Unlicensed wireless communications and uses for RF/microwave signals that do not require licensing have long appealed to developers of cost-sensitive applications. Around the world, government organizations set aside bands of frequencies for “free,” unlicensed use, with some rules to help minimize interference between different applications in the same frequency range. Throughout the world, these open frequencies are known as the industrial-scientific-medical (ISM) bands, since they were initially set aside for such applications as microwave heating and medical diathermy.
Indeed, the first encounter for many with an electronic product operating within an ISM frequency band is their microwave oven, which uses energy at 2.45 GHz for heating food. Any products that are developed and added to the ISM bands must be tolerant of the emissions from legacy applications. For RF/microwave hardware suppliers, the ISM bands around the world provide excellent opportunities for all of the components needed to assembly communications systems, from antennas and amplifiers to YIG oscillators.
Various organizations around the world help regulate the use of frequencies in the various ISM bands. In the United States, the Federal Communications Commission (FCC) is the regulating agency, with Part 18 of the FCC rules governing the uses of designated ISM bands in the US and Part 15 impacting unlicensed communications devices in those ISM bands. In other parts of the world, the International Telecommunication Union (ITU) sets guidelines for designated ISM bands according to ITU Radio Regulations 5.138, 5.150, and 5.280 and any appropriate national regulations in the affected area. The ITU’s ISM bands reach well into the millimeter-wave frequency range, with ITU ISM bands that include 902 to 928 MHz, 2.4 to 2.5 GHz, 5.725 to 5.875 GHz, 24.00 to 24.25 GHz, 61.0 to 61.5 GHz, 122 to 123 GHz, and 244 to 246 GHz.
In the US, three of the more popular ISM bands governed by the FCC Part 15 rules are 902 to 928 MHz, 2.400 to 2.4835 GHz, and 5.725 to 5.875 GHz. The Part 15 rules establish such operating parameters as maximum transmit power. The maximum transmit power that can be fed to the antenna within these frequency bands is +3 dBm (1 W). The maximum effective isotropic radiated power (EIRP) is +36 dBm (4 W). The EIRP value can be determined by adding the transmit output power (in dBm) to the antenna gain (in dBi). Any loss from the cable feeding the antenna must be subtracted.
ISM bands are often associated with lower-frequency applications, such as 900-MHz cordless telephones, near-field communications (NFC) devices, or 2.4-GHz Bluetooth devices. The aforementioned microwave oven, for example, coexists with numerous communications devices at or near that frequency band, including wireless local area networks (WLANs), wireless sensor networks, and cordless telephones. The number of manufacturers supporting these applications with a variety of components, including antennas, amplifiers, and radio-frequency integrated circuits (RFICs), is already quite large, making the market for lower-frequency ISM-and components extremely competitive.
By way of example: Several years ago, RF Micro Devices introduced its model RF3858 front-end module for ISM applications in the 900-MHz band. Priced at less than $2.50 USD in 10,000 piece quantities, the RF3858 contains a power amplifier, transmit/receive transfer switch, low-noise amplifier (LNA), and matching components. Designed to reduce the number of parts in an ISM product, the module includes a power amplifier that can deliver +31.5-dBm output power at 915 MHz, while the low-noise amplifier provides 27-dB gain with 1.3-dB noise figure. Not long ago, either of these amplifiers would be difficult to find for that price as a separate component. In general, markets for ISM applications at frequencies below 6 GHz are highly competitive and cost-sensitive, and require high levels of integration to minimize final-product costs.
Adoption of higher-frequency ISM bands may open some opportunities for companies with millimeter-wave engineering capabilities. The ITU’s designation of several millimeter-wave frequency bands for ISM use, for example, presents an opportunity for development of short-range communications links and high-speed data links for computer networks by taking advantage of the wide available bandwidths at those millimeter-wave ISM frequencies.
