Conformity Magazine - April 2008 - (Page 23)

n the recent years, scores of low power wireless devices (LPDs, also known as short range devices, or SRDs) in the unlicensed RF bands have emerged in the market, and their numbers are growing. New applications are being developed, as wires are continuously being replaced by radio waves. Unlicensed low power radio bands are getting crowded and design engineers are busy creating new devices. The regulatory bodies, including the Federal Communications Commission (FCC) in the United States, Industry Canada in Canada (whose requirements closely follow those of the FCC), and ETSI in the European Union (EU), classify LPDs and SRDs radio devices which radiate very small power (i.e., a fraction of a watt or close to a watt). However, the demand on radio operating range is continuously increasing, and RF engineer have to make vital decisions about maximum output power delivered to antennas for desired operating ranges without violating the regulatory limits in the given region. Further, the allowable output power limits for similar devices in the U.S. and the EU are often expressed in different units or terminologies, which can create confusion when it comes to finding equivalence. For example, power expressed in milliwatts may be conducted power or radiated power expressed as ERP, EIRP, or electric field in some cases. Also, the conducted power limit may be specified as peak, average or quasi-peak, which are all different. Often for similar devices, not only the magnitudes of emissions limits but methods of measurement may also vary between the regions. Understanding the relationship between limits of various regions and being able to compare them can make the task of the design engineer easier. In this article, we define the relationships in the output power limits expressed by FCC and ETSI for some very common short range, low power RF transmitting devices in popular unlicensed bands. Radiated RF Energy – A Typical RF Transmission Link The power radiated from a transmitting antenna is the function of total power input to the antenna, and the antenna properties. The design engineer has more control over the power developed at the output amplifier (antenna input), but less control over the radiated power, due to several uncertainties with the antenna, like its VSWR, radiation efficiency, and so on. The RF power transport path illustrated in Figure 1 shows an RF link from the LPD/ SRD to the antenna, and the radiated field I from it. P1 is the conducted power at the device output, and P2 is power available after cable loss. Both can be measured by direct connection with instruments. Once the antenna is connected, P3 radiates out, while P4 is reflected back to the source. So here are the complexities. Antennas for the devices under discussion are small, compared to normal dipole or monopole dimensions and are not very efficient, making it difficult to precisely estimate the radiated power P3. The last quantity in the Figure 1 is E1, the electric field at distance r, and the corresponding quantity Pd, the power density. The goal of the design engineer is to get the desired range within the regulatory power limits. Therefore, a valid question would be what power should be delivered at the antenna port by the power amplifier, if the maximum power limit is specified as ERP, EIRP, or e.i.r.p, plain mW or as electric field? Essentially, we wish to find some relationships between quantities in Figure 1 and the limits specified by the applicable regulatory bodies. All field measurements are assumed to be done in the far field (r >= λ/2π or r >= 2 d2/λ, for electrically small antennas). Thus, for measurement at 3 m and short device antennas (d << λ), the frequencies above approximately 17 MHz fall within far field definition. April 2008 Conformity 23 http://www.fair-rite.com http://www.fair-rite.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - April 2008

Conformity Magazine - April 2008 NewsBreaks Editor’s Note Mandatory Analog Cellular Service Ends FCC Releases Quarterly Reports on Consumer Inquiries and Complaints New Standards List Released for The EU’s Low Voltage Directive FDA Issues Notification on Unretrieved Device Fragments FDA Offers Guidance on Waivers for In-Vitro Diagnostic Devices ESD Open Forum Emerging Issues in Standards Challenges in Testing Design Fundamentals Focus on... New Products Product Reviews Premarket Approval Application Guidance Issued by FDA New Jersey Enacts Electronic Waste Recycling Regulations A New U.S. Standard for Workstation and Computer Design Might the CPSC Hold Retailers Accountable for Safe Products? CPSC Loses “Sole Toy Tester” CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - April 2008

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