The Computer-Aided Approach to Susceptibility Testing

Susceptibility testing is generally performed using manual methods which result in the following disadvantages and limitations: tests are time consuming, frequency and amplitude accuracy may not be satisfactory, operator errors can easily occur, and test setup inaccuracies are seldom taken into account and compensated. A new and low-cost method of performing susceptibility testing automatically is presented in this paper. With the advent of test instrumentation operated under digital control and the availability of standardized interface systems, it is now possible to build test systems to carry out automated susceptibility tests which are particularly advantageous when digital equipment are to be tested. By driving power amplifiers from programmable sweep generators, the repeatability and accuracy of the testing will be considerably increased as a result of accurate frequency and amplitude controls using control loops (AFC and ALC). A computer can be used to control and to manage the execution oftests as well as the processing of off-line test results. Major advantages given by the automation of susceptibility testing are increased measurement accuracy, reduction of testing costs and EMC-problem prediction capabilities.     

Graphical Harmonic Analysis

With the Fourier transform begin{equation*}F(omega)=int^{+infty}_{-infty}f(t)text{exp}({-j}omega{t})dtend{equation*} it is not difficult to evaluate a good approximation of the envelope of F(?) simply by maximizing the integral in the second member of the preceding equation. The purpose of this document is to use the maximization technique for the graphical harmonic analysis of complex waveforms.     

Insertion Loss of Mismatched EMI Suppressors

The insertion loss (IL) of EMI suppressors operating in systems with mismatched load and source impedances is studied. The design of an EMI suppressor is difficult because the actual impedances connected to it are often unknown. It has been recognized for a long time and by many authors [1]-[7] that the IL defined in a 50-ohm system (MIL-STD-220) is rather meaningless for general application. It may happen that the line voltage delivered to systems changes strongly after the insertion of the suppressor [8]. The IL under operating conditions may be quite different from that predicted. The source and load impedances are the variable parameters; for variations of one of them, the maximum and minimum IL's are calculated. Some general guidelines are also given in order to improve the perfonnance of suppressors.     

Signal processing in active shielding and direction-findingtechniques

On the basis of the analogy between time and space sampling electrical emitters can be identified and localized. The traditional method of screening (passive shielding) and the new technique (active shielding), based on the generation of interference signals capable of creating screening barriers, are compared. By using signal processing techniques the minimization of the mean square error (MSE), taken as the optimality criterion, shows that active screening has performance comparable with passive screening, as long as interference signals are related to the signals which one wants to protect     

Broad-Band and Narrow-Band Measurements

EMC specifications consider two kinds of signals (broad band and narrow band). Special care is to be taken to determine the nature of the signal if meaningful measurements are desired. Tests specified in MIL-STD-462 and in other EMC specifications to decide if there are narrow-band or broad-band emissions are not conclusive, and many times it is difficult to make a decision about the nature of the signal. The decision is less difficult to make when bandwidth effects are understood. This paper tries to show the implications of performing EMC measurements using different receiver bandwidths. It appears that broad-band measurements are valid only in the special case in which the signal has a continuous flat frequency spectrum; in the other cases their meaning is rather questionable. In the conclusion some remarks are made in order to clarify this difficult situation.     

Interaction between test site and amplifier nonlinearities insusceptibility tests

Susceptibility tests performed with nonlinear power amplifiers may give nonrepeatable test results or even show susceptibility levels at the spurious frequencies which may be wrongly related to the frequency characteristics of the specified emitted signal. The control of the emission system can be performed in three different ways: local-field AGC, net-power AGC, and incident-power AGC. The first two modes may interact with the test site forcing the amplifier to work in saturation and therefore to generate a large amount of spurious signals which do not represent the wanted test signal. The test site interacts with the instrumentation because of reflections from the environment (mainly the ground plane) and-the high values of VSWR of the transmitting antenna. The incident-power AGC which is adopted in most recent EMC specifications may overcome these problems but only if the test site is carefully standardized-at least in the same manner as done for emission tests     

Experimental evaluation of the spherical near field test range

In aircraft manufacturer firms there is the need for measuring antenna patterns of large beamwidth communication/navigation antennas with the scale modeling technique. A series of tests has been carried out to verify the use of the spherical near field (SNF) test range for this purpose. The comparison is made between measured and calculated data of stub and waveguide antennas installed on a cylinder. From the results it appears that the agreement is reasonable. The advantages and disadvantages of the SNF test range for this particular application are discussed.     

The Computer-Aided Approach to Emission Testing

The use of the computer to obtain an Emission Measurement Automatic System (EMAS) is described. Major advantages are increased measurement accuracy and reduction of testing time and effort. Frequency accuracy better than 0.1 percent and amplitude accuracy (assuming an ideal calibrator) better than 0.5 dB have been demonstrated. The computer is used both for controlling the test execution and for off-line test results processing.     

Shielding effectiveness of apertures in rectangular cavities

The shielding effectiveness of a rectangular cavity with an aperture in near-field conditions is calculated by means of the methods of moments. Theoretical results are compared to experimental values with good agreement     

Radome design and measurements

Radome performance (transmission loss and boresight error) is evaluated by means of a computer code the input data of which are the geometry (radome shape, layered structure configuration, and antenna/radome relative position) and the antenna aperture magnetic current distribution. The latter is measured directly, thus avoiding the difficulty of characterizing the antenna under test. The spherical near field (SNF) measuring technique is applied. A radome has been designed using this approach and then manufactured and tested by means of a compact range. The comparison between experimental and theoretical results is quite good