Conformity Magazine - February 2009 - (Page 14)

Some EMC testing experts suggest there are large and unpredictable uncertainties associated with the use of anechoic chambers [13] [14]. Reverberation chambers can provide much more realistic tests [15] [16] and, for this reason (plus their lower costs), they are used by many manufacturers of flight-critical avionics. 2. RF Modulation Types and Frequencies Are Not Realistic Traditional radio-frequency (RF) immunity tests use 1kHz sinewave modulation for ease of testing, low costs and repeatability, although some vehicle manufacturers employ pulse modulation to simulate digital cell phones and radars, at frequencies above 600MHz or so. But real-life transmitters use a wide range of analog and digital modulation types and frequencies. References [17] and [18] show that immunity can be significantly degraded (e.g., 20dB or more) when EMI modulation corresponds with frequencies or waveforms used in internal processes, or resonates with circuits, cables, transducers or loads. Therefore, testing with 1kHz is too simple where safety issues are concerned. Designers of military electronic warfare/countermeasures have known about the importance of modulation to immunity for many decades, but it is only now just starting to be addressed in standards (see [19] and [20]). 3. DC Power Disturbance Tests Are Not Realistic or Thorough ISO 7637 [21] specifies conducted transient tests to simulate noise on a vehicle’s power supply distribution network. The tests use waveforms based on simplifications of the transients that occur in real vehicles, so they can easily and repeatably be generated by low-cost test equipment. Reference [22] describes tests of the DC power supply on a variety of real vehicles, and shows that the use of even the highest level pulses in [21] can be insufficient for some vehicles. Reference [22] also includes examples of real-life conducted transients in vehicles for which there are, as yet, no corresponding tests. Varying the timings used by Pulse 2b of Reference [21] can delete the firmware in some ESAs, and varying the test settings can cause some ESAs to switch on or off without command. However, most vehicle and Tier 1 manufacturers do not vary the timings. Instead, they choose settings to reduce testing cost and time, or even to achieve a pass, possibly failing to detect latent unreliabilities that could increase safety risks. The Ford Motor Company is unique in that its EMC test specification [23] deviates in part from [21] by using chattering relay tests that should produce transient tests with waveforms closer to what is probably experienced in real life. 4. Simultaneous Disturbances Are Not Tested In real-life operation, ESAs are exposed to simultaneous EM disturbances, for example, two or more RF fields at different frequencies, a radiated field plus a conducted transient or electrostatic discharge, etc. But EMC immunity tests only apply disturbances one at a time. Simultaneous disturbances that have different frequencies can cause EMI through intermodulation (IM), which (like demodulation) occurs naturally in non-linear devices like semiconductors. Figure 2 shows a simple example of two RF fields at different frequencies, which can cause EMI by: • Direct interference from each frequency independently; • Demodulation of the amplitude envelopes of either frequency, or both mixed together; • Intermodulation, in which new frequencies are created. Equipment that passes individual immunity tests can be much more susceptible to lower levels of the same disturbances when they are applied two at a time [24]. Vehicles have many independent sources of EM disturbances that can occur at the same time. A simple analysis, based on reasonable assumptions for a 6-cylinder engine at 2000 rpm with spark-ignition transients lasting 50ns, shows that, if there was an average of one unrelated 100ns transient per minute (e.g. due to the actuation of an electric motor or solenoid), there would be a 0.001% likelihood that the 100ns transient would overlap with a 50ns spark-ignition transient. If this vehicle were driven for 1 hour/day, 5 days/week, 40 weeks/year, the likelihood of it experiencing an overlapping pulse event would be 12% per year. And, if the overlapping pulses caused an ESA to malfunction and caused a 1% chance of death (the official rate of death due to runaway vehicles in the United States over recent decades), the driver would have a risk of death of 0.12% per year. This might not sound much, but it is comparable with the risk of death knowingly accepted Figure 2: Example of demodulation and intermodulation 1 Conformity february 2009

Table of Contents Feed for the Digital Edition of Conformity Magazine - February 2009

Conformity Magazine - February 2009 Contents Editor's Note FCC Clarifies Closed Captioning Requirements for DTV Receivers Commission Cracks Down (For the Fifth Time!) On Junk Fax Marketer FDA Requests Comments on Global Harmonization Documents Australia Now Requires Solar Panel Testing and Certification EU Drafting New Directive on Toy Safety EU Commission Revises Standards List for EMC, R&TTE Directives Additional Standards Published For the EU's Medical Devices Directive EU Commission Publishes Additional Standards for Directives on Active Implantable, In Vitro Medical Devices EMC For the Functional Safety of Automobiles ESD Open Forum The ILAC Arrangement: Facilitating Trade Efficient Methodology for the EMI Analysis of Optical Transceivers Including PCBs and/or Flex Circuits The EuP Directive - Implementation, Design Requirements and Issues Chambers, Shielding, and Accessories Buyer's Guide CPSIA Lead Requirements Apply to Existing Inventory, Says CPSC CPSC Clarifies Certifcation Requirements 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 - February 2009

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