A very gross introduction to Electromagnetic Interference(EMI) (Originally posted on Saturday, March 21, 2020)

 

EMI is quite the topic for any discussion, like consider this scenario, for example, you are discussing with your old-timey school friends (who actually does that?), and you tell to one of them about your cool project that you did for measuring signal integrity in a double Molex line by modeling it's EM parameters and slowly loaded your model with all the insanity of small RF and electrical coupling that can happen inside the system.

 

So, ranting aside, how do we understand EMI and what EMI can actually do your systems(intentional and unintentional ones) and the badass of the questions what is EMI and how does it happen?

 

Image result for maxwell's equations

 

So what is Electromagnetic Radiation?

So by science/physics,  Electromagnetic Radiation means the waves refer to waves that are actually an Electromagnetic Field that is propagating through space. So electromagnetic wave is of very broad category this includes Visible Light, X-Rays, Gamma and the regular waves that are used for communication purposes as well.

 

The explanation that I gave is rather a very shortened version of EM, people study these waves for years together and do some intelligent problem solving with the data obtained, let's say you are building an antenna post building the antenna in order to understand how radiation parameters look like FDTD(Finite Difference in Time Domain) and FEA(Finite Element Analysis) and build a mesh on the 3D structure and solve the famous Maxwell's Equation inside every small part of this mesh and sum it up all in the end and compute near and far-field and get the results, all these with the help of Maxwell's equations and some more modeling math done in Antenna Design and Microwave Engineering.

 

So in conclusion, the only factor that distinguishes between all these radiations is the wavelength, the shorter the wavelength the higher the frequency. This is what electromagnetics is all about in the end.

 

What is EMI?

So before we go into the specifics of EMI let's understand it first. When a cell phone is placed on top of FM radio and if we are getting a call the FM starts beeping "beeeep bbeep bbeeep" or some shit like that. I mean this area of science and engineering is a roller coaster of emotions just like a sine wave one point everything is working fine and you understand something and you do it with GRC and see the data and think "What the fuck just happened I thought this is what I just read and practiced all this time".

 

So in summary EMI is an unwanted electrical signal that interferes with the operation of other normal electrical and electronic circuits, this can occur in two ways, one is by induction like the ones we observe when a microwave transformer is taken near a WiFi router and the ones that happen by radiation. The radiation type is usually observed in long-range systems like the old WWII RADAR's and this is also observed when retroreflectors are used by planes so that the same RADAR signal is reflected back to the source. This is a form of self-interference due to radiation that is observed in RADAR's. Apart from this capacitive and inductive coupling is a form of Radiated EMI.

 

So, how does EMI is generated in the first place?

 

When a current-carrying conductor is placed in a magnetic field it experiences a force, the direction of this force is explained using thumb rule and left-hand rule, but the point is not to create an electromotive force, but our idea is not generating an EMF, but rather to understand it's effects in Electrical and Electronic Circuits.

 

So, in its core, a jammer is also an interfering device that radiates EM at a much higher power which introduces intermodulation products at two different frequencies thereby causing severe non-linearities in the system, this can lead to adverse effects I mean not in civilian communication(Frankly I really don't care if a few million bozos take it out on each other when they have poor mobile net) systems, but in places where maintaining air superiority is important. So there we have it let's see how to build a jammer with an SDR in a later tutorial, but in this text matter, we can concentrate on EMI alone.

 

 

Intentional and Unintentional EMI

Intentional EMI when someone fucks up the working knowingly, this is sort of technique that is used in Electronic Warfare Systems, before actually deploying. The other one is unintentional, due to long wires or PCB traces that we can see in some systems.

 

A system can give unintentional EMI due to the following reasons when we have long PCB traces or wires in the system, any sharp bends in the PCB, improper impedance matching, terminal leakage, not having proper capacitors.....

 

But this is where the fun starts, as one my friend asks it "how do we hack it?", and helpfully there is an entire area of research called Side Channel Analysis(SCA). This type of analysis usually tells about the circuit characteristics, be it power consumption or unwanted emanation that come from a processor or an oscillator this type of analysis is called SCA, this kind of unwanted of emanation phenomenon was first observed in back in WWII in Bell Telephone systems, and in the recent days TEMPEST(https://en.wikipedia.org/wiki/Tempest_(codename)), so this can be classified into two types for research purposes, one is passive emission research, like TEMPEST and our SCA methods and the other one is active research(https://www.eff.org/files/2014/01/06/20131230-appelbaum-nsa_ant_catalog.pdf) lie RF Retroreflectors(SURLYSPAWN) and other stuff. This is how an unintentional EM interference can compromise the security of the system.

 

I'll attach a sample image with this to show how these unwanted emanations look like and how to characterize EM using open source software like GRC and some random devices found in amazon and other shit. But for the major part of next few tuts I'll be using an Arduino and an oscilloscope probe that came with Hantek when I bought it. This probe will be used as an sniffer or as an E Probe as we call it.

 

 

 

 

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