You can learn a lot about how RFI is conducted by studying how EMI filters work. Let’s check one out in detail.
As we have discussed, switched mode power supplies are a major source of RFI as they can emit rich harmonics all through the short wave bands. Pictured above are the details of the CORCOM 10EG1 High Performance EMI Filter for Switching Power Supplies. These are designed to filter conducted EMI on the AC power lines.
All EMI seeks a return path. Differential or normal mode conduction means that there are two wires, one of which provides a return path. Examples of differential mode conductors are coax and twisted pair. Current flows are independent of ground. Common mode conduction means that signals seek a sneaky return path through ground, typically coupled by stray capacitance. Examples are RFI traveling on the outside of a shielded cable and ground loops.
Let’s see how EMI filters work on differential mode noise. DM interference signals travel on two wires 180º out of phase. You will notice in the schematic above I have marked two capacitors and one inductor with “DM”. The inductor, typically a ferrite bead is placed on one of the lines. It will present a high impedance to high frequencies. The capacitors will couple the two lines at higher frequencies to neutralize the interference signals. At AC power frequencies they are an open circuit (invisible).
The rest of the components are for common mode filtering. The transformers are bifilar wound toroids. The magnetic flux inside the core creates high impedance to block higher frequency signals. You will find these are invisible to differential mode signals. The Y capacitors bypass high frequency signals to ground and ignore lower frequency AC.
That’s it. Pretty simple really. But great care is required to select the right components that block the right frequencies and, in particular, ensure safe operation. Take a look at this video tear down of a professional EMI filter.
How EMI Filters Work – Price and Performance
If all SMPS contained these filters, RFI would fade into the background. But you will find these devices are expensive. The 10EG1 costs more than $100. Pretty hard to embed one of these in an SMPS selling for $5.
Here is an LTSpice FFT simulation of a good AC line filter in action, similar to the CORCOM device above, filtering a 100 kHz SMPS switching wave and harmonics.
One word of warning, though. In the real world, you will probably not achieve the 70-80 dB specs. The product data sheets are based on 50 ohm terminations to the power lines. In your home, AC line impedance varies widely and this effects performance.