or Research on the Effectiveness of Absorbing Clamp Measurement Method Hong GuoChun Fujian Inspection and Research Institute for Product Quality Abstract For the effectiveness of disturbance power measurement by using absorbing clamp measurement method(acmm), the principle of ACMM is analyzed by establishing a theoretical model and calculating the theoretical values separately for ACMM and field strength measurement method. According to these two methods separately, the measurements were carried out to simulate realistic circumstances. Through comparing the results from these two tests, it can be observed that ACMM is only valid for the particular circumstance, and the measurement deviation between these two methods is caused by different cable routings and lengths of EUT, or caused by existing an RF disturbance circuit inside. Therefore, it must be considered that ACMM should be substituted by field strength measurement method gradually. Keywords disturbance power, ACMM, radiated emissions, radiated disturbance field strength, field strength measurement method, deviation Foreword Radiated emission is one of the most important part of EMC, field strength measurement method is well known as the most representative method which is consistence with trealistic circumstance. Whereas facilities of field strength measurement method is expensive, and the test duration is too long. Regarding to this situation, Meyer de Stadelhofen, a French scholar and his co-workers developed a new method called Absorbing Clamp Measurement Method (ACMM) to estimate radiated emission by using a power clamp to perform disturbance power test. This new method greatly reduces measurement time and cost. In 1967 at Stresa, Italy, CISPR agreed to adopt ACMM as a substitution for radiated emission test. ACMM is generally used in GB 13837 (IDT CISPR 13), GB 4343.1 (IDT CISPR 14-1), GB/T 6113.202 (IDT CISPR 16-2-2) and ANSI C.63.4. This paper analyzes the effectiveness of ACMM by theoretical model and practical test. In practical, such as GB 13837, GB 4343.1, etc., ACMM is just used on frequency range from 30 MHz to 300 MHz, in this paper, only above frequency range was discussed. The Evaluation of the Effectiveness of ACMM Different from the results obtained by field strength measurement method is field strength, the results obtained by ACMM are power, they are two different physical parameters, so they can not to be compared immediately. This paper tries to establish a kind of relation between these two measurement methods by comparing the results obtained by these two different methods and their limits respectively. If these two methods are equivalent, the thresholds between these two measurement results and their respective limits are completely equal. Because of uncertainty in measurement process, there is deviation in the actual measurement results, even though these two measurement methods are completely equivalent in theoretical. According to GB 6113.402, when performing SAFETY & EMC 2013
conformity test in a qualified laboratory, the measurement uncertainty of ACMM U cispr (P) should not exceeding 4.5 db, the measurement uncertainty of field strength measurement method U cispr (E) should not exceeding 5.2 db. Therefore the deviation of the thresholds between these two methods and their respective limits may reach (-9.7 ~+9.7 db) while performing measurement in a qualified laboratory, even though these two measurement methods are completely equivalent in theoretical. This paper tries to investigate two representative standards, GB 13837 and GB 9254. In GB 13837, it describes limits of disturbance power for associated equipments and limits of radiated disturbance at 3 m for receivers at non-local-oscillator frequencies (these limits are adopted from GB 9254). For the same kind of equipment, it may be regarded that the EMC protection level is consistent in this paper, so the measurement results obtained by whichever method must be consistent, therefore the limits of disturbance power in GB 13837 correspond to limits of radiated disturbance in GB 9254, as shown in Figure 1. In other words, for the specified equipment, the deviation between these two thresholds shall be in acceptable range, one threshold is disturbance power measurement results correspond to their limits, the other threshold is radiated field strength measurement results correspond to their limits. To simplify the analysis process, this paper only consider of the quasi-peak limits. is supplied to its mains and other cables, the power is nearly equal to that supplied by the EUT to a suitable absorbing clamp placed around any of these cables at the position where the common-mode current is at its maximum value. The equivalent circuit diagram of ACMM is shown in Figure 2, the interference source