Developing compact antenna couplers for TransferJet wireless data-transfer technology
TransferJet technology enables data transfer at a peak speed of 560 Mbps with an effective throughput up to 375 Mbps – although the system can adjust the transmission rate depending on the wireless environment. The maximum range of operation is in the order of a few centimeters and the network topology is always point-to-point between two active (powered) devices. The technology has been described as fast data transfer with the ease of using a USB cable – just without the cable.
A key aspect of TransferJet is the close proximity required for data transfer. If two TransferJet devices are more than a few centimeters apart, then they should not connect or even detect each other. But it is not necessarily simple to restrict operation to such a short range. This is a difficult task for a conventional antenna, as a typical antenna device is designed to radiate a signal as far as possible. Therefore an important element of TransferJet concerns the development of antenna coupler technology. An antenna coupler is a device that transfers signals between couplers by using electric field induction, working like an antenna in general wireless devices. However, the antenna coupler characteristically achieves a high gain at close range and shows rapid attenuation when it departs from an antenna coupler in another electronic device.
The short range makes it possible to operate in the ‘near field’ of the radio signal using very little transmit power – in fact less than -70 dBm per MHz is required on average – which in conjunction with the point-to-point network topology helps to minimize overall power consumption. Additionally, as the antenna coupler’s coupling electrode generates non-polarized longitudinal electric field waves, two TransferJet-compliant devices do not have to be precisely oriented to achieve a good connection and fast data transfer. Table 1 shows the TransferJet wireless technology specifications.
Over many years, Murata Manufacturing has developed a variety of different antennas for mobile phones and other wireless communication devices – focusing on the miniaturization of electronic components and making it possible for high-density board packaging, resulting in the development of highly advanced, multi-functional devices. The company’s antenna coupler products have been developed based on this miniaturization concept; and specifically for TransferJet applications, Murata has developed the LDA5M_L series of antenna couplers (see Figure 1), which are believed to be the smallest in the industry. See Table 2 for series specifications.
Two key products in the series have been selected to meet a broad range of needs: the first, measuring 5.4- x 4.5- x 1.5-mm, is a low-height device intended for mobile terminals; the second, measuring 5.4- x 4.5- x 2.0-mm, is suitable for mobile terminals and for large couplers that require more advanced functionality. In developing the series, Murata has also made full use of simulation technologies to find an optimal design based on the unique electrode structure, in addition to developing wider design variations suitable for more diversified market requirements.
These devices have been developed using Murata’s unique high-frequency design technology and multi-layer technology, which makes use of advanced low-temperature co-fired ceramic (LTCC) technology, which the company has continually refined in the development of its multi-layer products. The technology enables a highly reliable structure and stable temperature characteristics over the -40 to +85 °C range. In addition, using innovative ceramic dielectric material that offers a high dielectric constant, Murata has achieved a compact and low-profile design that allows large attenuation outside the bandwidth of 4.2 to 4.76 GHz used by TransferJet, while keeping flat and high-level coupling characteristics within the required bandwidth.
The recommended specifications for antenna couplers are described in the standard developed by the TransferJet Consortium, in the form of the degree of coupling with a standard coupler specified by the consortium. Figure 2 shows the coupling characteristics of the series when mounted on a standard board, shown in Figure 3, and placed face to face with the standard coupler at a distance of 15 mm. The product satisfies the recommended specifications and shows good performance compared to a competitive product with a similar size.
Figure 2: Coupler characteristics. Click on image to enlarge.
The series controls attenuation using its unique electrode structure, thereby making it possible to keep flat coupling characteristics within the 4.2 to 4.76 GHz bandwidth, while simultaneously delivering high attenuation outside of the required bandwidth. This ability complements the functionality of the filters used in the front-end radio-frequency part of TransferJet.
As was previously mentioned, an antenna coupler transfers data by using the electric induction field between couplers. However, if a metal material is adjacent to the coupler then the input impedance and parasitic capacitance will vary and, as a result, change the coupling condition. In addition, the GND condition of the internal layer of the mounting board will change the coupling condition. To deal with this issue, Murata has a considerable amount of application data based on assumable conditions for actual devices, which it can make available for customers and which the company believes can help make a significant contribution to improve design efficiency in the early phases of the design process.
Beyond the development of the LDA5M_L series, Murata continues to develop leading-edge compact and low-profile antenna-coupler products to help customers get to market quickly with new multi-functional and highly advanced electronic devices that utilize the fast emerging TransferJet technology.