used most often in clinics and hospitals in large metropolitan areas. The electromagnetic environment in these hospitals is apt to be polluted by a wide variety of transmitters, appliances, and even by medical electronic devices. The Bureau of Medical Devices of the Food and Drug Administration has supported a study of electronic devices and the ambient electromagnetic environment in several major hospitals. The results of this study establish guidelines for acceptable emissions from medical devices and safe susceptibility thresholds. These guidelines were published in the report, "EMC Standard for Medical Devices," MDC-E 1385, (1976). The guideline for minimum radiated electric field susceptibility limit from the report is shown here as Figure 6. It covers frequencies from 100 kHz to 1 GHz. It was deemed unnecessary in the report to consider higher frequencies since it is generally known that devices which are safe from hazardous EMI at one frequency will be safe at a higher frequency. The report further recommends specific device performance standards at the industrial, scientific, and medical (ISM) frequencies of 2.45, 5.8 and 24.125 GHz. It is clear, from Figure 6, that the existing guideline for electric field susceptibility at 1 GHz would permit the manufacture of devices with thresholds (^7 v/m) that are lower than the field intensities expected near the SPS receiving area (19 v/m). The impact of this recommendation at 2.45 GHz is not clear since threshold levels of most devices increase with frequency. The threshold of devices at 2.45 GHz could conceivably be well above 19 v/m, but there is little data available to establish this. One manufacturer of medical electronic devices claims their equipment is safe in field intensities of 1 v/m (private communication) while others, without providing supporting data, believe their equipment is safe in field intensities of hundreds of v/m. Most of those people contacted for information about medical device EMI susceptibility believed that by 1990 or so improved manufacturing techniques can produce devices that would be safe in almost any realistic EM environment. A few believed that specific instrument types (diagnostic, for example) should be tested for susceptibility over a wide frequency range. 4.7 Medical Electronics Summary Demand pacemakers are known to be susceptible to EMI. Their susceptibility thresholds, however, are rapidly increasing because of design improvements, and at ultra high frequencies the average threshold can be in the hundreds of volts per meter range. For these reasons, it seems unlikely that pacemakers would fail in the SPS-cw power beam which is projected to be 19 v/m or less outside the receiving areas.
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