which has an inner impedance of Z s feed to a load Z L through a low-loss cable which has a characteristic impedance of Z 0. If the value of Z L is not equal to Z 0, the power supplied by the EUT to the suitable absorbing clamp placed around any of these cables varies with position of the clamp, and the maximum and minimum respectively correspond to resonance and reactive resonance of the system. Figure2 Diagram of the principle of ACMM The results obtained by ACMM is just the interference energy radiated by portions of mains and other connected cables near the EUT, whereas the results obtained by field strength measurement method represents interference energy radiated by the entire EUT, so it's considered that the results obtained by ACMM regards the portions as an entirety. Therefore, in principle, the results obtained by ACMM is correct only when the portions investigated by ACMM equal to the entire EUT, it will be deeply discussed in following paragraphs. The Effectiveness of ACMM in Theoretical Model Figure1 Limits of disturbance power in GB 13837 and limits of radiated disturbance at 3 m distance in GB 9254 Analysis of the Principle of ACMM It's assumed of ACMM that the disturbing energy is mostly radiated by portions of mains and other connected cables near the EUT while the unit dimension is less than a quarter of a wavelength of the highest measuring frequency (according to GB 6113.202 Appendix A.2, the volume of the EUT shall be 3.5~1 700 dm 3 ), it is therefore agreed to define the disturbing level as the power that When dimension of EUT comply with the descriptions of the former Chapter 2, it's assumed by CISPR that the interference energy is mostly radiated by mains cable. It is defined the interference level as the power that is supplied to its mains cable in ACMM; but field strength measurement method is defined the interference level as the realistic field strength that is radiated by its mains cable, this field strength depends on current distribution of the cable, current of the cable is associated with the power that is supplied by interference source. In following paragraphs, it's established a theoretical model in this paper to calculate current of the cable and field strength radiated by cable respectively. 1) Calculation of the wave loop's current of the cable under test According to the calibrated procedure of GB 6113.103, in the SAFETY & EMC 2013
range of 30~300 MHz, the impedance of each unit of cable is greatly less than its inductive resistance, the conductive resistance is also greatly less than its capacitive resistance. Meanwhile, because of influence on the assistant clamp, so the remote-end reflected wave may be ignored, therefore the cable under test can be regarded as an infinitive non-reflective transmission line. According to the theory of microwave transmission line, I m Where = Zs P + Z (1) 0 I m is the wave loop's current of the cable under test, P is the power of the interference source, Z s is the inner impedance of the interference source, the value is 50 Ω, Z 0 is the characteristic impedance of the transmission line. As said in GB 6113.103, during the calibration of the absorbing clamp, the cable under test and grounded plane can be regarded as a micro-strip system, the characteristic impedance may be calculated by Hammerstadt,E.O. formula: Where plane, 60 8H 025W Z0= ε ln(. + ) (2) W H r Z 0 is the characteristic impedance of the system to be tested, ε r is relative permittivity, H is the distance between the cable under test and the grounded W is the diameter of the cable under test. According to GB 6113.103, ε r =1, H=0.8 m, W=0.004 m;putting above value into formula (2), it can be obtained that Z 0 =442.7 Ω For convenience, making the power (P) of interference source at any frequency equal to their limit of the disturbance power, and putting P, Z 0 =442.7 Ω, Z s =50 Ω into formula (1), thus the wave loop current I m of the cable under test at corresponding frequency can be obtained. test 2) Calculation of the field strength radiated by the cable under It's assumed of ACMM that the disturbing energy is mostly radiated by portions of mains cable, and the mains cable usually fall down to floor. To simplify this model, we assume that the mains cable is straight to the floor, so it can be regarded as a monopole antenna that is straight to the floor. Because of influence of the plane, the monopole antenna and its image can be regarded as a pair of symmetrical antenna, the length of one arm of the symmetrical antenna is l=0.8 m (equals to the height of table). Hence, it can be abstracted out a theoretical model, see Figure 3. According to the Figure 3 Diagram of the principle of field strength measurement method theory of antenna, the field strength distribution of far area is as