Introduction
The Fischer Custom Communications, Inc. Common Mode Impedance Stabilization Network
(F-CMISN-CAT5) provides a common mode impedance in support of conducted emissions testing of Information Technology Equipment (ITE) as per CISPR 22 (Third Edition – 1997). This passive device provides a true broadband common mode impedance that is well defined with respect to the reference ground plane over the CISPR 22 frequency range of 150 kHz to 30 MHz. When used in conjunction with a FCC F-33-4 Current Probe and a FCC F-CVP-1 Capacitive Voltage Probe, very accurate common mode current and voltage emission measurements can be made per CISPR 22 (Annex C.1.3).

Use of the CMISN eliminates the need for the time consuming positioning of ferrites on the cable under test to effect the desired common mode impedance (CISPR 22 Annex C.1.4). Ferrites must be re-positioned for every emission frequency to obtain the desired common mode impedance. In certain cases, the ferrite approach will not be able to obtain the desired impedance. The CMISN eliminates the need to perform the common mode impedance measurements required in CISPR 22 Annex C.2 as the ferrites are positioned along the cable under test.

The F-CMISN-CAT5 is equipped with RJ-45 telecom connectors suitable for direct connection to Category 5 cables.

Specifications

Background
Annex C of CISPR 22 defines test setups for common mode conducted emissions measurements for Information Technology Equipment (ITE). In general, CISPR 22 requires the common mode impedance of the ITE cable under test to be 150 W with respect to the ground plane of the test setup. For shielded ITE cables, the 150 W common mode impedance is achieved via a 150 W resistor from the cable shield to the ground plane (as per Annex C.1.2).

For ITE cables having 1 or 2 pair of wires, the use of an Impedance Stabilization Network (ISN) in the test setup is specified, and the ISN provides the 150 W common mode impedance. For all of these test configurations, the point at which the 150 W impedance occurs is well defined.

For an ITE cable using more than 2 pair of wires, Annex C.1.3 requires measuring both the common mode conducted voltage and current. As shown in Figure C.3 of CISPR 22, no ISN is used nor defined. To provide the required 150 W common mode impedance, ferrite cores are placed around the ITE cable between the voltage probe and the AE. These cores are positioned up and down the ITE cable to effect the 150 W common mode impedance as defined in Annex C.1.4. Annex C.2 provides a method of measuring the common mode impedance as the ferrite cores are positioned.

This approach has two potential problems. First, the ferrite cores can only add impedance. If the ITE cable and/or the AE has a common mode impedance greater than 150 W, the ferrites cannot effect a lowering of this impedance. It is possible to find conditions under which the 150 W cannot be achieved.

Second, as per CISPR 22, the length of cable from the measurement location to the AE is undefined and can be any length. This raises the possibility of common mode voltage and current standing waves on the ITE cable. Making the voltage and current measurements in the presence of standing waves increases the risk of exceeding allowable CISPR 22 conducted emissions levels. Ferrites may or may not help this situation. The only recourse is to position the ferrites, measure the common mode impedance, re-position the ferrites, re-measure, etc until the 150 W is achieved. As per CISPR 22 Annex C.1.4, this procedure would need to be done for every emitted frequency by the EUT. This is a very time consuming task, with the distinct possibility of not being able to achieve the objective of a 150 W common mode impedance.

The F-CMISN–CAT5 provides a well controlled 150 W common mode impedance, provides isolation from common mode emissions from the AE side, and reduces the impact of the standing waves.

Using the CMISN for CISPR 22 Testing
A typical test setup is shown below in Figure 1. The CMISN is basically installed in the position of the ferrites in Figure C.3 of CISPR 22. All other aspects of Figure C.3 are maintained with the exception of the spacing which is not clearly defined in Figure C.3. In general, CISPR 22 specifies the distance (typically 80 cm or less) from the EUT to the location of the 150 W common mode impedance. In Figure C.3 however, this distance is shown from the EUT to one side of the voltage probe. The distance from the other side of the non-contact capacitive voltage probe to the ferrites is not specified. In keeping with the typical CISPR convention, Figure 1 below shows the 30 to 80 cm distance from the EUT to the 150 W common mode impedance location, i.e. the CMISN.

The input and output connectors of the CMISN are both unshielded RJ45. They are wired in the TIA/EIA 568A convention as follows:

Figure 1 shows a FCC F-33-4 current probe. This probe is recommended as it provides less than 1 W of series impedance loading to the ITE cable under test. Excessive series impedance loading will increase the effective common mode impedance, and may result in increased conducted common mode voltage from the EUT. Other current probes may be used keeping in mind the need to maintain a low series impedance loading.

Figure 1 shows the current probe current probe on the EUT side of the F-CVP-1. If the current probe was placed between the voltage probe and the AE, the voltage probe will be measuring the voltage across the current probe and the CMISN. However, since the F-33-4 current probe has a negligible series impedance compared to the 150 W common mode impedance of the CMISN, the F-33-4 current probe could be placed on either side of the F-CVP-1. If a current probe is used that has a high series impedance, it should be place as shown in Figure 1.

The current probe should be placed up against the foam spacer of the F-CVP-1 voltage probe. The ITE cable should be placed in the center of the current probe for optimum effect.

CMISN Calibrations
Two calibrations are performed by Fischer Custom Communications, Inc. on each CMISM. A total of 5 calibration plots are generated from these 3 calibrations. A proprietary calibration fixture is used to provide proper and consistent positioning of the CMISN. These calibrations should be repeated by Fischer Custom Communications, Inc. once every year.

The first calibration is measuring the common mode impedance at the EUT port from 150 kHz to 30 MHz using a Network Analyzer. For this calibration, all 8 wires of the RJ45 connector are shorted together, and the common mode impedance is measured with respect to the ground plate of the CMISN enclosure. This measurement is performed twice – once with all 8 wires at the AE port open circuited, and once with all 8 wires at the AE port shorted to the ground plate of the CMISN. For both calibrations, the magnitude and phase of the common mode impedance is measured and plotted.

The second calibration is the common mode signal attenuation from the AE port to the EUT port. This measurement is made from 150 kHz to 30 MHz using a Network Analyzer. For this measurement all 8 wires at the AE and EUT ports are shorted together.

The data shown in the figures is typical. Actual calibration data will vary and this data will be provided with each F-CMISN-CAT5.