PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 2 3 W orking Cover Page 4 TIA-TSB-31C 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Rationale and Measurement Guidelines for TIA-968 Draft 1 19-August, 2002 W arning: This document is a “work in progress” by TIA TR41.9 and as such it’s contents may change. i PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 2 3 Appendix F, Alternate Transverse Balance, Digital EUT A.1 Background 4 5 6 7 8 9 This annex offers a current based measurement technique, where a ratio of the stimulating metallic current to the resulting longitudinal current represents the effective transverse balance. This ''ratio of currents'' technique uses completely passive toroidal current probes as the detection elements in conjunction with a well-balanced balun transformer as shown in Figure F-1. 10 11 12 13 14 15 16 17 18 19 20 The test fixture exhibits substantial immunity to parasitic effects of capacitance, inductance, and laboratory RF noise, while maintaining an extremely accurate response in the non-traditional telephony frequency bands of ISDN and DSL. The magnetic devices are available as ''off the shelf devices'', and they are directly compatible with typical 50 ohm BNC connections to measurement equipment such as a network or spectrum analyzer. Additionally, the passive devices completely isolate the analyzer from the Equipment Under Test (EUT), leaving absolutely no paths through ground or isolation resistors. The transverse balance of the EUT easily plots as a logarithmic ratio of currents on the screen of an analyzer for immediate analysis. Transverse Balance = 20log 21 22 i 1+ i 2 i 1-i 2 - 6dB 23 24 25 26 27 In effect, the transverse balance is a ratio in dB of an applied metallic current and the resulting longitudinal current. The presence of longitudinal current is undesirable, and it is well known that such current may disturb adjacent circuits. Therefore it is highly desirable to identify equipment that emits longitudinal current. 28 29 30 31 32 33 34 35 36 37 38 A ratio of currents measurement easily and accurately achieves the primary goal of identifying current absent from a desired equal current flow from tip to ring during metallic stimulation, across the frequency band of interest. The absent or lost current, referred to as longitudinal current iL, is quantified by the toroid that measures i1-i2. The stimulating metallic current iM is quantified by the toroid that measures i1+i2. The term i1+i2 is nearly twice the metallic current, and could more accurately be expressed as (i1+i2)-iL, but the iL term is several orders of magnitude less than iM, and therefore iL is neglected in the numerator. Neglecting iL in the numerator of the transverse balance equation compensates for the analyzer’s measuring twice the metallic current, in effect the user subtracts 6dB from the analyzer's results to get the true transverse balance. 359 PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 2 3 4 5 6 7 The mathematical error introduced by this test configuration (where iL is neglected in the numerator) is insignificant, and can be shown mathematically to be less than 0.136dB for an analyzer plot that shows 36dB for a given frequency (transverse balance of 30dB). For the quantified range for pass fail criteria (35dB minimum), a worst case error of 0.136dB at 30dB is completely acceptable. Of course, as the quality of the EUT improves, the error reduces proportionately, so the error at 35dB is truly negligible. 8 9 A.2 Purpose 10 11 To determine transverse balance of digital EUT, by using a ratio of currents. 12 13 A.3 Equipment 14 15 16 17 (1) A precision 50 ohm to 100 ohm balun with greater than 60dB of longitudinal balance with respect to the center tap on the 100 ohm side (similar to North Hills 0311LB) SEL# XX. 18 19 (2) Two of the same model precision wound toroidal current monitors (similar to Pearson 4100) SEL# XX. 20 21 (3) A typical network or spectrum analyzer with a frequency range encompassing the desired frequency of test (See A.5(1) below.). SEL# XX. 22 A.4 Equipment States Subject To Test 23 24 Active state with appropriate grounding applied. 25 26 27 28 29 30 31 32 33 34 35 36 NOTE: Terminal equipment may require special attention to ensure it is properly configured for this test. For example, if the equipment would normally be connected to ac-power ground, cold-water-pipe ground, or if it has a metallic or partially metallic exposed surface, then these points shall be connected to the test ground plane. Similarly, if the EUT provides connections to other equipment through which ground may be introduced to the equipment, then these points shall be connected to the test ground plane. Equipment which does not contain any of these potential connections to ground shall be placed on a conductive plate which is connected to the test ground plane (see comment (1)); this applies to both non-powered and ac-powered equipment. 37 360 PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 A.5 Procedure 2 3 4 5 (1) Assemble the circuit shown in Figure F-1 and connect the equipment to the circuit as shown. The frequency range for the analyzer should be at least 200Hz < f < 2MHz. 6 7 8 9 10 11 12 13 14 (2) Set the analyzer's 50-ohm tracking generator output to +3dB (equivalent to 0dB into 100 ohms). With switch S2 set in position B, terminate the tip conductor to ground while the ring conductor is open. This represents the worst case transverse balance condition, and there is no metallic current flow. Switch S1 is then toggled and the ground termination is attached to the opposite conductor, again a worst case transverse balance condition. For both positions of S1 the analyzer should read about 0dB for any frequency in the specified transverse balance frequency band. Essentially this process verifies conductivity and wiring for the test circuit. 15 16 17 18 19 20 21 22 23 (3) With switch S2 in position A, the analyzer should read within 1dB of 36dB for the termination shown (Za=100, Zb=10K, and Zc=1158), which is a transverse balance of 30dB. Toggle switch S1 and the analyzer should read within 1dB of the previous measurement. Should this not be the case, it is then necessary to add the variable capacitors and adjust them to achieve at most a 1dB difference for the two positions of S1, for the termination shown. These caps, when properly adjusted, compensate for the imperfections in the transformer's windings and possible parasitic effects such as interwinding capacitance. 24 25 26 27 28 29 30 31 32 (4) After the test fixture is calibrated as described above, set switch S2 to position C and measure the transverse balance of the EUT. The pass/fail limits in dB for transverse balance versus frequency using the ratio of currents method should be the same as the limits specified for the voltage method described in the primary text of Part 68. However, as is true of any alternative method of test the burden of proof of correlation between the standard and alternative method lies upon the user of the alternative methodology.* Recall that the display on the analyzer must be reduced by 6dB to get the true transverse balance for the EUT. 33 34 35 36 37 38 39 40 *Test methodologies and illustrative circuits specified in TIA-968 have been determined by historically understood use and recommended practice to provide ease of test and lab-to-lab repeatability, while they may not always be the most expedient or technically appropriate way to perform a test in a specific situation. Correlating test results with this alternative current method might include lab data for both voltage and current methods under conditions which can be expected to yield clean data with each (i.e., at the low- 361 PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 2 frequency end of the test range and/or away from frequencies and/or levels which might suggest the voltage method is questionable). 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 362 PN-3-3602-RV3 to be published as TIA/EIA/TSB31-C 1 2 3 4 FIGURE F-1: TEST FIXTURE TO MEASURE TRANSVERSE BALANCE USING A RATIO OF CURRENTS 5 363