below: E θ Where point, 60I βl θ βl = m cos( cos ) cos r sinθ E θ is the field strength distribution of far area, I m is the wave loop current of the cable under test, l is the length of one arm of the symmetrical antenna, r is distance between the interference source and the observing β is phase constant, equals to 2π/λ (λ is the wavelength of corresponding frequency). Putting I m (that is attained from Clause 3 (1)), l=0.8 m, r=r 0 / sinθ=3/sinθ, β into formula (3), the results is shown in Table 1, we only considering the following 12 frequencies: 30 MHz, 50 MHz, 75 MHz, 100 MHz, 125 MHz, 150 MHz, 175 MHz, 200 MHz, 225 MHz, 250 MHz, 275 MHz and 300 MHz. For example, at the frequency of 200 MHz, the limit of disturbance power is 51.30 dbpw, since the former hypothesis that the power of the interference source is equal to the limit of the corresponding frequency, then the power of the interference source is 51.30 dbpw, so the threshold between the power of the interference source and the limit of the corresponding frequency is 0 db. Accordingly, the wave loop current of the cable under test is 16.54 μa, the field strength at 3 m distance radiated by the cable under test is 47.85 dbμv/m. The receiving antenna is at 1.00 m height, so the threshold between the field strength and the limit of the corresponding frequency is 7.85 db. According to this theoretical model, the theoretical deviation between field strength measurement method and ACMM is 7.85 db at 200 MHz. The last row in Table 1 are the data of the deviations between these two methods under the theoretical model (-11.23~13.12 db, except of 30 MHz, others fall into -2.18 ~13.12 db). Even though these two measurement methods are completely equivalent in (3) SAFETY & EMC 2013
Table 1 A comparison between ACMM and field strength measurement method under the theoretical model Frequency /MHz 30 50 75 100 125 150 175 200 225 250 275 300 Wavelength /m 10.00 6.00 4.00 3.00 2.40 2.00 1.71 1.50 1.33 1.20 1.09 1.00 The power of interference source/dbpw 45.00 45.74 46.67 47.59 48.52 49.44 50.37 51.30 52.22 53.15 54.07 55.00 Corresponding wave loop current /μa 8.01 8.72 9.71 10.80 12.01 13.36 14.87 16.54 18.40 20.47 22.77 25.33 Corresponding field strength /(dbμv/m) 28.77 37.82 44.65 48.89 51.38 52.33 51.53 47.85 51.32 55.76 58.59 60.12 Corresponding antenna height /m 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 2.68 2.25 1.96 1.73 The limit of field strength /(dbμv/m) 40 40 40 40 40 40 40 40 40 47 47 47 The deviation between these two methods /db -11.23-2.18 4.65 8.89 11.38 12.33 11.53 7.85 11.32 8.76 11.59 13.12 theoretical, the actual deviations may reach between -9.7 and +9.7 db, but the results of theoretical calculation are worse, which range from -1.53 db to 3.42 db. In consideration of the immensity of the measurement uncertainty of these two methods themselves, ACMM may be regarded as an acceptable method, though it is not an ideal method. In practical, according to formula (3), at a certain distance for observation the radiated field strength E θ is not only relate to I m, but also a function of (l/λ). By the analysis above, we can just consider the disturbance power of I m, in spite of any other influence. Therefore, ACMM is not a sufficient method in theoretical. The Discussion on ACMM and Field Strength Measurement Method in Actual Measurement ACMM is only consider of the disturbance power of a cable, in spite of the realistic cable routings and lengths of EUT, or the existence of an RF disturbance circuit inside the EUT, these three circumstances are very important factor for radiated field strength. Figure 4(a), 4(b), 4(c) represent three common actual cable routings: Figure 4(a) stands for the basic hypothetic circumstance in theoretical, Figure 4(b) stands for a circumstance of a shorter cable, (a) (b) (c) (d) Figure 4 Diagram of four kind of actual cable routings and Figure 4(c) stands for a circumstance of a cable crossing through circumstance in theoretical and existing an RF disturbance circuit two equipments,whereas Figure 4(d) stands for the basic hypothetic inside the EUT simultaneously. In this paper Figure 4(b), 4(c), SAFETY & EMC 2013
