Conformity Magazine - April 2008 - (Page 26)

P2 or Ps sent to the antenna. This fact will be apparent in some examples later in this article. The FCC also gives the following relation in [10]: EIRP (dBm) = E (dB μV/m) – 95.2 (For 3-m from antenna) Which is equivalent to Equation 2, but ETSI calls it effective instead of equivalent. Also note that Equations 1 and 2 give different results for EIRP. Let us now understand the conducted power (which is measured in peak, average, or quasi-peak) all can be related to each other mathematically or by some accepted definitions from the regulatory bodies. The instantaneous power of communication signals depends upon modulation type, and average power depends upon its time domain behavior (e.g., duty cycle) required by an application. Most low power devices are assumed to operate intermittently, and often higher power levels are allowed if the duty cycle is low. Basic relations between various power terms are given as: Pav = 1/T ∫T P(t) dt: The average power integrated over time T which can vary depending upon the signal type. For example, for time division duplex (TDD) devices (6) such as Bluetooth time T must be sufficiently larger than several transmit cycles to happen ([5] sec. 5.7.2.2). Pqp: Quasi peak transmitted power measured at the output of device through an R-C charge / discharge circuit doing physical integration (in a specified bandwidth) as recommended in sec. 4 of [6]. Ppk = max. P(t): Maximum instantaneous power generally measured by a spectrum analyzer or similar receiver. In case of Figure 1, a typical link showing powers at various points in a transmitter path, basic relations between various quantities are as following: P2 = P1 – PcL (PcL is the cable loss) P3 = P2 - P4 - PaL (P4 is the reflected power and PaL is antenna loss) The reflected power P4 can be estimated easily from antenna VSWR information, which can readily be measured by a directional wattmeter or network analyzer. The VSWR is related to incident and reflected voltage ratios as follows: P4 / P1 = Vr2 / Vi2 = |ρ|2 where Vr is reflected and Vi is the input or incident voltage, and ρ is reflection coefficient. And the ρ is related to VSWR as: |ρ| = (VSWR -1)/(VSWR +1) or P3 = P1- P4 – PcL- PaL writing P4 in terms of P1. For small antennas, for example, with typical VSWR of 2, P4 is not large (only 11% reflection), but PaL can be large due to poor radiation efficiency which is not easy to measure [9]. Radiated Power and Electric Field We assume that the power from an antenna radiates in a perfect sphere, such that the field is uniform, or isotropic, at its surface. The energy conversion relations between power and the field given in numerous texts are as follows: Pd = P / 4 π r2 = P1 (1- |ρ|2) - PcL- PaL (7) BeCu Contact Rings Available in High Volume Free Samples at www.tech-etch.com/shield/contactrings.html ISO 9001:2000 REGISTERED TECH-ETCH, INC., 45 Aldrin Road, Plymouth, MA 02360 2 Conformity April 2008 http://www.tech-etch.com/shield/contactrings.html http://www.tech-etch.com/shield/contactrings.html

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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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