ISM Band Transceivers: Enabling the Wireless Link for a Diverse Range of Applications
By Analog Devices
Governments regulate the use of different frequency bands and a license is required to operate in certain frequency bands. The ISM (Industrial, Scientific and Medical) frequncy bands were defined by the International Telecommunication Union Radiocommunication Sector (ITU-R). License-free operation is generally allowed in these bands though there are some variations in national regulations. Figure 1 shows frequency bands which can be used without a license1 though requiring compliance with local regulations, for example, in terms of emissions.
Wireless connectivity has grown substantially over recent years, with a very diverse range of applications, varying from RF remote controls to Smart Metering applications. The drivers for this growth include the ease and flexibility enabled by wireless connectivity, avoiding retrofiting cables to existing structures for example. Advances in technology have also enabled the development of newer applications. Additionally, changes in the global enviroment with for example, the emphasis on energy conservation which has emerged in recent times, have driven new applications. Applications for wireless connectivity in metering began with automatic meter reading (AMR), where a van could read meters by simply driving by the houses. Wireless connectivity in metering has evolved to include more sophisticated systems which enable utilities to more finely monitor usage with multiple readings each day. They may implement real-time pricing to promote better energy efficiency. Feeding this information back to the consumer enables consumers to save money by running major appliances, such as washers and dryers, during lower cost, off-peak periods; and utility companies can avoid building new power plants because less capacity is required during peak periods.
As might be expected, such a wide range of applications have varying requirements. For example, a remote control might be required to send a small amount of data within the range of a house, with perhaps a point-to-point connection between two nodes. An RF remote control offers an advantage over the traditional IR remotes in terms of not requiring a line of site and enabling the option of bi-directional operation. Recently developed protocols which are apt for such RF remote controls are the io-homecontrol standard and the ZigBee RF4CE (Radio Frequency for Consumer Electronics) standard.
A number of companies who produce products for the home have worked together to develop the io-homecontrol standard. This enables compatibility between io-homecontrol products such as roof windows and roller blinds from these companies. Thus, one remote control can control multiple products in the home. Refer to the following website for further information: http://www.io-homecontrol.com/en/homepage.html. Use of an existing protocol can enable faster development times. ADI’s ADF7022 is an io-homecontrol transceiver incorporating Layers 1 & 2 as well as time critical elements of Layer 3 of this protocol, further facilitating fast development. The ADF7022 enables robust connectivty with long battery life. It incorporates smart wake up features which allow it to wake at defined intervals, do a channel scan, and only wake the host when a valid packet is received.
The ZigBee RF4CE standard emerged from a consortium which included a number of home entertainment equipment manufacturers. This standard is based on the IEEE 802.15.4 PHY. ADI’s ADF7242 supports this application. Figure 2 shows a block diagram of the ADF7242. Again incorporating timing critical elements, this part facilitates short development cycles. Functions like address filtering can be implemented on chip via a firmware download supplied by ADI. This reduces the computational requirements on the host while maintaining flexibility for adaptation as standards evolve.
The IEEE 802.15.4 PHY is also that on which the ZigBee Pro protocol is based. This protocol, by contrast, is designed to be able to handle large networks such as might be found in a Smart Metering application. In a networked metering system, meter data is typically fed to a fixed data collector, which is often located on a pole at the end of the street or neighborhood. The data is then fed back to the utility via a broadband or cellular backbone. ZigBee is frequently refered to in relation to the Home Area Network connection which connects between the meter and in-home devices. There are now several in-home devces certified to interoperate with the ZigBee Smart Energy application profile. These include products such as thermostats, in-home displays and smart plugs. Figure 3 illustrates an example of an advanced metering infrastructure.
The aforementioned ADF7242 can also be used in this application. The zero IF architecture of the ADF7242 facilitates its robust performance in a potentially harsh 2.4GHz environment with the possibility of many interferers. It does not suffer from the inherent finite image channel rejection of a low IF architecture. The ADF7242 can sucessfully receive in the presence of another IEEE 802.15.4 signal in either adjacent channel, which is 49dB, higher or an IEEE 802.15.4 signal which is 2 channels away and is 62dB higher. With the signal level at -92dBm, reception is still sucessful when a CW interferer at 5MHz offset from band edge is as high as -26dBm. The transmitter typically achieves a 3% EVM in the IEEE802.15.4 mode. This means that, for example, if an external PA is added, the modulation quality at the PA output will only be limited by that of the external PA.
