Week #11 & #12: EMI - Electro-Magnetic Interference & Compatibility

[rjagodowski]

This week we will pick up the discussion of Electro-Magnetic Interference beginning with the content posted at the end of Week #10.

Then we will discuss the highlights of the pdf attached below on

ElectroMagnetic Compatibility.pdf

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This is a fairly lengthy (200+ pages) document which addresses the concept of dealing with EMI & RFI with a focus on maintaining compatibility among systems which occupy the same spectrum space. It is written at a "technician's level" with minimal emphasis on mathematical equations, numerous diagrams and clear and concise descriptions of problems and their solutions.
While we don't have time to cover the entire document in class (it could be a course all by itself), I encourage you to put this on your "reading list" to read in its entirety.

A concept that will come up in the early discussions of the relationship between conductors and frequency is known as the "skin effect". Here's a bit of explanation about Skin Effect. At high frequency RF & Microwaves, oftentimes the "conductors" look like "pipes" and "tubes" because the center of the conductor would not be used at those high frequencies. They are often referred to as "Wave Guides". Here's a bit of info and a good pic of some Waveguides.

One of the key concepts you should grasp from this reading is that wire which appears to be just a resistive element at low frequencies behaves like a complex impedance at higher frequencies (typically 5-10 MHz & above). This understanding is critical when dealing with cabling used for high frequency analog and digital signals such as cable TV and computer networks. The cable that is used for each application must meet specific requirements. In the case of video circuits, 75 ohm impedance cable and circuits are used and in ethernet applications the cables and circuits are 100 ohm impedance.

One of the concepts presented early on in this document is the concept of harmonics. Here is a simulator that we'll discuss in class which shows the relationship between waveform shape and harmonic content. www.Falstad.com/fourier.

In order to better understand some of the concepts presented, it is helpful if you have a fundamental knowledge of how waves travel.

Here is an animated presentation of an electrical signal travelling down a transmission line model (4:58) complete with inductance and capacitance. You should watch this video. This series also includes this video on Grounding and Shielding (7:25) which is pretty good as well and probably deserves a look. There are some good reinforcing simulations relating the concepts of voltage, potential difference and ground.

This YouTube Video from Bell Labs in 1959 (28:02) very nicely demonstrates various aspects of wave motion and impedance using mechanical waves. Watch this video.

An instructional video which deals strictly with Electromagnetic Waves is presented here, taught by Sir Lawrence Bragg (20:22). Watch this video.

YouTube video of Standing Waves on a Transmission Line viewed with an Oscilloscope (10:50). Watch this video.

This is a good YouTube presentation on Understanding VSWR and Return Loss. (10:09) Watch this video.


Here is a link showing the Characteristic Impedance of Free Space and this is another with a more mathematical presentation. Here is one on Antenna Impedance.

Omni vs. Directional Antennas and Antenna Patterns and their Meanings from Cisco systems. (These links were broken, they were fixed on 4/26). Much more detail than is expected to be presented at the level of this course (but there ARE some pretty color graphics showing antenna patterns), but I supplied these links as a general introduction to the complexity of antenna design. The Cisco link has some decent graphs showing antenna radiation/reception patterns, which could be useful when trying to understand how RF signals will propagate from an antenna, such as a cell phone or wifi router.

Presentations on Crosstalk from Wikipedia, Polar Instruments and ad-net.com.

Twisting and Cabling from Mercury Wire.
Twisted Pair Types from Networx Security.
Video Explanation of Twisted Wires with an Audio Demonstration of Twisted and Untwisted Pairs (4:28) from AudioUniversityonline.com. This is a pretty good video. One point to note is that a little after the 1:05 minute mark, he states that the voltage in the two wires is opposite polarity and that the current in each wire is as well. This is true ONLY if the signal is a balanced (differential) signal. If it's an unbalanced (ground referenced) signal, that IS NOT the case. An unbalanced signal has one wire (probably the Black wire) connected to GROUND or 0V potential, and the other wire (Red) would have the voltage on it (positive or negative) with respect to GROUND, with alternating current direction with the polarity. Since the voltages on the Red and Black wires are unequal, there is NO field cancellation possible with unbalanced signals.

(Digging a bit deeper into this...if you say for the unbalanced situation "But doesn't the current travelling down the Red wire have to return to the source via the Black (ground) wire?" you raise a good question. The answer is....maybe. Seeing as the signal is ground reference, the return path back to the source can be ANY ground connection between the two points, not necessarily the ground wire with the red wire. "But what if there are no other ground paths, wouldn't there be a cancellation there?" Another good question, and THIS is the case shown in the audio signal where the two wires are near each other, but NOT twisted. Some cancellation is realized, but nowhere near ideal. In these situations, where noise is a concern, you will see Coaxial cabling (or UTP/STP) used to provide the shielding and protection desired. Think about the BNC cables and Scope Probe cables used in lab. They are Coax for this very reason AND the added protection that the shielded ground provides to the scope technician by ensuring the scope and probe are at ground potential.)

Another video on Twisted Pair Noise Immunity (5:15)

Here is a link to the topic of Ground Loops from Wikipedia.

Using Differential Signaling to reduce noise due to ground loops and other grounding issues.

The Purpose of a Drain Wire in Shielded Cables.

Shielding and Grounding in Industrial Automation.

The 50 Ohm Question: Impedance Matching in RF Design.

In class mention was made of a TDR (Time Domain Reflectometer). Here is Wikipedia's description of TDRs and their operation.

This YouTube video shows how Ferrite Beads (11:51) are used to suppress EMI. Watch this video.

Electrostatic Discharge - Causes, Effects & Solutions.

Here's a few observations about ESD (Electro-Static Discharge) to lighten the mood.


If you scroll down a bit in the attached pdf, you'll see the Allen-Bradley information about the Micro-820's meeting Electromagnetic Compatibility Standards (EMC).

Rockwell PLC Wiring Guide - Micro820.pdf

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