Conformity Magazine - October 2008 - (Page 44)

1. Stray signals can enter an amplifier measurement setup, be amplified, and generate harmonic and intermodulation products; Stray signals can enter an amplifier measurement set up and, if at the appropriate frequency, be mistakenly identified as intermodulation or harmonic products. techniques if there is a likelihood of generating interfering signals. Effects of Equipment in Design and Test on Electronics In the amplifier test environment, it is also necessary to be concerned with the other types of products and frequencies that are being developed, produced, and tested. Most amplifier manufacturers develop and produce devices that operate for different applications at different frequencies and power levels. Also, it is not uncommon for some testing to involve high power amplifiers at one test station, and low noise testing of another product at another station. There may be, but most likely is not, coordination between testing activities to minimize the possibility of co-test interference generation. All of the aforementioned effects of the ambient environment interfering with the test environment can be applied to the cotest situation. Due to the proximity of the devices in test, the interfering power levels can be considerably higher than those from the ambient environment. Also, the frequencies involved may not be standard frequencies expected for particular device operation. As an example, the author’s company builds a broadband 220 GHz, 20 watt nominal solid state power amplifier that is used for numerous applications. During testing, this amplifier is routinely tested for gain using a network analyzer, which results in a swept high-power signal being generated from 2-20 GHz. Additionally, the unit is tested for saturated and one dB gain compression powers at multiple frequencies, and harmonic performance and intermodulation levels are characterized. Even with careful test configuration, some amount of signal leakage occurs. The leakage is almost always detectable at other test stations, and must be considered during the evaluation of other products. Mitigation Most, if not all, of the potential issues created by interference in a microwave amplifier facility can be constrained or mitigated through the use of proper procedures and techniques. A detailed discussion of these techniques is available in several excellent application notes by equipment manufacturers such as Agilent and Anritsu. Some of the common sense practices for minimizing EMC/EMI problems in an amplifier lab include: • Use of properly terminated transmission lines; • Use of instrument quality terminations; • Verification that all connections are properly tightened and torqued; • Limitation of current to avoid damaging and interfering oscillation. The use of solid-state amplifiers such as a solid-state power amplifier (SSPA), for example, the Giga-tronics GT-1000A, rather than a traveling wave tube amplifier (TWTA) as a driver or test amplifier can also reduce many potential issues. 2. Care must be taken to specifically identify fundamental test frequencies and to calculate and verify the frequencies of intermodulation and harmonic content, or to avoid the presence and introduction of potentially interfering signals into the test environment. Spurious and Oscillation Measurement Spurious signal measurements are probably among the most difficult to ensure that the measurements are truly from the device under test and not from amplified interference. Any suspected out-of-specification spurious signal present at the amplifier output must be identified and evaluated as a truly spurious signal. This task can be arduous due to the often specious nature of both internally-generated spurious and externally-applied interference. Additionally, since spurious signals can be present or generated both within and outside the operating bandwidth, the spectrum of research may extend well beyond either end of the desired bandwidth, resulting in even greater search and evaluation time. In addition to being affected by EMI, each of these measurements is also affected by the time, frequency, and modulation nature of the interfering signal. External signals can be steady state (or nearly so), such as a commercial radio or television transmitters; bursty, such as legacy push-totalk communications used in aircraft VHF systems; variable power, such as CDMA or other mobile telephony; cyclical, such as an airport radar sweep rotation; or, perhaps worst of all, drive-by one-shot, such as a mobile phone user driving through the neighborhood. Effects of Equipment in Design and Test on Ambient The effects of ambient electronic interference on the test and measurement process are not the only concern for designers and manufacturers. The potential issues created by the testing of amplifiers, particularly high-power amplifiers, upon the local RF environment must also be considered. RF amplifiers, especially during the development phase, are often tested without lids or shielding to facilitate the development and alignment process. While the alignment is proceeding, the amplifiers can and do radiate some level of RF power. (This can also be an issue for personnel safety, but that is another topic.) If the amplifier creates spurious signals, oscillates, or has high intermodulation or harmonic power levels, these signals, in addition to any fundamental signals, will invade the local RF environment, causing disruptions. When testing high power amplifiers, one should be aware of the frequencies being tested, and limit testing to the lowest extent possible, as well as implement other mitigation 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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