COMPARISON OF COMMON MODE IMPEDANCE MEASUREMENTS
USING 2 CURRENT PROBE TECHNIQUE VERSUS V/I TECHNIQUE
FOR CISPR 22 CONDUCTED EMISSIONS MEASUREMENTS


CISPR 22 (1997) defines the conducted emissions requirements and testing of Information Technology Equipment (ITE). Section C.1.4 of CISPR 22 requires measuring the common mode impedance of an ITE cable bundle with respect to its ground plane, and provides a test method for this measurement. The technique uses one current probe to inductively couple a current into a physically short loop loaded with 50 ohms. The second current probe measures the induced current in the 50 ohm loop. These 2 probes are then used in the same way on the ITE cable bundle to inject and measure a current. CISPR 22 indicates that the ratio of the current in the 50 ohm loop to the current in the loop formed by the ITE cable bundle and the ground plane times 50 ohms yields the common mode impedance of the ITE cable bundle. Section C.1.4 requires the cable bundle to be disconnected from the EUT, and common mode grounded to the reference ground plane for this impedance measurement.


Section C.1.4 does not provide any guidance as to what size of loop this test method is valid or not valid. The technique is applied as shown in Figure C.4 of CISPR 22. The length of the cable under test from the Equipment Under Test (EUT) to the Auxiliary/Associated Equipment (AE) can be any length. Section C.1.4 indicates that the ITE cable bundle common mode impedance can be fixed at 150 ohms (+/- 20 ohms) by the use of ferrites positioned along the cable between the EUT and the AE. If the ferrites provide proper impedance control (with respect to the ground plane), the minimum ITE loop length (circumference) can be 2.2 meters (80 cm + 80 cm + 40 cm as per Figure C.4). If the ferrites do not provide proper impedance control, the length of ITE cable from the ferrites to the AE may affect the total loop length, which may be much longer than 2.2 meters. (A related paper will be submitted that shows controlling impedance in this manner with ferrite cores is unreliable.)

This paper presents the results of this 2 current probe impedance measurement technique performed on 2 loops having a total length (circumference) of approximately 1.25 meters and 4.25 meters. This measurement was performed loading these loops with discrete impedances of 50, 150, and 550 ohms. The intent of performing these measurements was to determine if the 2 current probe technique could accurately measure these discrete impedances for loop sizes likely to be encountered in a typical laboratory setup.

For comparison, the same common mode impedance measurements were made using a traditional current probe and a capacitive (non-contact) voltage probe at the terminating impedance. The ratio of V/I from these probes was used to measure the discrete impedances cited above. If this method is shown to be valid, it would allow impedance measurements to be made more quickly than using the ferrite method, and would not necessitate disconnecting and common mode grounding of the cable under test.


Two Current Probe Impedance Measurements
The test setup used is shown in Figure 1. A simulated ITE cable loop was formed with #14 insulated wire over a 30 cm wide copper ground plate. One end of the #14 wire was grounded to the copper ground plate simulating the common mode grounding of the ITE cable bundle. At the other end of the loop, a Type N fitting allowed discrete impedances to be connected in series with the loop. Impedance measurements were made for discrete impedance values of 50, 150, and 550 ohms (34 dBohms, 44 dBohms, and 54.8 dBohms).

The current probe has less than 1 W of insertion impedance over the measurement range. The test setup for physically short loop loaded with 50 ohms is shown in Figure 2. It consists of a standard current probe calibration fixture (FCC Model BCICF-4) that has a short circuit on one of the fixture’s coaxial connectors, and the 50 ohm discrete impedance cited above on the fixture’s other coaxial connector. This constitutes a short loop with a series 50 ohm impedance as specified in the CISPR 22 procedure (Annex C.2).
Figure 1. Basic test setup for 2 current probe impedance measurements
Figure 2. Test setup for calibration into 50 ohm loop.

The drive and monitor probes were placed on the loop formed by the #14 wire with the same spacing between the probes as used for the 50 ohm loop measurement. The probes were placed at the same end of the loop. The drive probe was driven in the same manner as cited above. The resulting signal from the monitor current probe was measured and stored for #14 wire lengths of 0.5 and 2 meters which correspond to total loop lengths of 1.25 and 4.25 meters. These 2 loop lengths are herein referred to as “Short” and “Long”. These measurements were made terminating each loop length with each of the 3 discrete impedances cited above. The measurements were corrected for the calibration factor of the F-43 current probe.
Figure 3. Results of 2 current probe method of impedance measurement.
FCurrent Probe and Capacitive Voltage Probe Impedance Measurements
The test setup is shown in Figure 4. Impedance measurements were made on the Short and Long loops, terminating the loops in each of the 3 discrete impedances. The V/I measurements from the Network Analyzer were corrected for the calibration factors of the F-33-2 current probe and the Capacitive Voltage Probe.

Figure 4.The results of these measurements
Figure 5. Results of V/I impedance measurements.
The data in Figure 5 shows excellent impedance measurement repeatability that is clearly independent of the loop length to which the actual impedance is connected. These data do not show the significant swing in the measured impedance above 10 MHz that the 2 current probe method generated (Figure 3). Additional data will be provided in the presentation that shows the
slight roll-up and roll-down of the curves in Figure 5 are due to the discrete load not being perfectly flat in impedance.


Summary of Key Benefits
The CISPR 22 two current probe method for determining the common mode impedance of an ITE cable is not reliable above 1 MHz. Errors of many dB can occur at higher frequencies depending on the length of the cable and its actual common mode impedance to the reference ground plane. These errors can occur regardless of the location of the 2 probes along the cable.
Use of a current probe and capacitive voltage probe to measure common mode impedance based on V/I provides a much more accurate method. This approach yields accurate and consistent results for a wide range of common mode impedances and cable lengths. The V/I method has the advantage of not requiring disconnecting the ITE cable bundle at the EUT and common mode grounding of it to the ground plane. The V/I method can be used with the ITE cable bundle in-situ. The V/I method can be used to measure the impedance at any point along the length of a cable.
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