Time – the fourth dimension – is often overlooked in the conversation about intelligent systems and anticipated emergence of ambient intelligence (AmI). As the IoT evolves into AmI, intelligence will be applied to make systems smarter, predictive, adaptive and more helpful in our everyday lives. However, without a time base, these systems will not be useful and AmI cannot be achieved.
A heartbeat is essential for any human or electrical system to function. Clocks, which act as the system’s heartbeat, are fundamental on many levels. First, clocks bring systems alive and keep them operating at the required speed. Second, precise timing signals are essential to ensure information measured by sensors is accurately distributed to the intelligent systems. Time is central to these systems and timing solutions must leverage novel technologies to rapidly improve for AmI to become the reality that many envision.
Intelligence needs accuracy
Let’s start with time in an intelligent system in use today – the smart home. Accurate time is required for a home control system to be intelligent, to learn habits and make data-based decisions that anticipate future behaviors and needs. Data, collected by sensors, is valid only if the time bases are synchronized, and systems can only be helpful if the sequence of events is recorded in the right timing order. Take the simple example of triggering a door sensor to switch on a hallway light. If the time record of the door sensor or light switch is off by more than a fraction of a second, the algorithm will not learn the correct timing of required events, and the light will not turn on at the right time.
In today’s connected world, the accuracy of the real time clock (RTC) is becoming more and more significant, and this importance will increase as more objects join the Internet of Everything. Let’s look at an early smart device example – the smart meter, designed to provide more information and capabilities for better energy usage. In this case, if the meter clock runs fast or is based on the wrong time interval for billing rates, any savings or benefits could be negated.
Structural monitoring of buildings and bridges or oil exploration with deployed seismic sensor networks are other examples of the need for precise time stamping of measured data for analysis and prediction. The accuracy of these systems and the predictive data they provide is directly proportional to the accuracy of the time synchronization of the individual nodes. Today’s solutions use expensive and power hungry GPS receivers or high powered RF networks to keep time. Tomorrow’s systems will be enabled by new timing technologies that provide high precision without the power consumption, bulky size and high expense. These systems will use miniature, ultra-low-power, precision MEMS TCXOs (temperature compensated oscillators) that consume only 1.5 µA. For indoor navigation based on triangulation time of flight using periodic timing synchronization techniques such as IEEE 1588 or duty cycled GPS transceivers, timing accuracy must be within 100 ns to 1 µs.
Low power needs accuracy
The importance of precise time keeping is also evident in sleep mode wakeup timers. Autonomous battery-driven nodes require power down intervals between computing and data transmissions to save power. In current applications, Bluetooth Low Energy (BLE) chip sets enable extended sleep times to save power. Each node has a predefined timeslot when it must communicate with the host to keep the connection alive and transfer data. If the wakeup and communications functions are executed more efficiently and accurately through more precise time, system power is greatly reduced.
No longer is accuracy sacrificed for energy efficiency – in contrast, accuracy reduces power consumption at the system level by extending sleep mode time.
At present, 4G LTE systems use DRX (discontinuous reception) cycles that require precise 1.5 µs sleep time to keep mobile phones connected with the network. Frequency stability must be maintained at an accuracy of better than 1.5 ppm over a time period of 1s to 2s in an environment of fast changing temperatures. Loss of connectivity would lead to large overhead in RF traffic and additional computing effort to reacquire the link to the base station. In a future where data is seamlessly transferred to and from ubiquitous smart devices at volumes well beyond today’s levels, connectivity becomes tremendously important.
Timekeeping of 1.5 µs is a challenge for legacy quartz-based timing components. Temperature gradients of more than 10°C per second are seen in today’s electronic devices. Uncompensated 200-ppm quartz crystals typically have more than 3 ppm per °C temperature slopes that cause frequency errors of more than 30 ppm in 1 second. Some of this effect may be mitigated by electronic temperature compensation; however, quartz oscillators are limited by slow thermal coupling between the quartz crystal resonator and electronic temperature compensation, making quartz TCXOs unable to effectively respond to fast temperature gradients. In contrast, MEMS TCXOs can maintain frequency accuracy under fast temperature changes and are a simple solution for these fast temperature gradients.
