Conformity Magazine - October 2008 - (Page 48)

If it is done with a broadband, untuned low-noise amplifier, the noise figure will be reduced, but the susceptibility to overload will be increased, because the mixer is presented with all signals in the amplifier’s passband at the same time, and in amplified form. A strong signal—even one that is not where the analyzer is tuned—can cause the mixer to overload and generate harmonics and spurious signals. This is because the diode mixer acts like a multiplier at moderate signals, generating sum and difference frequencies between the local oscillator and incoming signals, but more like a switch at higher input levels. If preamplification is done with some form of frequency selectivity, overload is less likely because the mixer is presented with signals from a narrower range of frequencies. Traditionally, the term “receiver” was used for instruments which had tracking, tuned RF stages, while “spectrum analyzers” were often lacking these and optimized for graphic display of swept data. The distinction today is blurring. Many instruments sold as EMI receivers today incorporate panoramic swept displays. Tracking pre-selectors are available for some spectrum analyzers. What is important is what is actually in the instrument. Another issue that is often raised for these instruments is CISPR 16 compliance. Perhaps the biggest question concerns quasi-peak detection. Quasi-peak detectors employ asymmetric rise/fall times in the post-detection signal processing. This detector type has been designed to provide a rough nuisance equivalence between impulsive noise and less variable signals. As a consequence, dynamic overload range and impulse frequency response are important parameters. Antennas For commercial work (EN 55022, FCC, NEBS), receiving antennas are differentiated by frequency range, polarization type antennas (linear, circular), and the type of fields they are sensitive to (magnetic, electric, electromagnetic), as shown in Table 2. At lower frequencies, where measurements are made in the near field, the distinction between electric and magnetic sensing antennas is important, and called out in many standards. Some of these antennas (biconicals, log periodics, horns) are also suitable for transmitting as well, and can be used in the plane wave immunity tests of such standards as EN 61000-4-3. Note that for very high field strengths (above 10 V/m) specialized high-powered versions may be needed. For example, antennas using coaxial baluns for impedance matching may need to be designed to dissipate more heat in high-level susceptibility applications. The primary factors of interest are the frequency range, the polarization, and, to some extent, the antenna gain, as expressed in the antenna factor. An antenna with higher gain (lower antenna factor) will allow the detection of weaker signals in EMI measurement applications; in susceptibility applications, it will allow the generation of stronger fields for a given amplifier power. Gain does come at the price of size and directivity. It is also possible for too much gain to result in Primary Features: • • • • • • Frequency range Available resolutions (filter bandwidth) Sensitivity Amplitude accuracy Frequency accuracy Stability Secondary Features: • • • • • • • • • • • • • • • • • • Detector functions (peak, quasi-peak, average, FM) CISPR 16-1-1 compliance Computer interfacing – GPIB, RS232, USB Windows graphics compatibility Software (automation, control, reporting) Video features (trace memory, multi-trace averaging) Hi-resolution or color display. Time domain features – sweep rates, triggering On-board computational features (e. g., power density calculation) Spurious signal performance Ability to extend range with external mixer/accessories Input impedance (50/75 ohms; for LF analyzers may include a high impedance as well) Preselection HDD or flash media support Built-in preamplifier Tracking signal generator Spectrum analyzer mode (for receivers) Modulation analysis (cellular/wireless) Table 1: Key features of a spectrum analyzer/receiver 8 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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