Conformity Magazine - October 2008 - (Page 42)

orthogonal codes of the CDMA transmission appear as thermal noise to equipment not configured to receive that modulation. In most cases, the presence of the effectively higher noise floor presented by a CDMA system is not apparent to a user of other equipment. Alternate equipment affected by a high signal level environment either has adequate signal-to-noise level to function, or it does not. The user’s experience is either that his communication device works, or it doesn’t. Just because it can’t be detected doesn’t mean it doesn’t happen. A common example of annoying contributed noise is when a ringing global system for mobile communications/enhanced data GSM environment (GSM/EDGE) mobile phone is close to a digital land line phone that is in use. The return RF signal sent out by the mobile phone is of sufficient power and appropriate modulation that it interferes with the audio modulation of the land line. A similar example is audio noise induced in an AM radio by the close proximity of power transmission lines. Spurious signals are a common interference source, potentially resulting in loss of data or communications. Technically, noise power is a spurious signal, but spurious signals are generally considered to be undesired harmonically- or intermodulationally-related frequencies that generate repeatable, measurable power above thermal noise. Every device with a signal source (oscillator) will also produce harmonics. Usually the harmonic levels are sufficiently filtered so that the power is not a concern, but occasionally equipment conflicts will arise where one device has sensitivity to a second or third harmonic of another item. Rogue equipment, or equipment that is operating improperly with respect to its power or frequency output, is also a contributor to environmental interference. This source of interference is less likely to occur than either the increased noise floor or the spurious signals, but can still be a potential source of EM pollution. The author ‘s personal experience is with an old remote control for security gate access. A mechanical ferrite core became loose after many years, and shifted internally, changing the transmit frequency, which rendered the unit ineffective. The unit transmitted 50 MHz low until the discrepancy was resolved. While this situation likely did not cause any ill effects to the surrounding environment, it illustrates the potential for rogue devices to generate interference. The Effects of Interference in an RF Amplifier Manufacturing Environment Effects of Ambient on Equipment in Design and Test RF and microwave amplifier manufacturers are adversely affected not only by intended radiating devices operating correctly, but also by any and all devices generating power levels anywhere within the operational bandwidth of the amplifier. Also, due simply to the nature of the product, amplifiers tend to amplify both desired and interfering signals alike, if they are within the operational bandwidth capability of the device. Several key amplifier measurements can be easily affected by RF interference, including noise figure and noise power, intermodulation and harmonic performance, spurious level measurement, and amplifier stability verification. Noise Figure Noise figure is a figure of merit used to describe the amount of degradation to the signal-to-noise ratio induced by a component. Noise figure is often measured using a noise figure meter and noise source3. Since this measurement reflects the excess noise created by a system, keeping external, additional excess noise out of the measurement system is imperative in order to generate an accurate measurement. During the author’s career, he has observed several instances of increased noise figure measurements occurring between the hours of approximately 8:00 a.m. and 5:00 p.m. The noise figures of the same device, when measured outside of that time slot, in the same configuration, without change, are reduced by two or three tenths of a dB, a substantial amount for low noise amplifiers. Intermodulation/Harmonics Intermodulation and harmonic content are additional measurements where electromagnetic interference can introduce measurement errors. Errors can be introduced in at least two ways: Figure 1: Sample of the detected spurious RF picked up by an 1800 MHz low noise amplifier during tuning using a maximum hold to capture even fleeting signals. The signal just below the center of the band is WiFi from an adjacent computer. 2 Conformity oCtober 2008

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Conformity Magazine - October 2008 Contents Editor's Note FCC Releases Annual Report on National Do-Not-Call Registry Commission Cracks Down (for the Fourth Time!) On Junk Fax Marketer FCC Releases Quarterly Reports on Consumer Inquiries and Complaints Greenpeace Ranks Electronics Manufacturers Standards List For The EU’s Energy Consumption Directive Published EU Commission Publishes Standards List for Directives on Pressure Equipment, Pressure Vessels Standards List for the EU’s Medical Devices Directive Available The IEEE Product Safety Engineering Society: The First Five Years ESD Open Forum Improving Election Security and Accuracy: The Future of Voting System Certification Conformity Assessment and Accreditation: Their Role in the Global Market Design Issues in Extreme EMC Environment Focus On... Test Equipment Buyers Guide Product News Updated Standards List for EU Directive on Active Implantable Medical Devices FDA Warns of Effects from CT Scans on Electronic Medical Devices FDA Provides Report Card on Its PMA Review Process CPSC Releases Import Safety Strategy Other CPSC Actions in the News IEC Standards Update Product Reviews 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 - October 2008

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