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Electromagnetic Environment Generated in a TEM Cell Biological Dosimetry Applications. According to the Federal Office for Radiation Protection (FORP) (2010) biological Dosimetry refers to the quantification of the detected radiation exposure through a biological indicator. Biological Dosimetry supplements the physical Dosimetry (FORP, 2010). In the biological Dosimetry the chromosome analysis is often used in the quantification of the ionizing radiation exposure. This is particularly significant since the ionizing radiation damages the human chromosomes (FORP, 2010). The biological indicators used in the biological Dosimetry thus estimate human exposure to the radiation and can be useful in the determination of the medical care necessary. The main way of access to the radiation into the biological systems is often through the ingestion and inhalation.
Due to the harmful nature of the radiation to the human chromosome the effect of such radiation to biological material is often studied in controlled testing environment (Miclaus and Morega, 2007).The study of the radiation of the biological material is often concerned with the effects of the radiofrequency and microwave radiation on the material. The controlled experimental testing environment is often in form of the TEM cells. The Transversal Electromagnetic (TEM) cells or Crawford cells are 'rectangular enclosures that act as coaxial transmission lines and provide inside Transversal Electromagnetic (TEM) field distribution assimilated to planar waves' (Satir, Christensen, Søren & Christensen, 2008). The source of the microwave radiation is connected to the TEM cell through a coaxial cable (Morega, 2007). Another cable connects to the 'matched load impedance' (Morega, 2007). There are several cautions taken in the design of the TEM cells including the minimization of the wave reflections as well as higher order modes of occurrence. To minimize the chances of 'higher order modes of occurrence' the TEM cells are limited in operations to a set frequency range (Morega, 2007). The efficiency of the TEM cells in any biological Dosimetry experiments is based on several variables on the biological material's radiation exposure (Michie & Löwe, 2006). Some of these variables include maximum test volume, minimum reflections of the waves, uniformity of the electromagnetic waves, minimum higher orders and maximum microwave range (Morega, 2007). Other optimal conditions for the experiment are the stability of the electromagnetic exposure conditions and a quality electric field distribution (Morega, 2007).
The advantages of the TEM cell based research in the biological Dosimetry are numerous. These advantages lie in the different aspects that can be measured by the TEM cell including the specific energy absorption rate (SAR) of a sample and the scattering parameter (S-parameter) values of the sample (Miclaus and Morega, 2007). These aspects may require special calibrations of the TEM cell. Experiments on the electromagnetic waves generated in TEM cell biological dosimetry yield several results. The Electromagnetic Field (EMF) distribution is mostly significant along the Ez direction (Miclaus and Morega, 2007). The Ez component refers to the distribution of the electric field along the Z component in x, y, z axis. This finding was established by measuring the electric field strength of the Ex, Ey and Ez components (Miclaus and Morega, 2007).
Do-it-yourself Fabrication of an Open Tem Cell For EMC Pre Compliance
Electromagnetic compatibility (EMC) refers to the condition in which the electronic devices are able to function in the existing electromagnetic environment without interfering with each other (Clemson University, 2010). There are three basic ingredients to an EMC problem that includes the electromagnetic source, the electromagnetic receptor and a connecting path between the two (Clemson University, 2010). To eliminate the EMC problem at least of the three ingredients is eliminated or redesigned. There are four methods of eliminating the path between the electromagnetic source and electromagnetic receptor. These includes conducted coupling for the electric current, radiated coupling for the electromagnetic field, inductively coupling for the magnetic fields and capacitively coupling for the electric field (Clemson University, 2010).Due to the EMC problem electronic devices are supposed to be tested for the electromagnetic compatibility.
The transverse electromagnetic (TEM) cell is ideal for the EMC pre compliance testing due to it's easy use, diversity in susceptibility tests in radiation and measurement of radiation emissions (Sterrer, 2002). Its other advantage is the ability to generate a wide frequency range of consistent and accurate electromagnetic waves (Satav and Agarwal, 2008). The electromagnetic waves generated by the TEM cell have plane wave characteristics and are transverse propagated (Satav and Agarwal, 2008). The uses of a TEM cell include testing of RF devices for EMC and measurement of emissions from electromagnetic radiations (Satav and Agarwal, 2010).
In essence a TEM cell consists of an electromagnetic field established between two parallel rectangular plates with tapered input and output ports to assist in impedance matching (Satav and Agarwal, 2010).The tapering is limited to 'characteristic impedance of 50' (Satav and Agarwal,2010). The construction of the two plates is should be a manner that confines the electromagnetic field between the plates (Orgel, 1998).There can be open and closed TEM cell with the former having its sidewalls open. The closed TEM cell completely isolates the Equipment under Test (EUT) from any electromagnetic interfere and as such is used for the final EMC tests. The construction dimensions of the open TEM cell that is the height, tapering, width and length are mostly determined by two aspects; the characteristic impedance of the TEM cell and the appropriate applicable frequency range (Satav and Agarwal, 2010).
