the sample being able to withstand fields greater than 200 volts/meter. Since the SPS power beam outside of the receiving area is projected to be 19 volts/meter or less (0.1 mw/cm^2 ), pacemakers are not likely to fail based on the results of these particular tests. 4.3 The Effects of Pulse Width and Repetition Rate Pacemakers are more likely to fail in low intensity pulsed rf fields if the pulses are wide (at least to a point) and at lower pulse repetition frequencies (PRF). Data taken by Mitchell and Hurt (1975) indicate that the average failure threshold decreases from 117 v/m at a pulse width at 10 usee, to 33.8 v/m (1 m sec) and to 18.6 v/m (20 m sec) when the PRF and frequency were constant. Denny et al., (1977) found that failures are more likely to occur if the rf source is pulsed at a rate of 10 pps or less. Some demand pacemakers, by design, revert to an asynchronous mode of pulsing if they are subjected to an intense rf source pulsed at a high rate. These observations regarding the effects of pulse width and pulse repetition frequency, though not directly applicable to the problem of possible interference from the cw SPS power beam, could be important when considering the effects of communication signals used in control of the SPS beam. 4.4 Effects of Modulation It is a prevalent opinion among workers in the pacemaker industry and with researchers, that continuous wave emitters are very unlikely to produce EMI related failures, and little testing with cw sources has been done. Bonney et al., (1973), after testing, considered a continuous wave source at 915 MHz to be hazardous if the field intensity was 75 v/m or more. 4.5 Historical Trends in Pacemakers The existence of EM hazards to pacemakers has been well known for several years. As a result, researchers have identified hazardous EM sources and manufacturers have worked to harden the devices against those sources of EMI that users might encounter. The success of these manufacturers’ efforts is reflected in a study by Denny et al., (1977). This five year study, involving hundreds of pacemakers showed that the average resistance to interference of pacemakers to a pulsed 450 MHz source increased from 35 v/m in 1973 to 144 v/m in 1976. 4.6 EMC Standards for Medical Electronic Devices The general class of medical electronic devices contains many sensitive instruments likely to be susceptible to EMI. The Electrocardiograph, for example, is vulnerable to 60 Hz line interference (Huhta and Webster, 1973) . Medical devices are
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