How to test the radiation emission of medical products?

　　Radiated emission (RE) testing is mainly to measure the electromagnetic interference energy of the equipment under test to the environment. As electronic equipment is developing day by day, there are more and more electronic equipment around, and the electromagnetic environment is becoming more and more crowded. The purpose of RE testing is to control the electromagnetic radiation of the equipment under test so that it does not interfere with the surrounding electronic equipment.

　　Medical equipment can be divided into Class A and Class B equipment according to the use environment: Class A equipment refers to non-household equipment or equipment used in facilities that are not directly connected to a residential low-voltage power supply network; Class B equipment refers to household equipment and equipment used in facilities that are directly connected to a residential low-voltage power supply network.

　　For Class A equipment, CISPR11 stipulates that the test of its radiated emission can be carried out either in a test field or on site.

　　CISPR11 explains this: "Due to factors such as the size of the equipment under test, the complexity of its structure and operating conditions, certain industrial, scientific and medical equipment can only be determined through on-site testing to determine whether it complies with the radiation emission limits specified in this standard." MRI and DR products are basically Class A devices, so their radiation emission tests can be carried out in the form of on-site testing.

　　1. Test limits

　　In addition to the classification of Class A and Class B, industrial, scientific and medical equipment can be divided into Group 1 and Group 2 equipment according to the purpose of their radio frequency energy. The definition of Group 1 and Group 2 equipment in the standard is as follows:

　　Group 1 Industrial, Scientific and Medical Equipment: All industrial, scientific and medical equipment that intentionally generates and/or uses conducted coupled radio frequency energy for its own functional needs.

　　Group 2 ISM equipment: includes EDM and arc welding equipment, as well as all ISM equipment that intentionally generates and/or uses electromagnetic radiation radio frequency energy for material processing. Most types of equipment and systems generate or use RF energy only for their internal functions, so they belong to Group 1. Such as electrocardiogram and magnetocardiogram equipment and systems, electroencephalogram and magnetoencephalogram equipment and systems, etc. Some other equipment and systems that are expected to transfer energy to patients in non-RF electromagnetic forms also belong to Group 1 equipment, such as medical imaging equipment and systems - X-ray diagnostic systems, CT systems, nuclear medicine systems, ultrasound diagnostic systems, etc.; treatment equipment and systems - X-ray treatment systems, ultrasound treatment systems, infusion pumps, ventilators, etc.; monitoring equipment and systems - impedance plethysmography monitors, pulse oximeters, etc. Only a few equipment and systems apply RF energy to materials (medical equipment is to patients), and belong to Group 2 equipment. Common Group 2 equipment includes: magnetic resonance imaging systems, diathermy equipment (short wave, ultrashort wave, microwave treatment equipment), thermal therapy equipment and high-frequency surgical systems, etc.

　　The emission limits for Group 1 Class A and Group 2 Class A devices are shown in Tables 1 and 2. There are some differences between the limits for the two groups.

　　

　　

　　For Group 1 Class A equipment, the field test limits are the same as those in Table 1. For Group 1 Class A and Class B equipment that is to be permanently installed in an X-ray shielded location, an increase of 12 dB is permitted in the electromagnetic radiation emission limits when measured in the test field.

　　For the two groups of Class A test equipment measured on site, as long as the test distance D in Table 2 is within the jurisdictional perimeter, the test distance is calculated from the outer wall of the building where the test equipment is installed, D = (30 + x/a) (in meters, m) or D = 100m, whichever is smaller. When the calculated distance D exceeds the jurisdictional perimeter, then D = x or 30m, whichever is larger.

　　x is the closest distance in each measurement direction between the exterior wall of the building where the EUT is installed and the perimeter of the user's jurisdiction;

　　a=2.5 (frequency below 1MHz)

　　a=4.5 (frequency greater than or equal to 1MHz)

　　In order to protect special aviation services in a specific area, relevant national authorities may require that 30m distance be met as a fixed limit.

　　2. Environmental requirements

　　During on-site testing, since it is not a dedicated shielded test site, it will receive various environmental interferences, such as mobile phone communication signals, broadcast signals, etc. The impact of these environmental noises should be considered in the test. The test site should be able to ensure that the emission of the equipment under test can be distinguished from the environmental noise.

　　In order to distinguish the influence of environmental noise from the emission of the device under test, the environmental noise level should be measured before the test. Before the test, place the antenna and receiver according to the normal test layout, turn off the device under test, and then measure the environmental noise level.

　　The test results should ensure that the environmental level is at least 6dB lower than the specified limit of the EUT to facilitate measurement. Therefore, when testing, the surrounding electronic equipment and other equipment that may affect the radiation characteristics should be turned off as much as possible to ensure a relatively pure test environment.

