Regarding RF Isolation for small Enclosures

Transcription

1 Regarding RF Isolation for small Enclosures IEEE electromagnetic society and IEEE standard board has published standards for measuring the shielding effectiveness (SE) of chambers. The measurement methods are introduced for single or double wall shielded enclosures. The standard IEEE STD , provides methods to measure the SE of shielding enclosures. This standard replaced MIL-STD 285 which its applicability lessened by technological changes and canceled by U.S department of defense in 1997 [1]. This standard and earlier versions cover SE measurement methods for chambers with minimum dimension greater than 2m. There is not any approved specific standard for measuring SE for smaller size chamber [1]. In this paper, measurement method for 300 MHz 6 GHz in chambers with dimension greater than 2.0m is investigated. Also, methods for SE measurement in small chambers are introduced. Shielding Effectiveness: magnitude ratio of signal received from transmitter antenna without shielding enclosure to the signal received within enclosure [1]. Shielding Enclosure: It is a structure that prevents any electromagnetic field (EM) penetrate to its interior, and it also prevents environments from the effect of internal EM fields. In general, a shielded enclosure is constructed from metals with a continuous electrical connection in between metallic parts [1]. Test plan for any shielding enclosure must be provided for SE measurement. Test plan includes frequencies that owner will use the enclosure. There are suggested frequencies for low frequency (magnetic), resonant frequency, and plane wave ranges. Table 1: Test plane recommended frequencies suggested by standard IEEE STD 2006 [1]. Standard frequency Antenna type Low range 9 KHz -16 KHz Loop 140 KHz -160 KHz Loop 14 MHz -16 MHz Loop Resonant range 20 MHz MHz Biconical 100 MHz MHz Dipole High Frequency range 300 MHz GHz Dipole 0.6 GHz - 1GHz Dipole 1 GHz 2 GHz Horn 2 GHz 4 GHz Horn 4 GHz 8 GHz Horn

2 It has to be stressed that the frequencies above are only suggestions made by the standard, and owners can request test plans with their frequencies [1]. Dynamic Range: The dynamic range of testing system is determined by strength of exciting signal, performance of transmit and receive antennas, cable loss, attenuators and amplifiers in case used, and noise floor of receiving instrument. In the case of measurement target for SE >120 db, a higher field strength EM sources is needed. Antennas will not affect the DR in significant matter. The noise floor for modern EM sources and receivers is below -120dB. The DR for receiver system is the difference between highest signal strength (1dB compression point) and minimum detectable signal strength: DR receive = P 1 P 2 (db) Eq. 1 P 1 : maximum input signal; causes 1dB compression point P 2 : instrument detectable minimum signal level; the noise floor When measuring SE for shielding enclosure, the DR should only exceed the SE value by 6dB [1]. Source of EM field: The source of EM field can be Dipole, Horn, Biconical, and Yagi antennas. A continuous wave without modulation is used to power the antennas. Measurement of SE for Chambers dimension greater than 2.0m In this study, the SE method for frequency range 300 MHz - 6 GHz is investigated. In the first step, leakage test should be performed and any leakage source must be repaired. The parts of enclosure that need to be checked are vents, door area, power line filters, seams, coaxial cables, etc. Based on the preliminary leakage test results, owner will decide to do a full SE measurement or do repairs [1]. The preliminary test method is done by rotating or placing antennas in a place to have highest signal strength and performance of shielding is determined. If leakage test passes then full SE test can be performed. According to standard [1], leakage test is not mandatory in SE measurement. The SE measurement includes setups for: reference level Figure 1 and attenuation measurement [2], as shown Figure 2. Figure 1: Reference level measurement; using two horn antennas. Attenuators must be used if high fields are applied and no internal attenuators to EM meter. The distance of 2.3m is mandated by the standard [1].

