Selecting crystals for use in wireless connectivity functions
The past ten years has seen the demand for wireless connectivity between devices rise significantly. A vast range of consumer electronics devices are now incorporating the popular wireless standards of Bluetooth and Wi-Fi. While earlier trends have been towards adding a variety of accessories to mobile phone and tablet computers, the advent of the Internet of Things driven the demand for connectivity even higher. Increasingly smartphones are becoming used as the gateway or connection hub for consumer ‘tech’ such as sports fitness monitors and domestic appliances while home gateways will need to potentially host multiple wireless protocol standards and many different types of edge nodes and consumer home appliances. Both Bluetooth and Wi-Fi are the preferred wireless candidates for such applications. With a slower transmission rate compared to Wi-Fi it is designed for sending relatively small amounts of data over a short distance. Wi-Fi tends to complement Bluetooth by ideal for sending much larger volumes of data over longer distances.
Any wireless transceiver requires an accurate frequency source in order to provide reliable communication with other systems. In almost all cases a crystal is used to provide the reference signal from which the transmission and receive frequencies are derived. Crystals need to be extremely accurate in order to meet the wireless standards they are used for and would typically require accuracy in the order of ±20ppm. Factors affecting the accuracy include temperature, aging and frequency deviations.
Also, because of the rise in the number of accessories, such as wearable devices that incorporate wireless communication functions, an increasingly strong demand for miniaturization of components to be incorporated, including high-accuracy and small crystal units, are being observed.
The equivalent series resistance (ESR) is one of the most important characteristics of a crystal unit. If the ESR of a crystal unit is low this really helps the design engineer when selecting and matching an IC to the crystal unit during circuit design. Essentially, the ESR of a crystal unit is inversely proportional to the size of its crystal element, and therefore when crystal units become miniaturized, the size of their ESRs increases. An example of achieving this is the Cap Chip structure that Murata innovated in 2009. Used in their CERALOCK ceramic resonator devices Cap Chip is a simple structure where a plain ceramic plate is covered by a metal cap with the inside of the package providing a high space efficiency, making it possible to incorporate a larger crystal element than a general crystal unit. In this way ESR was reduced as compared to other crystal units with a comparable size. For example, Murata’s XRCFD/XRCMDseries crystal units have been designed for wireless communication use and have an airtight structure where the connection between the metal cap and the substrate is hermetically sealed using alloy. This minimized temperature-related frequency characteristics and aging changes as compared to the firm’s conventional products, allowing high accuracy and helping to achieve the frequency accuracy required by clocks for wireless communication.
Figure 1 – Construction of an example crystal unit
When selecting a crystal resonator for their wireless design engineers should be mindful of a number of criteria. Clearly the size is an important consideration and this is greatly influenced by the space efficiency of the crystal packaging. A device that has a flat substrate is the ideal choice as is one that employs metal sealing to achieve an airtight structure. Both these techniques contribute to achieving a high degree of frequency accuracy.
Figure 2 – Murata’s XRCED series
Figure 2 illustrates the exterior appearance of Murata’s XRCED Series products. Measuring just 1.2 × 1.0mm these crystals have a frequency accuracy of ±20ppm and suit use in a wide variety of wireless and processing applications.
Wireless communication functions are being increasingly incorporated into smartphones and tablet computers as well as into any kind of devices such as A/V and OA equipment and consumer electronics. As these devices are becoming more highly functional, electronics components in their IC-peripheral circuits are becoming more densely arranged. Moreover, as for wearable devices and the like, a reduction in their body sizes is highly demanded. In order to respond to such requirements next generation timing devices will also be in demand. Engineers can expect crystal units to continue to get smaller, meeting the demands that the fast-paced consumer and wearable electronics industry require.