　　Due to the limitations of the field test site, sometimes the interference cannot be eliminated manually and the 6dB margin requirement cannot be met. For this situation, CISPR11 provides the following solutions:

　　1. When the ambient level plus the emission of the equipment under test still does not exceed the specified limit, there is no need to reduce the ambient level to below 6dB of the specified limit. In this case, the equipment under test can be considered to have met the specified limit.

　　2. If the field strength cannot be measured at the specified distance due to the ambient noise level or other reasons, it can be measured at a follow-up distance. In this case, the distance and test conditions should be recorded in the test report. In order to determine whether it is qualified or not, the measured data should be normalized to the specified distance by an inverse factor of 20dB for every 10 times the distance. When measuring large test products at a distance of 3m, pay attention to the influence of the near-field effect when the frequency is close to 30MHz.

　　3 Test Arrangement

　　3.1 Antenna Arrangement

　　According to the requirements of CISPR11 Clause9: "For equipment that is not measured in a radiation test field, the equipment can be installed in the user's jurisdiction and then measured. The measurement should be performed outside the outer wall of the building where the equipment is installed at the test distance specified in Section 5." Taking DR products as an example, the DR product is placed in an X-ray shielded room and the antenna is tested outside the outer wall.

　　The antenna used for field measurement is the same as that used in the test field. Other types of antennas may also be used if the difference between the measurement results and the results of the balanced dipole antenna measurement is within 2dB. The 3m method is used for testing, but the antenna for field measurement is no longer raised or lowered within the range of 1~4m, but the center of the antenna is fixed at a height of 2.0m±2.0m above the ground. The position of the antenna is selected for measurement in at least four directions orthogonal to the shielded room, as shown in Figure 1. In addition to these four points, as many test points as possible should be selected with the shielded room as the center, if practical conditions permit. In addition, measurements should be made in any direction that may have a harmful effect on the radio system.

　　

　　3.2 Equipment Layout

　　When measuring radiated emissions, the equipment settings should be adjusted as much as possible to obtain the maximum value of the disturbance level. The specific arrangement depends on the inherent mobility of each specific equipment. When testing on site, for specific equipment, it is necessary to take into account the changes in the location of the cables and the different functions in the equipment, as well as the degree to which the equipment can be moved in the premises on site. The arrangement of the equipment under test should be accurately recorded in the test report.

　　The length and type of the interconnecting cables between the devices during the test should be consistent with the specifications in the technical requirements of the individual devices. If the cable length can be changed, the cable that can produce the maximum field strength should be selected when measuring the field strength. If shielded cables or special cables are to be used in the test, they should be clearly specified in the instruction manual. Where there are multiple interfaces of the same type, if increasing the number of cables does not significantly affect the measurement results, just use one cable to connect to one of the interfaces of this type.

　　Any set of measurement results should be accompanied by a complete description of the location of the cables and equipment so that the measurement results can be reproduced. If there are conditions for use, they should be specified and included in the instructions for use as a backup. If a device can perform several functions separately, the device should be tested when performing each function. For systems consisting of several different types of equipment, at least one of each type of equipment should be included in the evaluation. If the system contains several identical devices, only one of the devices needs to be evaluated. If the initial evaluation meets the requirements, no further evaluation is required. When evaluating equipment that is connected to other equipment to form a system, other equipment or simulators can be used to represent the entire system for evaluation. Both evaluation methods for the equipment under test should ensure that the other parts of the system or the simulator influence meet the requirements of the standard for environmental noise levels. Any simulator used to replace the actual equipment should fully represent the electrical and in some cases mechanical characteristics of the interface, especially the RF signal and RF impedance, cable layout and model. Taking DR products as an example, during field testing, the equipment is basically fixed after installation and will not move again, so the equipment itself does not need to be moved. DR interconnection cables are relatively long. Under the premise of ensuring that the cables are fully assembled, they should be placed in different ways during testing to achieve the maximum interference requirements. Some components of DR products have functions such as mechanical movement and exposure. During testing, they should be tested under different functional conditions, and the maximum value of radiation emission should be taken as the final test result.

　　3.3 Test steps

　　Based on the above analysis, during the field test, the following process was followed for measurement:

　　1. Arrange the equipment under test according to the typical installation requirements at the client end, and the length and model of the wires must comply with the specifications;

　　2. Place the antenna and receiver 3m away from the outer wall of the EUT (equipment under test), and adjust the center of the antenna to a height of 2m from the ground;

　　3. Turn off the equipment under test first and test the ambient noise level;

　　4. Turn on the device under test, adjust the wiring method, perform different functions, and find the maximum radiation mode;

　　5. Rotate the antenna to test the horizontal and vertical radiated disturbances respectively;

　　6. Move the antenna around the device under test and select as many test points as possible for measurement;

　　7. Select the highest radiation level as the measurement result;

　　8. Turn off the device.