The characteristic impendence is dependent on various variables such as the width and thickness of the parallel plates as well as the distance between the plates. The other variable is the fringing capacitance per unit length of the plates. The 'FR4 grade glass epoxy double Printed Circuit Board (PCB) sheets ' are used in the construction of the TEM cell. There are several advantages to the use of the PCB sheets to the overall design of the cell including easy fabrication in the laboratory; it's significantly light weight, lesser field fringing and better voltage standing wave ratio (Satav and Agarwal, 2010).
Mode Suppressed TEM Cell Design for High Frequency IC Measurements
Integrated circuits (ICs) are complex circuits engraved on silicon based semiconductor chips. The EMC of the Integrated circuits is very critical in the working of the modern electronic devices. While there are established EMC standards as well as the measurement methodologies for the electronic devices the same has not been established for the ICs which form a component of them. The IEC is working on two standards that try to address the issues; IEC 61967 and IEC 62132 .The IEC 61967 addresses the characterization of the electromagnetic emission while IEC 62132 addresses the characterization of the susceptibility of the IC. The standard IEC 61967 addresses the use of the TEM cell for the measurement of the electromagnetic waves. The other method for the measurement of the same is the surface scan method.
The evaluation of the electromagnetic field coupling is done using the TEM cells (Morega, 2007). There are several factors affecting the applicable frequency range in the evaluation of the electromagnetic field coupling. These include 'resonance of higher order modes' including both its propagation and reflection (Pommerenke et al, 2007).This has the effect of limiting the frequency bandwidth of the TEM cell. There is thus a need for the modification of the TEM cell in the evaluation of the electromagnetic coupling to negate the effects of the higher order resonance (Pommerenke et al, 2007). One of the desired effects of the modified TEM cell is the extension of the frequency bandwidth by a margin of 1.5 GHz up from 1.0 GHz (Miclaus & Morega, 2007).
The suppression of the higher order resonance while maintaining the TEM mode and shielding efficiency at a constant level is critical to the increase of the frequency bandwidth of a TEM cell (Pommerenke et al, 2007). The commonly used higher order resonance suppression methods are not effective on the modified TEM cell due the frequency bandwidth of the 2.5 GHz. The resonances of the Transverse electric (TE) modes are the ones that affect the frequency bandwidth of the TEM cell. The increase in frequency in the TEM cell leads to the generation of the TE modes. The TE modes differ from the TEM modes in the orientation of the electromagnetic vectors (Pommerenke et al, 2007). In relation to the Z (Longitudinal) direction, the TEM mode has both magnetic and electric field vectors perpendicular to the Z direction which isn't the case for the TE modes. The TE modes have the electric and magnetic field vectors perpendicular to each other with the latter in the longitudinal direction (Pommerenke et al, 2007). The regulation of the TE modes lies in the correlating the frequency and the resulting cutoff frequencies to be a manner in which the latter is lower than the former (Pommerenke et al, 2007).There are several methods of the TE suppression in a TEM cell including use of slotted walls and spectrum in TEM cell, use of a two layer PCB spectrum with resistors and the use of resistors between the slotted traces (Pommerenke et al, 2007).Other methods include use of magnetic loops near the walls, use of an absorbing material and ferrite tiles in the corner of the TEM cell.
In the measurement of the electromagnetic radiation from an integrated circuit an IC test Printed Circuit Board (PCB) is used. The test PCB is joined to a specially cut portion on top or bottom of the TEM cell and hence forming a part of the cell wall. The connecting leads are connected outside the cell as opposed to the inside of the cell so as to give space for the operating IC. The emissions from the IC are measured using a spectrum analyzer which is connected to a 50 Î© port of the TEM cell. The spectrum analyzer can be substituted by the use of EMC receiver. Two methods are used to evaluate measured electromagnetic emission (EME) from the ICs; the 1 Î© and 150 Î© methods. The difference between the two methods lies in their measurement of the Radio Frequency (RF). The 150 Î© method uses a 150 impedance matching network while the 1 Î© method uses the resistive probe of 1 Î©. The 150 Î© method has another dimension that can be evaluated; the effect of pins to integrated circuit's emission (Ménétret, Schaletzky, Clemons et al. 2007).
The structure of a modified TEM cell only varies in minimal aspects to the standard TEM cell. The difference lies in the type of the material used as well as the design aspects. Aluminum box is placed on the outer part of the modified TEM cell (Pommerenke et al, 2007).The use of the Aluminum box is to act as a shielding box which together with the absorber reduces any external interference. The absorbing material placed between the wall and the aluminum box. The modified TEM cell also uses a two layer PCB spectrum as opposed to one.