Henry Ott Consultants

Electromagnetic Compatibility Consulting and Training

Measuring Common-Mode Currents on Cables

The radiation from a cable is directly proportional to the common-mode current on that cable.  The common-mode current is the unbalanced current (current not returned) on the cable.  If this current is not returned on the cable, where does it go?  Into radiation, that's where!  In the case of intentional signals that flow down one wire of the cable and return on another wire, the net current is zero.

Since cables are always a major source of product radiation, measuring the common-mode current is one of the most useful thing that you can learn to do.  The common-mode current can easily be measured with a high frequency clamp-on current probe (such as the Fischer Custom Communications Model F-33-1) and a spectrum analyzer, as shown in the figure below.  The F-33-1 current probe has a flat frequency response from 2 to 250 MHz.  The transfer impedance of the current probe is five ohms (+ 14 dB), therefore, a one microamp current will produce a five microvolt output voltage from the current probe.

Make it a habit to measure the common-mode currents on all your cables.  Do it early on prototype models (while it is still easy to make a change to the product), and prior to performing EMC compliance testing.  If you fail the common-mode current test you will also fail the radiated emission test.

For a Class B product, the current must be less than 5 microamps (15 microamps for a Class A product).  Use the above limits for cables that are one meter or longer.  For cables shorter than one meter, the allowable current can be increased proportional to how much the cable is shorter than one meter.  For example, for a half-meter long cable the maximum current would be 10 microamps for a Class B product (30 microamps for a Class A product).

This technique works equally well on shielded or unshielded cables.  This is also a good way to determine the effectiveness of your cable shield termination.  If you use common-mode filters on your cables or use ferrite cores to suppress common-mode radiation the current probe measurement will indicate their effectiveness.  Just measure the current before and after inserting the filter (or ferrite).

All cables should be measured regardless of their intended purpose.  Measure the signal cables, the power cord (ac or dc), fiber optic cables, monitor cables, I/O cables, telecom cables, and any other cables that are attached to the product.  If it's connected to the product it can be a source of radiation!

What you want to do is reduce the common-mode current on each cable to below 5 microamps (15 microamps for a Class A product).  The cables will interact.  If you reduce the common-mode current on one cable, it will often increase on another cable.

Test Procedure:  Monitor one cable at a time with the common-mode current clamp.   Use common-mode filters, ferrite chokes, cable shields, etc. to reduce the current to under 5 uA (15 uA for a Class A device) then go on to the next cable.  When you get through all the cables, start over again, since the currents may  have increased on some of the previously fixed cables.  Keep this iterative process up until the currents on all the cables are under 5 uA (15 uA for a Class A device).  You may have to go through the process two or three times on each cables.  When you are finished, the current on each cable should be under 5 uA (15 uA for a Class A device) and the cables should no longer present a problem when you do a radiated emission test.

Of all the various types of EMC measurements that you could possibly do, the common-mode current measurement is the most useful -- learn to do it, and do it often.

  © 2000 Henry W. Ott                                                                                                                                            Henry Ott Consultants

Additional information on this plus other simple EMC measurements (both emission and susceptibility), that can easily be performed in your own laboratory, is contained in our one-day course on Workbench EMC Measurements.

For more information on the Model F-33-1 common-mode current probe, contact Fischer Custom Communications, 2917 West Lomita Boulevard, Torrance, CA, (310) 891-0635, FAX (310) 891-0644, e-mail: Sales@Fischercc.com.


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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793,   FAX: 973-533-1442

February 20, 2004