DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

160 ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICESincreasing the physical size of the coil. However, this solution will again result in reducedspectral response due to increased self-inductance. It is clear, then, that loop geometry mustbe chosen for every specific instance based on a solution of compromise. In general, it is agood idea to keep a variety of probes handy to tackle different problems.Another convenient H-field probe can be constructed similar to ac tongs. In this case, amagnetically permeable material concentrates the magnetic flux lines created by the circuitunder test. The resulting magnetic flux is then detected by a coil with multiple turns.If the tongs were to enclose completely the conductor through which a current is flowing,the voltage developed across the coil would be proportional to the vector sum of the currentsthrough the conductor. This is, of course, impractical for the needs of sniffing H-fields, and a structure with open-ended tongs is more suitable for probing a circuit withoutmodifying it.The probe can be built as shown in Figure 4.8, using a small ferrite bead (e.g., 0.1 in.thick, 0.3 in. in outside diameter) that has been sectioned in half. The actual constructiondepends on the actual ferrite that you select, but in general, 40 to 50 turns of thin enameledcopper wire provide suitable sensitivity. The terminals of the coil should be solderedto the center and shield of coaxial cable. After insulating the central conductor connection,a portion of the braid should be used to cover the assembly, thus E-field-shielding the coil.The assembly can then be mounted at the end of a small plastic tube and embedded withina glob of epoxy. For the prototype probe, a virtually flat bandwidth was measured fromaround 600 kHz to approximately 10 MHz.Better bandwidth can be achieved by using a VCR magnetic head instead of the ferriteassembly. Video heads are designed for broadband detection of magnetic fluctuations, andfor this reason they can be used for sniffing H-fields from 2 MHz up to approximately120 MHz with relatively flat response. To construct the probe, carefully remove one of theCoaxPlastic TubeEpoxy GlobE-Field Shield~0.3Ferrite “U”Figure 4.8 A useful H-field probe can be constructed from a small ferrite bead that has been sectionedin half. Approximately 40 turns of thin enameled wire are used to detect the magnetic fluxconcentrated by the ferrite. A small portion of the coax cable braid is used as an E-field shield forthe coil. The assembly is mounted at the end of a small plastic tube that serves as a handle andembedded in a glob of epoxy.