XCXO: An Ultra-low Cost Ultra-high Accuracy Clock System for Wireless Sensor Networks in Harsh Remote Outdoor Environments

As the world ecosystem comes under increasing duress from the effects of global warming, volatile organic compounds (and other air pollutants), oil spills, pesticides, stronger coastal storms, and more frequent tornadoes and forest fires, early warning of and effective response to micro, local, and regional ecological problems is critical. Capturing this data is an immense challenge requiring long-term study over very large areas. Further, such a system must operate virtually free of infrastructure (power and hardline communications networks) to reduce the cost and environmental impact of deploying and maintaining it. Such systems, as envisioned by the scientific community, consist of a large quantity of sensors each attached to a small battery operated microprocessor replete with a radio communications transceiver. Each of these sensor terminals is called a node and the entirety of nodes is termed a Wireless Sensor Network (WSN). WSN’s are, therefore, emerging as a vital element in humanity’s response to a changing climate. The single biggest impediment to a node’s battery-powered lifetime is the energy spent during radio communication, and secondary to that, the time spent in its “awake” (as opposed to its low-power shutdown “sleep” state). Reduce these times, and lifetime improves substantially. However, as soon as nodes in the network begin sleeping and are correspondingly offline, other nodes in the network that are still awake can no longer use them as a communications hub to route sensor data back to a command-and-control station (referred to as a “gateway” node). To optimize sleep time and network performance simultaneously all of the nodes must synchronize their internal clocks and sleep and wake at the same time (we are aware that this statement is somewhat of a generality and numerous works on WSN scheduling exist, but our axiom to follow – that better synchrony yields better lifetime – still holds in these cases). Commodity time references in WSN’s, and indeed consumer and industrial products in general, typically consist of a quartz crystal driven by a Colpitts oscillator. This configuration is popular because it offers substantially better performance than switched resistive-capacitive networks