One company strongly associated with a diversity of ICs for lower-frequency ISM-band applications, Hittite Microwave Corp., has already seized the opportunity for a millimeter-wave ISM-band transceiver solution with their HMC6000LP711E transmitter and HMC6001LP711E receiver silicon-germanium (SiGe) chipset for 60-GHz short-range ISM applications. These antenna-in-package (AiP) ICs combine SiGe chips built around frequency synthesizers with 60-GHz antennas in 7 x 11 mm QFN plastic packages for low-cost, surface-mount printed-circuit-board (PCB) assembly. Perhaps as important, the devices require no special PCB fabrication measures and knowledge of handling millimeter-wave components, to ease the transition for ISM product suppliers to higher frequencies at 60 GHz. To further ease the way, Hittite offers a complete AiP transceiver evaluation kit, model EKIT01-HMC6450, with both ICs, configuration software, and everything needed to build a bidirectional millimeter-wave link at 60 GHz with a range of 4 m. A universal analog in-phase (I) and quadrature (Q) interface translates baseband analog I and Q signals with single-sideband (SSB) bandwidth to 880 MHz to and from the 60-GHz ISM band.
For international equipment suppliers considering higher-frequency ISM applications, Frankfurt, Germany-based IHP Microelectronics offers circuit-prototyping services for communications and other applications at 60 GHz and other ISM millimeter-wave frequencies. The firm has already fabricated and successfully tested radio transmitter and receiver front ends for ISM applications at frequencies as high as 245 GHz, as well as a complete radio front-end device capable of data rates to 4 Gb/s at 60 GHz. The 60-GHz demonstration circuit (see figure) was developed nominally for video streaming applications.
The firm is also developing a silicon-based system-on-chip (SoC) radio device for use in the 122-to-123-GHz ISM band. The SoC employs a direct-downconversion transceiver architecture and is suitable for short-range distance and speed sensing, as might be used in industrial applications and in automotive electronic systems. The firm, which is funded by the European Union (EU) under grant reference number FP7-ICT-248120, is working on the project with an impressive cast of partners. These include Silicon Radar, Evatronix, Robert Bosch GmbH, STMicroelectronics, Karlsruhe Institute of Technology, Selmic, HighTec, and the University of Toronto.
As ISM applications extend from lower frequencies through millimeter-wave bands, demand grows for some of the pieces needed to build these final ISM products [iincluding printed-circuit-board (PCB) materials and antennas]. At higher frequencies, circuit materials should provide good dielectric stability with temperature, and typically require a different physical makeup than at lower ISM frequencies. For applications extending to 60 GHz and beyond, PCB materials should be thinner than at lower frequencies, with a typical rule of thumb calling for dielectric PCB materials that are about one-eighth the measure of the wavelength of the frequency of interest. Similarly, the copper conductor on these PCB materials should be thinner at these higher frequencies, with minimal copper roughness to trim circuit losses at millimeter-wave frequencies.
As examples, Taconic Advanced Dielectric Division (ADD) has developed its TacLamPLUS PCB material for millimeter-wave applications. The non-reinforced substrate material is a cost-effective building material for higher-frequency ISM-band circuits, with typical thickness of 100 μm. It features a dielectric constant of 2.10 in the z-direction at 50 GHz and dissipation factor of 0.0008 at 50 GHz.
Similarly, the RT/duroid® 5870 and 5880 PCB materials from Rogers Corp. offer the mechanical and electrical characteristics that suit ISM applications at higher millimeter-wave frequencies. These polytetrafluoroethylene (PTFE) composites are reinforced with glass microfibers to provide good dielectric stability, and the materials exhibit low values of dielectric constant: 2.33 for RT/duroid 5870 in the z-direction (thickness) at 10 GHz and 2.20 for RT/duroid 5880 in the z-direction at 10 GHz. The materials have dissipation factors of 0.0012 or less at 10 GHz.
Millimeter-wave signals tend to provide shorter-distance communications links than their lower-frequency counterparts, due to atmospheric losses, but they also offer wide available bandwidths for high-speed data and video services. Certainly, as the demand for these higher-frequency ISM bands grow, the supply of components and such needed building blocks as PCB boards will grow—and their prices will drop accordingly.