4(d) will not be calculated in theoretical, if necessary, they can be calculated by simulating the calculation of Figure 4(a). In the following paragraphs, this paper will investigate the relationship between these two methods through performing measurements using AV connection cables by convenience. Selecting a standard signal generator as the interference source, connecting a 6 db attenuator, a 50 Ω/75 Ω converter and an AV connection cable are terminated by a matching load. To performdisturbance power measurement in an appropriate shielding room according to the related standards, the output level of the standard signal generator is adjusted and the receiver indications equal to the limits of disturbance power is set at corresponding frequencies. Then remain the output level of the standard signal generator unchanged, the standard signal generator, 6 db attenuator, 50 Ω/75 Ω converter, AV connection cable, and matching load are moved into a 3 m anechoic chamber, configured as Figure 4(a), 4(b), 4(c), to perform radiated field strength measurement and to compare the results and their limits, filled into Table 2. Table 2 The comparison between the measurement results of disturbance power and the measurement results of radiated field strength that are configured as Figure 4 Frequency /MHz 30 50 75 100 125 150 175 200 225 250 275 300 The power of interference source /dbpw 45.00 45.74 46.67 47.59 48.52 49.44 50.37 51.30 52.22 53.15 54.07 55.00 The limits of field strength /(dbμv/m) 40 40 40 40 40 40 40 40 40 47 47 47 Figure 4(a), field strength /(dbμv/m) 23.9 38.9 29.9 32.4 32.7 46.9 42.1 39.7 41.3 54.8 55.9 52.3 Figure 4(a), deviation /db -16.1-1.1-10.0-7.6-7.3 6.9 2.1-0.3 1.3 7.8 8.9 5.3 Figure 4(b), field strength /(dbμv/m) 20 20.7 32 28.1 40 32.2 53.9 46.4 50.3 51.3 52.1 49.1 Figure 4(b), deviation /db -20.0-19.3-8.0-11.9 0.0-7.8 13.9 6.4 10.3 4.3 5.1 2.1 Figure 4(c), field strength /(dbμv/m) 19.1 27.5 13 24.1 29.2 39.4 43.6 41.6 46.3 60.5 59.9 56.3 Figure 4(c), deviation /db -20.9-12.5-27.0-15.9-10.8-0.6 3.6 1.6 6.3 13.5 12.9 9.3 Figure 4(d),Ф=10 cm, field strength /(dbμv/m) 44.7 55.1 43.0 40.3 50.1 53.2 49.9 59.8 61.8 67.0 71.7 72.8 Figure 4(d),Ф=10 cm, deviation /db 4.7 15.1 3.0 0.3 10.1 13.2 9.9 19.8 21.8 20.0 24.7 25.8 Figure 4(d),Ф=20 cm, field strength /(dbμv/m) 33.6 37.8 36 45.6 62.6 66.0 57.1 60.3 68.4 72.1 75.9 73.4 Figure 4(d),Ф=20 cm, deviation /db -6.4-2.2-4.0 5.6 22.6 26.0 17.1 20.3 28.4 25.1 28.9 26.4 In Table 2, the second row "the power of interference source" is the power that is supplied by the interference source of the EUT and equals to the disturbance power limit of corresponding frequency, so the threshold between the measurement result of disturbance power and its limit is 0 db; the fourth row "Figure 4(a), field strength" is the actual measurement result of radiated field strength that is configured as Figure 4(a); the fifth row "Figure 4(a), deviation" is the deviation between field strength measurement method and ACMM, that is equal to the threshold between the measurement results of the radiated field strength and their limits (because the threshold of ACMM is 0 db). Otherwise, the meanings of the others column of Table 2 configured as Figure 4(b), 4(c), 4(d) are similar with Figure 4(a). The fifth column "Figure 4(a), deviation" of Table 2 is the actual measurement deviation between field strength measurement method and ACMM (-16.1~8.9 db, except of 30 MHz, others fall into -10.0~8.9 db). The actual measurement deviations of these two methods are smaller than their theoretical deviations (except of 30 MHz). Since the cable under test is straight to the floor in theoretical model, it is not straight to the floor in actual, so the efficiency of antenna is lower, and the actual measurement results are smaller than the theoretical ones, this reduces the negative influence of the function of (l/λ) in formula (3) to some extent. Therefore, the actual measurement results of Figure 4 (a) prove that ACMM can yet be regarded as an acceptable method of radiated emissions. However, when the EUT cable routings configured as Figure 4(b) and Figure 4(c), the actual measurement deviations increase obviously, the actual measurement deviations of Figure 4(b) range between -20.0 and 13.9 db, the actual measurement deviations of Figure 4(c) range between -27.0 and 13.5 db. The model shown in Figure 4(b) means a shorter cable (from 0.8 m shorten to 0.4 m), the cable length is not similar to the wavelength of low frequency bandwidth, the efficiency of antenna decreases, the actual field strength decreases in pace with decrease of antenna's efficiency, so the actual negative measurement deviations of the low frequency bandwidth increase; At 175 MHz, the wavelength is 1.71 m, so a quarter of the wavelength is 0.43 m, the length of the cable under test is 0.4 m, it is easy producing resonance, then the actual field strength increases, and the actual positive measurement deviation SAFETY & EMC 2013
