Conformity Magazine - October 2008 - (Page 50)

the antenna’s pattern being too narrow for some applications, such as the generation of an even field over a large product. An important convenience factor to consider in testing is the frequency range covered by an antenna. A wider range is certainly more convenient, especially in automated or semi-automated testing, because it minimizes the amount of stopping to switch from one antenna to another. In the mid-frequency range of 30 MHz to 1 or 2 GHz, this has led to the popularity of the combination antennas composed of log-periodic and biconical/bowtie elements, because one antenna can cover the entire range. At higher frequencies, the double ridged waveguide horn is popular because it covers a wide range (e.g., 1 – 18 GHz units) in a compact unit. Also used at microwave frequencies are “standard gain” horns. However, one pitfall in using broadband and combination antennas is lack of sufficient gain flatness across the operating band. When using these types of antennas for RF susceptibility testing, the associated amplifiers must be sized based on the worst-case antenna characteristics. This may mean purchasing an oversized amplifier to deal with a combination antenna that yields below average performance in only a very narrow range of spectrum. Since good RF amplifiers are often at least an order of magnitude more expensive than the antenna to which they are connected, the price of broadband convenience can become quite high. Power Amplifiers When one thinks of amplifiers to use in generating the strong fields or voltages required for radiated and conducted immunity testing, the two obvious parameters that come to mind are power and frequency range. There are a lot of secondary factors to consider, such as: • Overhead, or reserve power; • Stability with varying source and load impedances, including forward power levels well as resistance to oscillation; • Ability to operate into open and shorted loads (important for durability). The first item listed above warrants a bit more detail. Some amplifiers are tightly specified—they’ll put out a continuous carrier at the specified level, but don’t have any headroom. In other words, the specified level is a peak level. You can’t run a high, or even a modest, peak to average signal through these amplifiers without reducing the signal. For example, you couldn’t amplitude modulate a carrier run at the rated power without compression and distortion if there wasn’t any headroom between the specified power and the deliverable peak power. This is an important consideration when choosing an amplifier, as current standards (IEC 61000-4-3 Ed 3 in particular) specify that the amplifier must be capable of producing a clean output at 1.8 times the specified field strength. Clean means with minimal harmonic content. In the case of IEC 61000-4-3, that’s -6 dBc when performing the Uniform Field calibration. Although filtering and antenna response may help to improve harmonic levels, generally speaking, the amplifier must stay in its linear region - often defined as the 2 dB compression point. The 1.8 multiplier is simply to compensate for the 80% depth of the amplitude modulation often used during testing. Since calibrations are routinely performed using an unmodulated signal, by calibrating the system at 1.8 times the wanted field strength (+5.1 dB), there is reasonable certainty that, during product testing, the full field strength will be produced under modulation. Signal Generators • Flatness through frequency range, both in gain and power; Frequency Range Below 30 MHz Antenna Type Shielded loop monopole 30 – 1000 MHz Dipoles biconicals log periodics “bi-conilog” combos The main use for RF signal generators in an EMI/EMC test lab is as low-level signal sources for RF immunity testing to be boosted by amplification to the level required by radiated. Of course, there are also a lot of other uses for signal generators around the lab. As with many other EMI/ EMC instruments, a key parameter for a signal generator is its frequency range. Most Comments RF signal generators produce sinusoidal H-fields outputs. However, at lower frequencies E-fields (usually up to 15 MHz, or for some Active and passive models available instruments, 80 MHz), there is overlap for both between function generators and sinusoidalonly RF instruments. Table 3 provides Combos have broadest range. a summary of primary and subsidiary All types can be used for susceptibility (see comments) specifications and features to consider. As we see from Table 3, there’s a lot more to a signal generator than just its frequency range. For example, in immunity testing, a frequency range must be covered. It can be covered in a smooth, analog fashion, with 1000 MHz and up Log periodics/combos Horns At lower end of range, overlap between log period/combo and horns. Table 2: EMC antennas 50 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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