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What Is Interference?

Friday, September 7th, 2012 by | Filed Under: I've Always Wanted To Know...
 
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I’ve Always Wanted To Know:
What Is Interference?

Answer:

If you’ve ever used a wireless device and had signal issues someone has probably told you that there must be a lot of “interference.” But what is interference? What types of interference are there? Is it even interference or something else?

Let’s start by talking about how a wireless signal works. A wireless signal operates at a certain frequency (ranging from 3Hz to 300Ghz) and who can transmit at a specific frequency is controlled by the government and generally follows the international telecommunications union guidelines. In the USA the FCC controls what frequencies are allowed to be transmitted on but there are unlicensed radio frequencies which are open for use. Wifi devices transmit in these frequencies.

So let’s talk about interference: Interference is something that causes undesireable results of the receiving unit. The radio signal might be blocked, altered or undecipherable. This signal may be affected by another device emitting signals outside of it’s correct frequency, may be emitting signals in a shared frequency or may be emitting on harmonic sidebands which are at the frequency the receiver is picking up causing interference. Interference can also be used to describe radio absorbing materials or obstructions which prevent the signal from being received or sent.

Types of interference:

Physical: Certain materials absorb or scatter radio signals. The degree to which they scatter or absorb them varies but a building with large amounts of steel or lead in the walls generally has lower signal penetration then a building made of thin drywall.

Environmental: The sun emits radiation of all different types (visible light, ultraviolet, radio waves, gamma rays, etc) and during periods of high solar activity there can be interference caused. The rain can also cause interference, especially in outdoor wireless networks, which degrades performance.

Other Devices: Microwaves and some industrial heating applications use the 2.4 Ghz frequency to heat items up and can cause signal problems. Devices transmitting on the same frequency (for example two sets of toy radios, two RC cars set to same channel) can also interfere with each other and pick up signals from one another.

Electrical: High powered electrical lines or unshielded cable can cause interference. Example would be AC electric cable running next to a speaker cable causing a buzzing sound.

Harmonic Sidebands: Devices can produce harmonics on frequencies they are not transmitting on thus interfering with devices on those other frequencies. A popular example is a cellular telephone when placed over a headphone cable you can hear pulses when the device is transmitting even though the cell phone does not transmit on the frequencies the headphones are sensitive to.

Multiple Cases of Interference/Design Limits: If a device has a theoretical range of 800 feet people will often blame “interference” on why at 725 feet they are not able to get reception. The physical terrain, specific atmospheric conditions, exact power of the transmitter, sensitivity of the receiver and other factors all go into how far a signal can reliably be received and transmitted. It’s always a good idea to take maximum range as theoretical numbers not absolute guaranteed range.

Does this cover article all the possible interference methods or go into circuit design or RF shielding/interference reduction circuitry? No, but there is tons more to learn on the subject! RF engineers are a very well paid position at communication companies and often have Masters or Ph. D. degrees to be able to do their job!

-Tim

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