increases either. The directions of the current of the two vertical portions of the "U" shape in Figure 4(c) are contrary, this means the effective length of the cable under test shortens, at low frequency bandwidth the efficiency of antenna decreases, at high frequency bandwidth it is easy producing resonance, so the negative deviations at low frequency band increase, and the positive deviations at high frequency band increase too. Therefore, ACMM is not appropriate as a substitution of field strength measurement method at these two circumstances. Otherwise, when there exists an RF disturbance circuit inside the EUT, it may establish a model like Figure 4(d) for simulation. Selecting a standard signal generator as the interference source, connecting with a 6 db attenuator and a 50 Ω/ 75 Ω converter, passing through a "T" shape connector, then connecting to a loop antenna which has a diameter of 10 cm and an AV connection cable terminated by a matching load, performing radiated field strength measurement as the methods of Figure 4(a), 4(b), 4(c). Finally the measurement results are filled into Table 2; Replacing Ф10 cm loop antenna with Ф20 cm loop antenna, then repeating the above mentioned process, the measurement results are also filled into Table 2. According to Table 2, the actual measurement deviations of Figure 4(d) are also very large, reach to (0.3~25.8 db,ф=10 cm) and (-6.4~28.9 db,ф=20 cm). It is influenced by loop antenna, the actual field strength get higher, the measurement deviations increase with 15~20 db. Therefore, ACMM is obviously not an acceptable method for Figure 4(d). Overall, there are negative deviations of these two methods at low frequency bandwidth and positive deviations of these two methods at high frequency bandwidth. In other words, the actual field strength is lower at low frequencies and higher at high frequencies, this is because the wavelength is from 10 m to 1 m correspond to the frequency range from 30 MHz to 300 MHz, the length of the cable under test get more similar with the wavelength of the investigated frequency, and the efficiency of the antenna increases, then the actual field strength get higher. It can lead to a conclusion by the above mentioned actual measurement results, for a small, simple EUT only with a mains cable (like as Figure 4(a)), there is a good relation between ACMM and field strength measurement method, yet ACMM can be regarded as an acceptable and simpler substitution of the measurement of radiated emissions. Therefore, for an EUT shown in Figure 4(a) (such as household appliances), ACMM is still useful in practical. However, ACMM is not able to consider of different cable routings and lengths of EUT, or the existence of an RF disturbance circuit inside, so the results obtained by ACMM are quite different from the results attained by field strength measurement method. ACMM is not obviously a substitute method of radiated emissions. Otherwise, even the EUT complies with the assumptions of CISPR, the deviations between these two methods are too large at the frequency of 30 MHz. Conclusions Overall, it is time for estimation of ACMM, it should be considered that ACMM should be substituted by field strength measurement method gradually. This is the reason the newest edition of CISPR 32 has abandoned ACMM. It may be considered of keeping ACMM as a pre-test method for the continuity of standards. [1] GB/T 6113.103-2008 Specification for radio disturbance and immunity measuring apparatus and methods - Part 1-3: Radio disturbance and immunity measuring apparatus-ancillary equipment - Disturbances power[s]. Beijing: Standards Press of China, 2008. [2] GB/T 6113.202-2008 Specification for radio disturbance and immunity measuring apparatus and methods - Part2-2: Methods of measurement of disturbances and immunity-measurement of disturbance power[s]. Beijing: Standards Press of China, 2008. [3] GB/T 6113.402-2006 Specification for radio disturbance and immunity measuring apparatus and methods - Part 4-2: Uncertainties, statistics and limit modelling - Uncertainty in EMC measurements[s]. Beijing: Standards Press of China, 2006. [4] GB 4343.1-2009 Electromagnetic compatibility. Requirements for household appliances, electric tools and similar apparatus. Part 1: Emission[S]. Beijing: Standards Press of China, 2009. [5] GB 9254-2008 Information technology equipment - Radio disturbance characteristics - Limits and methods of measurement[s]. Beijing: Standards Press of China, 2008. [6] GB 13837-2003 Sound and television broadcast receivers and associated equipment - Radio disturbance characteristics - Limits and methods of measurement[s]. Beijing: Standards Press of China, 2003. [7] Chufang Xie, Wenjie Qiu. The theory and design of Antenna[M] Xi'an: Northwest Telecommunication Engineering Institute Press, 1985. [8] Chengen LIAO. The theory and technology basis of microwave [M] Xi'an: XiDian University Press, 1990. [9] IEC/CISPR 32:2012 Electromagnetic compatibility of multimedia equipment - Emission requirements[s].2012. [10] Meyer de Stadelhofen.J. A new device for radio interference measurements at VHF: the absorbing clamp[c]// IEEE. EMC symposium Proceedings. Piscataway: IEEE, 1969:189-193. SAFETY & EMC 2013