A Rapid, Non-Destructive Method for Measuring Connector Surface Transfer Impedance

Connectors are typically a weak link in the radiation tendencies of cable assemblies. A test procedure to characterize the quality of the connector shielding is needed for the manufacturing environ­ ment. The proposed stovepipe test is inexpensive, convenient and non-destructive to the cable assembly under test. The stovepipe test determines the connector surface transfer impedance, a parameter which characterizes the external voltage generated from an internal common-mode current. INTRODUCTION Connectors contribute significantly to the radiated emissions and susceptibility of cable assemblies. A rapid, non-destructive test for evaluating the surface transfer impedance of a connector in a ca­ ble assembly is presented here. Since the transfer impedance instead of the shielding effectiveness [1] is determined, the test results are independent of the test bed, within the limits of the model. In addition, a stovepipe replaces Martin’s “milked braid,” [2], facilitating the ease of the test pro­ cedure. Due to the geometry of the test bed, the test has a wide but finite range of frequency applicability. The test bed need not be electrically short. The fixture radius must be much less than a quarter wavelength. This constraint implies that the frequency must be less than 400 MHz. The lower end of the frequency range is constrained by the current probe, which can have a useful range of applicability of 30-1000 MHz. For our tests we consid­ ered 50-250 MHz. This range avoids the lower end of applicability of the probe and allows resolution around 150 MHz, a frequency of practical interest. In addition we make the realistic assumption that the cable of the assembly under test is much less leaky than the connector, leading to essentially lumped leakage at the connector. ANALYSIS The analytical basis of the stovepipe test involves the surface transfer impedance phenomenon and transmission line theory. The concept of surface transfer impedance was introduced by Schelkunoff in 1934 [3]. Schelkunoff also developed a transmission line analysis based on Green functions [3]. Smith [4] presented equa­ tions for the current distribution in a lossless transmission line due to a lumped excitation at one end of the line (Appendix A). The surface transfer impedance concept arose historically from the diffusion of electromagnetic energy through homogeneous conduc­ tive shields. It is defined as the ratio of the voltage generated on one side of an interface due to a common-mode current on the other. where V, is the voltage which results from ICM, the common-mode current in the internal line at the connector. The transfer impedances of practical (non-homogeneous) construc­ tions have resistive and linearly frequency dependent reactive com­ ponents (Vance [5]). Z, = R0 + j w Mt, where R0 is the transfer resistance and M, is the transfer induct­ ance. The resistance is caused by skin effect, and is a negligible part of the surface transfer impedance at the frequency of interest. The transfer inductance dominates at high frequencies. The trans­ fer inductance is caused primarily by apertures which have a volt­ age generated across them when current must divert around them (Chang [6 ], Taylor and Harrison [7], Taylor [8 ]). An additional cause is from excessive “pigtail” length of bonding wires [9], As per Vance [5], the surface transfer admittance is taken to be negligible, since most connectors used for shielded cables have es­ sentially 100% optical coverage. The voltage which is generated as a result of transfer impedance may be measured by using it to drive a transmission line. In the stovepipe test, a common mode current is set up in a cable assem­ bly, flowing down the center conductor and back the inside of the sheath. The transfer impedance of the connector is excited by this current, and a voltage source is generated on the outside of the connector shield. The voltage source drives a transmission line, the external line, which is comprised of the outside of the cable sheath and the stovepipe as a return path. For convenience we invoke Smith’s analysis of the current distribution due to a voltage source at one end of a line (see Appendix A). The line is essentially lossless, and has a known characteristic impedance (see Appendix B) and termination impedance (see Appendix C). Smith’s analysis is pursued for a voltage source as implied by equation (1). After imposing appropriate constraints, the applicable relation for transfer inductance is developed as IM I = 1 Zf cos(ffs) + j Z„ sin(ffs) I 10RdB/2o (A-l 1) 1 CO Zf is the terminating impedance of the external problem. Zc is the characteristic impedance of the external problem, fl is the propaga­ tion constant, s is the length of the external line. o> is the angular frequency of interest, or 2irf. RdB is the ratio of the unbalanced current at the current probe in the external problem to the com­ mon mode current at the connector in the internal problem, ex­ pressed in dB. For a discussion on how RdB is measured, see Appendix D. Our proposed benchmark for connector leakage is that it be no worse than RG-58, a relatively leaky single-braid cable. RG-58 has a nominal shielding effectiveness of 58 dB (Parker [9]). A connec­ tor with the same amount of coupling would have a transfer in­ ductance of Mt = 0.12 nanoHenries at 150 MHz (see Appendix E). C H 2 1 16-2/85/0000-122 $1.00 © 1985 IEEE 122 DESCRIPTION OF TEST SET-UP The set-up is fundamentally composed of the test bed and the RF generation and detection instrumentation. The specifics of our set­ up are described in Tables 1 and II, diagrammed in Figure 1, and pictured in Figure 2. Table I Test Bed Equipment Stovepipe Fixture Absorbing Clamp Connector Adaptor