetic interference (EMI) that would typically cause drift and other performance issues.

Figure 5: Shown are the SiT5721’s frequency stability (left) and frequency slope (right) versus temperature. (Image source: SiTime)

The SIT5721AI-KW333-T-10.000000F features:

• ±5 ppb frequency stability over temperature
• ±0.04 ppb/°C typical frequency slope versus temperature
• -40°C to 85°C operating temperature range
• A 1.4 × 10^-11 Allan deviation at 10-second averaging time
• Exceptional dynamic stability under airflow and rapid temperature changes
• Excellent holdover over a wide range of conditions
• Integrated regulators for on-chip power-supply noise filtering and excellent power-supply rejection ratio (PSRR)
• GR-1244 Stratum 3E compliance
• Resistance to shock and vibration
• A 3.3 V supply
• An LVCMOS output; other configurations offer clipped-sinewave output
• A 10-lead, 9.00 × 7.00 × 5.6 mm high package

Conclusion

Meeting the timing requirements of datacenters, network infrastructure, and advanced communications systems is challenging. By using MEMS oscillators from SiTime, designers can benefit from accuracy, stability, relative insensitivity to mechanical and thermal perturbations, and high resistance to shock and vibration. These miniature devices show what silicon-based resonator and oscillator technology can achieve, while offering engineers ease of use, simple design-in, and legacy component replacement.

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2. Shock and Vibration Performance Comparison of MEMS and Quartz-based Oscillators
3. Endura Low Phase Noise Super-TCXOs: Time Synchronization and RF Systems
4. MEMS-Based Resonators and Oscillators are Now Replacing Quartz
5. Do you know when to use a crystal or an oscillator? The wrong answer can cost you.