Low SWaP MEMS OCXOs Prolong Holdover in A-PNT—Even in Rough Environments

Available anywhere, anytime, around the world, GPS provides precise positioning and navigation for the armed forces. Military operations are built around GPS-based positioning, navigation and timing (PNT) technology because GPS has greater accuracy, availability and reliability compared to legacy systems. Resilient and reliable PNT is critical to mission success.

Unfortunately, GPS signals are vulnerable to jamming, spoofing and cyber-attacks. These attacks have increased over the years. In fact, GPS World states that in March of this year the PNT systems of over 1,600 aircraft were impacted across Eastern Europe by GPS jamming. In addition to GPS attacks, unintentional environmental factors can corrupt or break GPS signals. Cloud cover, tree canopies, rough terrain or urban canyons, ionospheric storms and solar flares can all disrupt GPS signals. Whether signal disruption is intentional or unintentional, it throws off the accuracy and reliability of PNT, potentially imperiling defense operations. Assured PNT (A-PNT) systems augment GPS to mitigate these disruptions for more resilient and reliable PNT.

Keeping accurate time is cornerstone for reliable positioning and navigation and enables the synchronization of critical tactical systems. This article covers why a novel commercial-off-the-shelf (COTS) MEMS oven-controlled oscillator (OCXO), low in low size, weight, and power (SWaP), is an attractive option for military A-PNT systems when compared with quartz OCXOs or even chip-scale atomic clocks (CSACs).

The Local Clock Becomes the Core of A-PNT

GPS provides the reference clock, the source of timing, for PNT systems. Reference timing is critical not only for position and navigation but also for synchronizing systems used in military operations such as command and control, communications and intelligence, surveillance and reconnaissance (ISR).

In A-PNT systems, if the primary timing reference cannot be trusted or is not available, the local clock becomes the source of timing accuracy. The local clock needs to maintain accurate time during GPS signal loss whether it is for seconds, minutes, hours or days. The ability for PNT systems to operate to a specified level of accuracy even during GPS signal loss is known as holdover. Holdover is measured as a time error over a specified time interval. Holdover performance is defined by the local clock.

Local clocks need to be resilient to environmental conditions, including temperature change, acceleration and vibration. They may also drift over time after calibration to the external reference clock. Because of all of this, selecting a local clock requires special care. For instance, a local oscillator clock would need to meet holdover requirements within expected stress parameters for the duration of a mission, such as temperature changes and vibrations. The holdover performance is determined by the oscillator stability over temperature, Allan deviation and daily aging, as well as under mechanical forces such as shock and vibration.

Finding a low SWaP oscillator that provides holdover of 1 µs time error over 24 hours, under most military mission profiles, is not an easy task. Stringent requirements would traditionally dictate a custom, vibration-compensated OCXO which can have long lead times and be expensive. Another alternative is a miniaturized atomic clock, which also has long lead times, is even more expensive, performs poorly under vibration and is large and heavy. Today, however, there is a new superior holdover alternative for A-PNT systems: the Endura Epoch Platform™ OCXOs. The new MEMS Endura Epoch OCXO delivers A-PNT resilience and low SWaP.

Delivering Industry Benchmark Performance with MEMS Timing Technology

The Epoch Platform combines advanced silicon MEMS resonator technology with optimized control circuitry and innovative packaging to overcome the limitations of traditional quartz OCXOs. Since the introduction of silicon MEMS oscillators to the market 15 years ago, SiTime has continually improved performance of MEMS timing technology through continuous and focused innovation.

SiTime uses a silicon MEMS resonator as the frequency source. The MEMS resonator is about 1,000 times smaller than a typical quartz resonator and is made of high-strength silicon material encapsulated in an ultra-clean cavity, eliminating contaminations that cause frequency jumps, aging or failure. Silicon MEMS technology leverages high-purity semiconductor processes for unmatched quality and reliability. The small weight, superior material strength and structural design of the MEMS resonator enable shock- and vibration-resistant designs. Quartz OCXOs by comparison, typically sacrifice SWaP and performance to achieve the same levels of shock and vibration resistance as an Epoch OCXO.

Endura Epoch OCXOs outperform commercial vibration-rated OCXOs in the following areas:

• 0.01 ppb/g g-sensitivity - 70x better
• ±1 ppb stability over -40 to +95°C temperature range - 20x better
• ±0.01 ppb/°C ΔF/ΔT, stability over temperature slope - 10x better
• 5E-12 Allan deviation (ADEV), 0.1 to 1,000 sec - 4x better
• 0.1 ppb/day daily aging - 10x better

Endura Epoch OCXOs solve the size and power issues of traditional vibration rated OCXOs:

• 9.0 x 7.0 x 3.6 mm3 surface mount package - up to 20X smaller
• 0.35 grams - up to 100X lighter than vibration compensated quartz OCXO designs
• 420 mW typical (3.3V) power consumption - up to 2X lower power

The Epoch OCXOs use SiTime proprietary DualMEMS® technology which combines a TempFlat® MEMS resonator and a TempSense MEMS temperature sensor on the same die. While the TempFlat resonator is optimized for an ultra-low frequency temperature coefficient, the TempSense sensor is optimized for temperature sensing. The difference in frequency between the two MEMS resonators co-located on the same die provides an extremely accurate reading of the temperature-in the order of 30 micro-Kelvin-and it allows very accurate heater control and fast temperature compensation. Proprietary mixed-signal circuits can be optimized to provide the tight controls needed for maximum stability with excellent immunity to external noise.

Endura Epoch MEMS OCXOs leverage high-volume industry standard semiconductor and assembly manufacturing for unmatched quality and reliability, short lead times and availability. Manufacturing is managed according to statistical process control and Six Sigma specifications to deliver part-to-part consistency that customers see in their system performance.

The Endura Epoch OCXO offers performance and reliability in a compact easy-to-use package that promises to enable a new level of A-PNT system performance in GPS-contested environments. SiTime maintains close relationships with our customers to understand use cases and is committed to providing market-leading solutions that meet their needs now and into the future. The Endura Epoch MEMS OCXOs will be readily available over a range of frequencies from 10 MHz to 220 MHz with I2C or SPI frequency pull options.

Learn more and download our Endura Ruggedized Timing Selector Guide.

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