Wednesday, March 30, 2011

Ground Loops and Other Spurious Coupling Mechanisms and How to Prevent Them Part 1

This is a two part blog post is based off a great tutorial I read titled “Tutorial on Ground Loops” by P. M. Bellan. A link to the tutorial can be found at the end of the post. The subject of the two part post is something you won’t find in textbooks from your EE courses, spurious coupling mechanisms in circuits and test systems and how to prevent them. A spurious coupling mechanism is any outside unwanted voltage or current source entering a signal path. There are five main spurious coupling mechanisms: direct conduction, capacitive coupling, inductive coupling, radiated electromagnetic field pickup, and ground loops. This first post will focus on the first four types of spurious coupling and the second will focus strictly on ground loops.

Direct Conduction:
Direct conduction is caused when an unwanted conductive path is created between a circuit or signal path of interest and a neighboring signal path. This type of spurious coupling is shown in the figure below. The arrows in red represent the current from a neighboring signal entering the circuit of interest through an unwanted conductive path. The cause of this could be an un-insulated crossed wire, a shorted PCB run, sloppy solder job, or a malfunctioning switch in a test system. The only way to prevent it is to identify the low resistance path and correct it.
Used by permission from P. M. Bellan

Capacitive Coupling:
Capacitive coupling, also known as stray capacitance, occurs due to the capacitance that exists between two otherwise isolated adjacent signal paths (see figure below). The current coupled from one signal path onto the other signal path can be represented by I = C * dv / dt. Where “I” is the unwanted current coupled into the signal path, “C” is the stray capacitance that exists between the adjacent signal paths, and “dv/dt” is the rate of change in voltage on the adjacent signal path. The closer the signal paths are together the higher the value of “C” is and therefore the higher the “I” value in the adjacent signal path. Also the larger the rate of change of the voltage (dv/dt) the higher the coupled “I” value in the adjacent signal path. Higher signal frequencies mean higher dv/dt. High impedance signal paths are especially vulnerable to this type of spurious coupling because they are low current already so any added current can have a big effect. In test systems capacitive coupling affects can be seen when using low frequency switch cards close or over their upper frequency limits. The same is true when using dense interface and bus connection types in test systems at their upper frequency limits. In these types of cases the capacitive coupling is often referred to as crosstalk. This capacitive coupling can be reduced by using shielded cable and connectors, larger spacing on PCB runs, and higher frequency rated switching products in your signal paths.
Used by permission from P. M. Bellan

Inductive Coupling:
Whenever current flows through a conductor it creates circular lines of magnetic flux (remember the Right Hand Rule). Any signal path adjacent to a current carrying signal path will have magnetic lines of flux coupled onto it creating a current flow in the signal path equivalent to the amount of linked flux lines that are in contact with it. This same principal is how transformers work. One way to reduce inductive coupling affects is to space out adjacent signal paths because the more space between the signal paths the weaker any linked magnetic flux lines will be. More practical ways include using twisted pair wiring or shielded cable. Twisted pair wiring essentially cancels out magnetic lines of flux by creating an alternating pattern where the magnetic flux lines oppose each other. Twisted pair wiring can be purchased or you can make it yourself. To make twisted pair take two wires of the same length. Secure one end of each wire in a vise and place the other ends inside a drill where the drill bit goes. Stretch the wires out, fire off the drill, and you have twisted pair. Shielded cable has a conductor as its outer shell which catches any flux lines created by the internal conductor essentially blocking them from coupling on adjacent signal paths.

Radiated Electromagnetic Field Pickup:
In today’s wireless world we are surrounded by transmitters. On top of that devices, such as electric motors, can act like transmitter even though that was not their design intention. These intentional and non-intentional transmitters fill every part of our environment with electromagnetic waves. Radiated electromagnetic field pickup is when your circuit or signal path acts like an antenna and picks up radiated electromagnetic waves. To get a first hand example of this kind of spurious coupling, place your mobile phone close to a stereo speaker (make sure the speaker is on). From there send a text message or some other data transmission and you will hear interference on the speaker created by the transmitted packets coming from the phone. The amount and frequency range of electromagnetic waves a circuit or signal path picks up depends on its length compared to the electromagnetic wave lengths in the area. For instance if your circuit or signal path is close to a transmitter that is operating at frequencies whose wavelengths are the same length, twice the length, or four times the length of your circuit or signal path the pickup will be much stronger. To reduce radiated electromagnetic field pickup use shielded cabling and conductive casing around any exposed signal paths. The idea is you want to create a Faraday Cage around your signal paths.

Stay tuned for part 2 next week


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