# How are electromagnetic waves differentiated?

I would like to know how the signals for remote controlled cars, radios, etc.. That use radio waves are told apart from each other. I know that the radio waves are modulated to encode data and the frequency or amplitude are changed, so then the waves are propagated through the air and received at another location via a receiver that is tuned to a certain frequency that the waves were emitted, but I'm sure in most places in the world by now there are numerous amounts of waves traversing at any point, why doesn't the receiver of this device happen to catch another wave of the same frequency instead of the one that was intended? or is there anything that stops me from having a device that emits a wide range of frequencies or amplitudes that would manipulate nearby electronics?

why doesn't the receiver of this device happen to catch another wave of the same frequency instead of the one that was intended?

It does catch other waves at the same frequency. This is called noise. Communications are engineered so that the signal is significantly stronger than the potential noise such that it can still be reliably demodulated.

More generally, all wireless communications use the same medium (the electromagnetic field), and the problem of allowing multiple users access to this shared medium is multiplexing. While frequency division multiplexing is most common, it is not the only way: for example ultra-wideband communications may use time-division multiplexing, and spread-spectrum may use code-division multiplexing.

These methods can also be used in combination. For example, we may tune a receiver to a particular frequency (frequency-division multiplexing), and simultaneously use a directional antenna to reduce sensitivity in directions where there is only noise (space-division multiplexing). A system's ability to distinguish the desired signal from noise is called selectivity, and the more known about the desired signal (frequency, timing, phase, polarization, etc), the more selective a receiver can be.

is there anything that stops me from having a device that emits a wide range of frequencies or amplitudes that would manipulate nearby electronics?

Regulatory bodies, such as the FCC, in conjunction with international bodies such as the ITU, establish licenses and laws which grant specific users or classes of users exclusive access to allocated spectrum. Violators are fined, or made by force if necessary to stop transmitting.

A device that transmits over some range of frequencies with the intent of disabling other radio devices would be called a jammer. There's no technical reason you couldn't build one, but ultimately the government's monopoly on violence will discourage you. A military, having no reason to fear these consequences, makes jammers all the time.

• Technically, picking up other "waves" at the same frequency is not noise. Unless it's random, it's not noise. There's terms like hiss, hum, pickup, etc to describe other phenomenon. – user3814483 Aug 6 '14 at 23:38
• @user3814483 I think that really depends on your definition of noise. In communications engineering, any thing that isn't signal is noise. The first two sentences of wikipedia on noise are "Noise means any unwanted sound. Noise is not necessarily random." Technically, if you mean random noise, you'd specify "Gaussian noise" or whatever particular kind you mean. – Phil Frost Aug 7 '14 at 0:48
• Ironically, I made that comment from the perspective of an analog IC/RF engineer. Second, we're talking about "noise" in the context of electronics, not as your everyday lay person would use it (which is your wikipedia reference). My sources are colleagues and practitioners of the art at MIT. See: web.mit.edu/ryz/Public/… Look at the section for noise. – user3814483 Aug 7 '14 at 3:11
• Here's the actual wikipedia link for noise: en.wikipedia.org/wiki/Noise_(electronics). It's all laid out there. – user3814483 Aug 7 '14 at 3:22
• @user3814483 Sure, you can use that definition. Or you can use Noise (radio): "In radio reception, noise is the superposition of white noise and other disturbing influences on the signal, caused either by thermal noise and other electronic noise from receiver input circuits or by interference from radiated electromagnetic noise picked up by the receiver's antenna." But I'm not at MIT, so I'm probably wrong. – Phil Frost Aug 7 '14 at 10:57

Radio wave receivers are designed to resonate at a particular frequency. If you look at the response of a resonant device as a function of frequency you get something like (this image is from the Wikipedia article):

This is a rather busy plot, but the point to take away is that the response of the resonant system is greatest when the frequency matches the resonant frequency. The more resonant the system is (the higher its Q factor) the more sharply peaked the response is.

So if you want your radio to pick up just 98.4MHz (the frequency of the radio station I'm listening to at the moment) you tune your receiver to resonate at 98.4MHz. It will still pick up other radio frequencies as well, but because of the resonance it's far more sensitive to the resonant frequency than to the other frequencies.