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 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.

The impedance control is specified by Section C.1.4 to be achieved by first establishing the
nominal conducted emission frequencies on the ITE cable under test. Next, at each frequency of
emission, ferrite core(s) are to be positioned up and down the length of the ITE cable. At each
position, the common mode impedance is to be measured. This procedure is to be repeated until
a position yielding a measured common mode impedance of 150 ohms (+/-20 ohms) is found for
each emission frequency. The ferrite cores to be used are not defined by CISPR 22.
The purpose of this application note is to provide the results investigation into the viability of
controlling the common mode impedance of a cable using ferrite cores as per the technique cited
above. An example of a common mode impedance stabilization network is also shown.

Test Setup
The test setup used is shown in Figure 1. A #14 gauge wire was used to simulate a cable bundle,
and was placed over a 30 cm wide copper plate simulating a ground plane. The wire was 2.5 m
long, and was positioned 7 cm above the copper plate. A HP 4396B configured as an Impedance
Analyzer was attached to one end of the wire, and the other end was terminated in a 50 ohm
resistor.
The impedance seen by the HP 4396B was measured for 3 sets of ferrite cores placed at 3
different positions along the length of the wire. The impedance was also measured with no cores
present. The 3 sets of cores had relative permeabilities of 125, 850, and 10,000.Figures 2, 3, and 4
show the results of the impedance measurements for these 3 sets of ferrite
cores. It can be seen that achieving the CISPR 22 required common mode impedance of 150
ohms (+/- 20 ohms) at any arbitrary frequency between 150 kHz to 30 MHz is difficult.
Data was also taken (but not shown here due to space limitations) for terminating resistances of
10 ohms and 150 ohms. Results follow the same pattern seen for the 50 ohm termination case.
Measurements for terminating impedances above 150 ohms were not attempted due to the
inherent behavior of ferrites to increase impedance, not reduce impedance.
Common Mode Impedance Stabilization Network
A common mode impedance stabilization network was constructed to provide the 150 ohms (+/-20
ohms) required by CISPR 22. Figure 5 shows the common mode impedance presented to a
Category 5 cable with the load on the output of the network set as an open circuit and as a short
circuit. The desired CISPR 22 common mode impedance over the specified 150 kHz to 30 MHz
range is properly maintained.

Summary of Key Results
Ferrite cores provide limited flexibility to control the common mode impedance of a wire or
cable bundle.It is difficult to know in advance that a given combination of ferrites is available to effect the
required common mode impedance for any arbitrary frequency between 150 kHz and 30 MHz.
A properly designed common mode impedance stabilization network is much more effective
than using the ferrite method in achieving the common mode impedance required by CISPR 22.