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Procedures

Because of near-field effects, small changes in the position of a handset may sometimes, result in unexpected changes in energy absorption in the head. The SSI/DRB-TP-D01-034, standard describes handset-positioning procedures for evaluating portable communication, devices using the Universal Head-arm (SSI/DRB-TP-D01-031) with its integral alignment, aids., A device normally used next to the ear is positioned in a normal operating position with, the vertical center-line of the body of the handset aligned with the central line on the hand, simulator as well as the central line of the head simulator (Universal Head). This will, result in the earpiece being lined up with the head simulator’s virtual ear canal. The, central line on the head simulator represents the imaginary line created by the intersection, of the plane consisting of the three lines joining both ears and the tip of the mouth with the, side of a human head. The symmetrical design of the Universal head accommodates both, left and right side of the head testing in the one setup. With the earpiece pressed against, the head, the next step is to back off the device by the thickness of the compresses human, ear (~4mm). It is recommended that a picture of the setup position be used to document, the test positions used to demonstrate compliance., For handsets that are designed to operate like a push-to-talk transmitter, the typically used, test position is to align the device as above but back it off by the distance from the face to, the tip of the nose (~25mm). In this case the central line on the head simulator represents, the imaginary line created by the intersection of the plane containing the tip of the nose and, the mouth that would bisect the front of a human head. The mouthpiece of the portable, communication device will thus be aligned with the virtual mouth of the head phantom, with the device in contact with the tip of the nose in an upright position., For devices that are carried next to the body, such as shoulder, waist or chest-worn, transmitters, SAR compliance can be evaluated in the appropriate operating position, defined by the manufacturer, which offers maximum RF energy absorption in the, respective regions of the body. Appropriate operating positions include manufacturer’s, suggested operating positions and other typical usage positions where maximum RF, energy coupling to users or nearby persons are possible, The measurement system used for evaluating SAR usually consists of a small diameter, isotropic electric field probe, a multiple axis probe positioning system, the instrumentation, and computer equipment for controlling the probe and making the measurements. Certain, supporting equipment may be required for calibrating the electric field probe, validating, the measurement system and characterizing the tissue material., Several types of electric field probes are currently used for SAR measurements. Typically, probes are on the order of 3-5 mm in diameter and about 25-30 cm long. They use three, miniature dipoles, typically about 1.5-2.5 mm long, loaded with a diode sensor at the gap, of each dipole for measuring electric field strength in three orthogonal directions. The, detectors, consisting of the dipole and diode, are deposited and bonded on a substrate that, offers minimal perturbation to the incident field. The substrates may be arranged in, several configurations, such as I-beam, triangular or other designs to allow each detector to, measure the field component parallel to its axis and with minimal effects from the other, two. High resistance lines are used along the length of the probe to prevent RF pickup,, which may lead to inaccurate readings at the sensor. The other end of the probe is usually, fastened to a custom holder on the robot arm of a positioning system where the leads are, connected through EMI-shielded leads to the instrumentation amplifiers. The amplified, signals are processed with precision A/D converters or voltmeters connected to the, computer., The electric field probes are usually calibrated together with the system instrumentation., The sensors of the probes are designed to operate as true square-law detectors where the, output voltage is proportional to the square of the electric field. The probes must be, calibrated in the type of tissue media formulated for the test frequency and at that, frequency. Probes may be calibrated in two stages, in air and then in tissue media, to, obtain calibration factors that can be used to convert the output voltages of the detectors to, SAR (see Procedure SSI/DRB-TP-D01-032). Alternatively, a one step approach is used, where a waveguide is filled with the appropriate tissue material and the output voltages of, A head model is usually placed on its side that allows a handset to be placed underneath, the head to facilitate field measurements. The field probe is inserted into the liquid from, above and measurements can then be made on the inside surface of the head next to the, phone. SAR measurements usually start with a coarse measurement at 1-2 cm resolution, where the electric field probe is scanned throughout the entire region of tissues next to the, handset and its antenna. This provides a SAR distribution near the surface of the phantom,, closest to the phone, where the approximate location of the peak SAR can be identified. A, smaller region centered around the peak SAR location, is then scanned with a 1-5 mm finer, resolution to determine the one-gram average SAR., The fine resolution scan may take 20 minutes to more than an hour to complete. In some, cases, a pause in the testing may be necessary in order to replace batteries in the device to, maintain the test signal level. The measurements obtained from this fine resolution scan, are averaged over a 1 cm3, average SAR (a 10 cm3, average density of most high water-content tissues is about 1020-1040 kg/m3, requires the tissue volume to be about 1 cm long on each side (2 cm for 10 gram average, SAR). The number of measurement points required in the fine scan to provide accurate, one-gram average SAR is dependent on the field gradients at the peak SAR location. In, smooth gradients, the one-gram average SAR can be correctly predicted with only a few, measurement points. When steep field gradients exist, many measurement points evenly, volume in the shape of a cube to determine the one-gram, volume for a ten-gram average SAR for the extremities).  Which distributed within a cubic centimeter of the tissue material may be required to correctly, predict the one-gram average SAR. To overcome this problem, a curve-fitting process, may be applied to the measured data to allow more points to be used in the average. A, description of the procedures used to compute the one-gram average should be included., The measurements provided by electric field probe normally do not correspond to the, location at the tip of a probe because the detectors are located behind the tip. For, homogeneous phantoms, the peak field values are at the surface of the phantom 100% print inspection!!, but the, detectors of the probe are generally 2.5-7.0 mm behind the tip of the probe. Therefore the, field measurements must be extrapolated to the surface of the phantom to compensate for, field attenuation introduced by this offset distance. This can be done by taking a number, of measurement points in a straight line perpendicular to the phantom surface at the peak, SAR location and applying a curve-fitting process for the extrapolation., The margin of error for typical measurement systems is directly related to the latest, technical developments for SAR evaluation. Systems that use unreliable techniques or that, do not produce repeatable results should not be used to test devices for Industry Canada, compliance., Measurement uncertainties are the results of errors due to system instrumentation, field, probe response and calibration, tissue dielectric property usage and characterization., Uncertainties due to measurement procedures include test device placement, probe, positioning, procedures used to extrapolate measurements to the surface of a phantom and, methods used to determine one-gram SAR averages (see Procedure SSI/DRB-TP-D01-, 035). Information on such uncertainties is relevant to SAR evaluation and should be, included in order to support compliance with SAR exposure limits.,