Networking a trillion connected devices
Today’s smart gadgets and phones are only the beginning. A future with a trillion connected devices will need various technologies to be connected. As the number of autonomous nodes rapidly grows, more devices will compete on the RF spectrum, creating bandwidth issues. To maximize bandwidth and dynamic range of RF channels, precise low phase noise frequency references are required. Many applications such car-to-car communications will require very precise frequencies and timing references to avoid interference, maintain reliable connections and enable seamless handover techniques by very accurate, synchronized cells.
In addition to making the spectrum more efficient, network densification is another way to add capacity to handle increased traffic. More cell sites will be strategically added in high concentration areas. But with the dense deployment of small cells, these compact low-power base stations will be placed in uncontrolled environments such as rooftops, basements, curbsides, and on poles. These locations will introduce added stressors such as faster and wider temperature changes, thermal shock, unpredictable airflow, and vibration – all of which can affect component performance, especially the stability of legacy quartz timing devices.
MEMS timing solutions will be essential in building robust systems by enabling higher performance in crowded, dynamic environments. New MEMS architectures, such as DualMEMS™ structures from SiTime, incorporate new sensing techniques to provide the required dynamic performance. Figure 2 shows the frequency stability performance of a MEMS-based TCXO over temperature.
Phase noise and its time domain counterpart jitter are often considered the most important characteristics of an oscillator after frequency stability. These parameters have an impact on system performance and quality of service because they directly affect data link/connectivity and data throughput speed. Figure 3 shows 20x better phase noise performance under vibration of a MEMS-based Elite Super-TCXO compared to a quartz-based TCXO.
Embed video: https://youtu.be/Bttd1f1wo6g
Video: Dynamic performance of an Elite Super-TCXO is compared to a best-in-class 50-ppb quartz TCXO with a side-by-side test of the two devices simultaneously subjected to air flow, temperature ramp, tap test, and VDD fluctuation. Results of the quartz TCXO are shown on the left; results of the Elite MEMS Super-TCXO are shown on the right.
Continued quest for low power and small size
In addition to higher accuracy and dynamic performance, ultra-low-power technologies will be essential in the development of AmI. Low power is especially critical in real time clocks used in autonomous battery-powered systems for timekeeping because they are always on. Here again, MEMS and the CMOS technologies supporting them, will play a vital role. Modern programmable MEMS kHz clocks consume less than 24 µW and have unique features to further reduce system power. Programmable NanoDrive output voltage is one such feature. Significant power is saved by programming the oscillator output voltage swing to a reduced level to match the downstream chip input voltage. Using lower frequencies, programmable down to 1 Hz in MEMS clocks for true pulse-per-second (PPS) timekeeping, is another feature that significantly reduces power consumption compared to fixed 32.768 kHz quartz crystals.
For intelligent systems and sensors to be adopted and embedded into our environment, and perhaps our bodies, they must be very small. Micro and nano technologies will enable the requisite miniaturization. MEMS resonators are less than 500 µm on each side and less than 200 µm high (as shown in Figure 5), making them 90% smaller than quartz-based resonators, which are limited by physics in further size reduction.
Chip vendors have started integrating high-precision MEMS resonators into their products to offer more integrated multi-chip modules (MCM) – a development made possible with encapsulated silicon MEMS technology. If the MEMS resonator is stacked on the IC, it consumes zero board space in the target system. Integration has many benefits beyond shrinking size. By integrating the clock, not only will devices have fewer external pins, they consume less power, have better performance and accuracy, and increase reliability and tamper resistance.
Technology working together
As ambient intelligence materializes, the number of digital objects interwoven into our environment will surge. These devices must be made inconspicuous and unobtrusive through miniaturization and integration. MEMS technology will play a critical role moving forward, making systems smaller and more integrated, as well as more reliable and lower power. And importantly, MEMS timing will make smart devices smarter by making them more accurate and higher performing – a necessity for systems to be predictive, adaptive and responsive. AmI will encompass a coalition of many technologies working together, and advanced timing solutions will be at the heart of these systems.