3 Figure 2: SE measurement for chambers dimensions (all xyz) bigger than 2m within frequency range of 300MHz- 6GHz. The power levels has to be paid attention because of possible hazardous to testing personnel. The shielding effectiveness can be expressed by [1]: SE = 20log 10 E 1 E 2 (db) Eq. 2 E 1 : is the received electric field strength in the absence of enclosure in Figure 1. E 2 : is the received electric field strength within enclosure as shown in Figure 2. Figure 3: SE measurement for shielding box with dimensions smaller than 2m. A and B demonstrate the attenuation and reference setups for electric field [2]. Measurement of SE for smaller enclosure dimensions less than 2m The restriction of 2m is because of antenna dimensions that limits the measuring location to the far field of antennas. As it was stated earlier, SE is the difference of EM field strength inside and outside of an enclosure [2]. Because of dimension limitations proposed by standard [1], antennas cannot be placed inside the small shielding enclosures for full SE investigation [2]. In

4 [2], authors suggested to minimize the size of antennas for smaller enclosures, however they also point out that this will degraded the dynamic range of system. On the other hand, the walls (reflection from walls) of enclosure affect the antenna pattern. The setup for electric field measurement in smaller shielding enclosure is shown in Figure 3 [2]. As it can be seen, measurement is repeated for door open (standardization or reference) and close (attenuation) without moving the antenna positions. The difference in signal level is SE of the shielding enclosure. A log periodic or Yagi antenna can be placed outside 1m away from box as transmitter. It is recommended that the entire setup can be placed inside an anechoic chamber in order to reduce interference. Ferrite beads are used in end points and midpoints of coaxial cables to avoid common-mode noise to couple to the lines [3]. Figure 4: Electric field measurement for sheilded enclosure inside anechoic chamber. Ferrites are shown on coaxial cables to avoid common mode currents to flow on lines. Antenna can be a log periodic or Biconical antenna. Foam is placed under shielded box to minimize reflections from bottom of chamber. SE of a shielding enclosure using comb generator In this case the signal transmitter includes comb generator, antenna, and accumulator. Comb generator is a signal generator that generates vector of frequencies. A spectrum of frequencies is generated with descending amplitude by increasing frequency. The receive chain is composed of antennas, antenna commutator, receiver and a display.

5 Figure 5: SE measurement setup with comb generator: reference and attenuation measurements with enclosure door open and closed, respectively. Measurement is done inside anechoic chamber to reduce reflections and noise. The received signal from antennas in Figure 5 is routed to measuring receiver. The angle of antennas can be automatically adjusted using antenna controller. SE measurement with HEMP generator High level electromagnetic pulses (high voltage pulses) also can be used to measure SE of an enclosure. For this measurement a set of portable measuring signal transmitter and field intensity probe is needed. Figure 6: SE measurement using EMP generator. The oscilloscope is placed inside shielded unit in order to avoid high power electromagnetic field damage its functionality.

6 As it is shown in Figure 6, the receive chain includes a power sensor, matching circuit, attenuator and oscilloscope. The EMP generator creates high voltage pulses about 80 KV/m in short time sequences, as low as 5ns. In this way, the HEMP generator creates very high voltage pulses at the input of transmitting antenna. Data from power sensor is shown in oscilloscope in time domain. A device with Fourier transform function can be used to export frequency spectrum from data. The measurement takes place in two parts as shown in Figure 6: reference without shielding enclosure, and attenuation with shielding enclosure. For each particular frequency measurement should be done for reference level and attenuation levels and SE can be calculated by subtracting numbers. Conclusion In this paper, a general standard is introduced for measurement of SE in shielding enclosures but this standard covers shielding enclosures with dimensions bigger than 2m. Unfortunately, there is not an approved standard written and available for enclosures with dimensions less than 2m. In this paper, methods are introduced that can be used to analyze SE of smaller boxes. Using anechoic chamber is recommended in the measurement to reduce reflections and noise from environment. References [1] IEEE Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures, IEEE Std Revis. IEEE Std , pp. 1 52, Feb [2] F. Ustuner, A. Akses, I. Araz, and B. Colak, A method for evaluating the shielding effectiveness of small enclosures, in 2001 IEEE International Symposium on Electromagnetic Compatibility, EMC, 2001, vol. 2, pp vol.2. [3] L. Nowosielski, M. Wnuk, and R. Przesmycki, Methods of Measuring Shielding Effectiveness of Small Shielded Chambers, Prog. Electromagn. Res. Symp. Proc. Mosc. Russ., p. 1437, Author: Saeed Javidmehr - Advanced System Division, Testforce saeed@testforce.com Address: Testforce Systems Inc, Ironstone Manor, Pickering ON, CANADA L1W 3W9 Contact information: Tell: (905) Fax: (905)