While the neighborhood area network (NAN) may also use ZibgBee, in many cases where an ISM solution (as opposed to, for example, power line carrier (PLC)) is deployed it has been a proprietary FSK solution. There is now an IEEE 802.15.4 Smart Utility Networks (SUN) Task Group 4g which was set up to create a PHY amendment to 802.15.4 to provide a global standard that facilitates very large scale process control applications such as the utility smart-grid network. This task group has been running since early 2009 and has formulated three complimentary, merged proposals for FSK/GFSK, OQPSK and OFDM. In addition to supporting the existing 2.4GHz IEEE 802.15.4 PHY on which the ZigBee protocol is based, the ADF7242 supports FSK/GFSK operation from 50kbps to 2Mbps. The robust operation seen in IEEE 802.15.4 mode is also present in FSK/GFSK mode. For a 2Mbps GMSK signal, for example, the ADF7242 can provide adjacent CW channel rejection of 51dB and alternate CW channel rejection of 56dB for a 5MHz channel bandwidth. This is with a signal level equal to Psense + 3dB, which is -82dBm for this condition. 50kbps is the mandatory data rate requirement for the 2.4GHz IEEE 802.15.4g FSK/GFSK proposal. Therefore, in addition to supporting ZigBee for the HAN, the ADF7242 could be used in FSK mode for the NAN.
For the neighborhood area network, range will be an important parameter. The ADF7242 is equipped with two fully differential RF ports. Port 1 is capable of receiving, whereas Port 2 is capable of receiving or transmitting. The availability of two RF ports facilitates the use of switched antenna diversity. For receive antenna diversity, the link margin is maximized by selecting the optimum antenna based on the RSSI level of the desired signal received with each antenna. It may also be desirable to increase range by the addition of an external PA and LNA. The dual RF ports also simplify the application circuit for a connection to an external LNA and/or PA. Connecting to an external PA and/or LNA is possible with a single external Rx/Tx switch, reducing the cost and loss associated with an additional switch. Figure 4 demonstrates some RF port connectivity options.
In Configuration A, a single antenna is connected to RF Port 2.
In Configuration B, a dual-antenna configuration is suitable for switched antenna diversity.
In Configuration C, the PA is configured to transmit on RF Port 2. RF Port 1 is configured as the receive input.
Configuration D is similar to Configuration A, except that a dipole antenna is used. In this case, a balun is not required.
Furthermore, it is possible to implement timing critical functions via firmware download. For IEEE 802.15.4-2006 mode, for example, ADI supplies code for the implementation of address filtering, auto acknowledge, and CSMA-CA. Particularly for applications with evolving standards the implementation of such functions through firmware downloads supplied by ADI facilitates adaptation as standards, are updated or new standards evolve.
The IEEE 802.15.4g proposal also incorporates a sub 1GHz FSK/GFSK PHY. Another sub 1GHz FSK protocol which has been popular in Europe for metering applications is the Wireless M-Bus protocol. This protocol is detailed in the EN 13757-4 variant. ADI’s ADF7023 supports these applications. Figure 5 shows a block diagram of the ADF7023.
The ADF7023 operates from 862MHz to 928MHz and from 431MHz to 464MHz and supports data rates from 0.1kbps to 300kbps. It provides excellent sensitivity, enabling longer range. For example, at 100kbps a sensitivity of -103dBm can be achieved. Additionally, it provides robust operation in the presence of interferers. Blocking resilience at 2MHz offset of 62.5dB is achieved. The ADF7023 also includes an image rejection calibration which does not require the use of an external RF source, nor does it require any user intervention once initiated.
Furthermore, the ADF7023 enables long battery life with a receive current of only 13mA in high sensitivity mode. Battery life can further be extended using a Smart Wake Mode. The receiver can wake up autonomously, do a carrier sense, and receive a packet. Similarly to the ADF7242, the ADF7023 supports firmware downloads. For example, a download is available from ADI which enables 128-bit AES encryption/decryption. A highly flexible packet handler supports a wide range of packet formats with the insertion/detection of Preamble, SWD, CRC, and Address.
Thus, it can be seen that there are a wide and growing range of ISM applications. There is a trend for interested parties for a particular application to come together and develop a standard. This enables interoperability between products from different manufacturers. It also allows a developer to use a protocol which has been tried and tested. ADI’s ISM band transceivers support a range of recently evolved standards. The ADF7022, ADF7023 and ADF7242 are latest generation parts in ADI’s transceiver portfolio. They support resilient operation with excellent sensitivity and low power consumption. They also provide support for timing critical elements, which reduces the burden on the host and enables faster development time. Furthermore, the option to implement such functions through firmware downloads supplied by ADI facilitates flexibility and adaptiveness in a fast changing environment.
1: Check for individual country exceptions
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