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 … [Read more...]
Mom! Dad! Do Open & Short Circuits Really Exist?
I debated whether I should introduce radiated pickup by the DM loop area next and then dismissed it so that I could continue with radiated emission from common mode (CM) currents. These tend to be very mysterious because they are typically caused by unbalanced differential mode (DM) currents. Often these are caused by a few extra grounds floating around somewhere in … [Read more...]
Radiated Emission from Green Electron Current
Well . . . I’m back. I hadn’t planned on being out of print so long and it will take a few paragraphs to bring everyone up to date.A number of you learned at the IEEE EMC Symposium in Pittsburgh that my family and I were taking our little travel trailer on an extended camping trip immediately after the symposium. There were several National Parks that we have wanted … [Read more...]
What is Differential and Common Mode Current?
Answering the title question will be a hot topic at the IEEE 2012 EMC Symposium in Pittsburgh. Hopefully many of you will have the chance to attend it this year. There are always some really great technical papers presented at this show – some more practical than others. If you’re there look me up. I’ll be spending some time at the Interference Technology booth, #817.The … [Read more...]
Unwanted Conductive and Radiative Emissions
The last post indicated that RF signal energy coupling from the emitter to the receptor can be by conduction or by radiation and that we would discuss that later. Well it’s later! The two coupling processes are very closely linked. Radiative coupling always starts out as a conductive signal and ends up as a conductive signal. In between and especially at … [Read more...]
Do I really need that signal?
We’ve been discussing signals from the perspective of their bandwidth relative to the bandwidth of the receptor and have not considered them from the EMC point of view. RF signal energy can get from the emitter to the receptor by conduction or by radiation. We’ll discuss that later. Regardless of the energy transfer process, using the FCC classification scheme we … [Read more...]
Abracadabra the Broadband Signals Disappeared
Almost all EMC engineers these days are familiar with MIL-STD-461 and its various revisions because it has influenced the procedures in virtually all present day EMC standards. The original document and the A, B and C revisions all had requirements for measuring both broadband (BB) and narrowband (NB) emissions. When MIL-STD-461 was first released (July 31, 1967), the … [Read more...]
Another Look at Broadband Bandwidth Bandwidths
The last comment I made was regarding those perfect rectangular filters that we always describe with such reverence. But as I said, they don’t exist. Taking a receiver as an example, the filter amplitude versus frequency characteristics are measured by sweeping a narrowband signal through the passband frequency range of the circuit. Typically the passband width is … [Read more...]
Random or Coherent Signals
In the previous post, I indicated that coupled Broadband energy levels are determined by whether the signals are random or coherent. Random/non-coherent signals originate from sources that are not time related. A couple of good examples would be receiver thermal noise and DC motor brush noise. Another would be the RF environment taken as a whole. Because the individual energy … [Read more...]
Narrowband and Broadband Signals
As I indicated in the previous post, this discussion begins examining signal characteristics from an electromagnetic compatibility (EMC) perspective. But first, a few words to describe EMC. This is a relationship issue. We want all the equipment items to get along in peace and in harmony, but if one of them is a big source and the other is a wimpy receiver, they may not play … [Read more...]