Footprinting
At Texas Children’s Hospital, the EMI testing program has evolved from years of experience and analysis. This program is not static. Sources of EMI and susceptibility characteristics of devices are in constant change. As new threats arise, the plan is periodically reviewed and modified to contend with them. Further modifications to the plan and procedures are based on a continuous review of wireless industry trends. They represent a proactive response to perceived future threats. At present the program consists of the procedural plan outlined above and three series of tests: (1) area characterization, footprinting (Figure 63-3), (2) device characterization, fingerprinting (Figure (63-4), and ad hoc susceptibility testing according to IEEE guidelines (Knudson and Bulkeley, 1994). Footprinting an area means performing a series of spectrum/amplitude scans for electromagnetic radiation in a defined or designated area. Footprinting is an ongoing technique that defines electromagnetic radiation in a clinical facility or facilities in a multibuilding campus. Footprinting is also done upon request from a department experiencing performance degradation of clinical, diagnostic, or therapeutic devices when EMI is the suspect. The procedure involves a series of 20-MHz-wide spectrum sweeps, beginning at 2 MHz and ending at 1 GHz with antenna(s) in the horizontal plane. This procedure is repeated with the antenna(s) vertically polarized. Tunable standard antennas are adjusted for the correct resonant length for the center of each 20-MHz window. The footprinting procedures yield two results: An overview of the radiated electromagnetic fields present at a specified location in the environment and the amplitudes and types of emission of those fields. Several incident investigations have been successful using this information. In cases where EMI has been site-originated, the source has been removed and the problem has been corrected. In some cases, the affected device required maintenance to correct the problem. An added value of footprinting is that the data obtained during the process meets the basic requirements for a site search similar to the OATS procedure for fingerprinting individual devices. The results of the footprinting scans are transferred to a storage medium and filed. They form a comparison database that is used to evaluate new devices before introducing them into a specified area.
Fingerprinting
The process of fingerprinting a device has several steps. Again, the spectrum analyzer and calibrated antenna system are the primary tools used to analyze the device under test (DUT) (see Figure 63-4). To minimize the loss of information from the DUT due to masking by other sources of electromagnetic radiation, areas within the clinical physical plant should be tested using the footprinting procedure until a relatively quiet area is found. For the fingerprinting procedure, as in the footprinting procedures, the standards antenna should be located as far as possible from any conductive material. The standards antenna should be located at a height equal to the center of the DUT. Due to constantly changing EM fields and frequencies, they execute the footprinting procedure in the selected area immediately prior to fingerprinting the DUT. Once the data from the latest footprint has been stored, place the DUT on a nonmetallic stand located as far as possible from any conductive material. The standard antenna(s) is then mounted on a tripod and, in a horizontally polarized mode, placed 1 meter from the front of the DUT. As in the footprint procedure, a series of 20-MHz-wide spectrum sweeps are performed and the results are recorded. This procedure is repeated with the antenna(s) vertically polarized. When using tunable standard antennas, adjust them to the correct resonant length for the center of each 20-MHz window. Upon review of the data collected, any emissions attributable to the DUT should be rescanned. To increase detail in the frequency range in which the emissions were observed, the spectrum analyzer window is narrowed to a 200KHz/division or smaller, (e.g., 5 KHz/division) sweep width.