When we were discussing green electron current (differential mode) in the last post, I mentioned that common mode (CM) currents (red electrons) are typically caused by unbalanced differential mode (DM) currents. It’s pretty easy to visualize that in an unbalanced differential transmission system, one of the traces is longer, has an extra via, needs a branch, the spacing between +signal and -signal had to be changed to get around a component or something else. This also happens in a single ended system. The signal leaves the source, goes to the load and, from the load, returns back to the source. The impedances of the signal path and the return path are not completely equal because of differences in path length and capacitive coupling to ground.
If both the source and load are hard grounded (such as to the safety ground) as shown in Figure 1, then an alternative return path is established through some mysterious parasitic coupling inductance. If they are not hard grounded, then the alternative return path is through some equally mysterious parasitic coupling capacitance. With the inductance, the CM impedance is increasing with frequency while with capacitance, the CM impedance is decreasing with frequency. This small amount of CM current flowing through a very large loop area creates a potential difference between the source and load.
In the post titled, “Radiated emission from green electron current”, I indicated that a simplified radiated emission model for the DM case could be developed by assuming that each wire of length L behaves as a small dipole, the two dipoles are out of phase (green current) and the two dipoles are separated by spacing (S). Because they are out of phase, the closer the spacing, the more field cancellation results. A similar configuration exists for the CM (red) current, except that the CM currents in the two dipoles are in phase. Because they are in phase, the closer the spacing, the more field enhancement occurs. It just looks like a single bigger wire.
The following relationship for the CM gives the electric field (E) at a distance (d).
E = 1.257×104 L I F / d
Where:
E = Field strength in microvolts/meter
L = Length of wire in cm
I = Current in Amps
F = Frequency in MHz
d = Distance to observation point in meters; usually the measurement distance.
Using the same example that was used in the previous post for the DM, let’s see what happens now with the CM. Assume that a replaceable electronic module for a space application has a 0.5 meter (20 inch) jumper cable with a 1 mm wire pair spacing that interconnects two subsystems. This cable is carrying a 3.5 ma data signal running at a frequency of 50 MHz. Will it meet the MIL-STD-461F RE102 requirement?
Calculating the electric field E at 1 meter gives:
E = 1.257×104 * 50 * 3.5 x 10-3 * 50 / 1 = 109,988 microvolts/meter
Converting to dBuV/m gives:
dBuV/m = 20 log 109,988 = 101 dBuV/m. The MIL-STD-461F spec limit at 50 MHz is 24 dBuV/m!
With the DM the level was 36 dBuV/m, which was out of spec by approx 11 dB. The CM from the same signal is out of spec by 77 dB . . . 66 dB higher.
With the DM, we only thought we had a problem. Now, with CM, we know we do.
– Ron Brewer
