Conformity Magazine - April 2008 - (Page 28)

requirements in the EU (ETSI) and in North America (the FCC and Industry Canada). Example 1: Short Range Devices Suppose the SRD is a common RKE device operating in North America in the 315 MHz band and in the EU in the 433 MHz band. For such devices, Table 1 shows the allowed limits for both regions: Industry Canada RSS-210 Annex 1, together with Table 4 and Table 5 in this regulation, specify same limits as the FCC in units and magnitude. If we assume the ETSI limit of 10 mW ERP as radiated power equivalent to P3 in Figure 1, and use Equation (4) to calculate equivalent E-field at 3-m from antenna, we get: E = sqrt (30 x10x10-3) / 3 = 0.182 V/m or E = 182,574 μV/m, isotropic which is orders of magnitude larger than the FCC limit (10,960 μV/m at 433 MHz), but is generally expected to be comparable or less. (See Table 1 for range of values of Efield.) Now we do a reverse calculation on the FCC limits of electric field at 3 meters to obtain equivalent power P3 from Equation 4 for 433 MHz band, as follows: P3 = (10960) 2x 10-12 x 9 / 30 = 35.5 μW = -14.5 dBm The ETSI limit is 10 mW ERP or 10 dBm which, when compared to the above results, can be misleading (a difference of 24 dB), unless methods of measurements used for ERP and the effects of the antenna are accounted for. Comments The measurement method of ERP, as specified in ETSI EN 300 220-1 sec. 8.3 using a substitution antenna, suggests that the ERP is actually the power fed to a resonant dipole. Therefore, it is equivalent to P2 in Figure 1, and not P3. In practice, a typical antenna used for SRDs might be only 2 or 3 cm long, which is very small compared to a dipole antenna for above frequencies (λ = 0.95 m for 315 MHz, or λ =0.7 m for 433 MHz), and therefore will be inefficient due to its small radiation resistance [8]. For a practical device, if the antenna is, say, 3-cm long, at 433 MHz its radiation resistance would be: Rr = 160 π2 (L / λ )2 = 160 π2 ( 0.03 / 0.7 )2 = 1.42 Ω which is very small compared to any resistance value in the RF power circuit. However, unless antenna loss resistance is known, the radiation efficiency cannot be determined and it is not easy to estimate relationship between P1 and P3. But, as can be seen Rr << 50 Ω, typical source resistance the actual transmitted power and electric field could be an order of magnitude less than the calculation above. On the other hand, power radiated from a resonant dipole antenna with 10 mW input will be significantly greater than 35.5 μW, required for the FCC field strength limit. Therefore, it may be argued that ETSI allows much higher transmitted power levels if one can use an efficient antenna, but it would mean a larger device size. The FCC specification of limits in this case makes more sense because it looks at the final quantity of concern, that is, the electric filed strength at 3 m. This can be measured with relative ease and only the receiver antenna-factor is the variable in such measurements. This is not difficult to measure, and most antenna manufacturers specify it. Example 2: Low Power Devices, or LPD II Reg. Body Test Conditions FCC ETSI 260 – 470 MHz, 3750 to 12500 uV/m, at 3-m (or 6040 μV/m at 315 MHz, RKE freq.) 25 MHz to 1 GHz, Power class 8, 10 mW (The ERC/REC 70-03 allows 10 mW ERP). Standard & section(s) FCC 15.231 ETSI EN 300-220-1 Table 2 shows output power limits for both the United States and the EU. Industry Canada RSS-210 Annex 2, A2.8 is applicable with same limits as FCC. (Refer to Table 2 for power and E-field limits values for LPD II devices.) Again, let us assume that ETSI limit of -43 dBm ERP is the power transmitted from the antenna and it is P3 as in Figure 1: Table 1: Short range devices transmitter power limits Reg. Body FCC ETSI Test Conditions Standard & section(s) 88 – 108 MHz, 250 μV/m at 3-m FCC 15.239 25 – 2000 MHz, Cordless audio, ETSI EN 301 357-1 LPD II, -43 dBm ERP Table 2: Low power device power limits 28 Conformity April 2008

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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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