Why don't signals of different frequencies interfere?

hi guys
i have a question. i might sound silly but this question has been bugging me alot and i couldnt find any explanation of it from any where; not even from my teacher

we say that frequencies interfere. so we separate signals in frequency so that they dont interfere. what i want to ask is why dont signals of diferent frequencies interfere? i mean to say is that two different frequencies travelling in the same medium, why would they not interfere?
when we talk of communication we say that these signals are distant in frequency so they dont interfere and this thing confuses me as they are travelling through same medium they why would they not interfere
why only signals having similar frequencies interfere?

please sort this out for me as i am tired of asking people about it and nobody seems to have a clarifying answer

Re: a stupid question

i think waves of different frequencies do interfere..... i remember teachers talking of beat used for tuning instruments to unison i.e the minute difference which results when signals of different frequency interfere....

check out the "check your understanding" part of **broken link removed**

there interference between two signals of different frequencies has been discussed.......

Re: a stupid question

thanks for the info

so my question becomes now why do we neglect it in communication of radio signals? why we always talk of only frequencies that are similar to the signal of our interest?

Re: a stupid question

hi friend !!!

take our atmosphere,, it is divided into various layer like stratosphere,ionoshere..etc,, all based on the behaviour of air molecules present in that particular distant range , we use ionosphere for our communication , its mean that it is more suitable for it , moreover each layer has its uniquiness ,,

as the same, for frequency , we have electomagnetic spectrum , each specified wavelegth and specified frequency ( λ ∞ height) which is located in a particular height range ,, each frequency is unique ,,, but then u may ask why do the FM channells located adjacent frequency tend to interfere ,we hear only noise in such case ,,, its because the frequency reponce is not exatcly straight line , it has rising gain , peak , decreasing gain, ..

can we expect stratoshpere to do the work of ionosphere,, as the same for frequency , each has its own specification !!!
it is the thing what i under stood!!!!!!

Re: a stupid question

we dont usually neglect it... there are a whole lot of topics and studies on these kinds of noise and interference (i recommend you to search about the researchs and other developments going on in fading, interference, noise etc)....

coming to radio communication that you are talking about, AM is more prone to distortion due to interference than FM.... actually interference is just amplitude modulation in a simpler sense.... In FM the frequency is directly proportional to the input signal and hence the noise which is amplitude modulation gets rejected on demodulation.... In the case of AM it is not so... the household sound themselves have certain effect on the AM radio reception......

Re: a stupid question

Hi, All signals do interfere !. Assume a light bulb with red, green and yellow light... what YOU see is "interference" of the three and your eyes combine all. The same is true for radio signals. But the difference is the receiver side !. The receiver is tuned ONLY to ONE frequency or in our light example, ONE light frequency/color. With light this is possible to use a red, green or yellow filter, that only filters ONE color out. In a radio its the filters in the radio that do the filtering and make sure you only receive ONE signal. 100 years ago pleople used spark transmitters and they all generated the same signals, interference was a big problem, not so big in the beginning since there was only two stations but when the number of stations increased it became a problem and it was demanded that people would only transmit at ONE frequency .. Hope its clear now. regards, Paul.

Re: a stupid question

i think all the previous answers were very good but i will try to answer in a more smple way.
at first let us ask why we study interference between signals in communication?
the answer is to see if our reciever will be able to detect the transmitted signal or not.

how then our reciever detect the signal?
by multilplying it by a frequency the same as the frequency of its carrier(oscillator frequency) so we can extract the data modulated by this frequency by a mean of filters.

so simple mathimatical analysis you will find that any signal with frequency not equal the carrier frequeny when reach the reciever will be multiplied also by the oscillator frequency giving high frequency components thta will be suppressed by the filter then the problem will be with signals with the same frequency of the carrier frequency which when reach the reciever and multiplied by the oscillator frequency it will give a dc or very low frequency component which won't be suppressed by the filter andcompine the wanted signal.

we conclude that no interference in communication is a fictious expression meaning that signal with different frequencies than the carrier of the wanted signal won't affect my wanted signal when they reach the reciever with it,although, they interfere it in the medium

Added after 1 minutes:

i think all the previous answers were very good but i will try to answer in a more smple way.
at first let us ask why we study interference between signals in communication?
the answer is to see if our reciever will be able to detect the transmitted signal or not.

how then our reciever detect the signal?
by multilplying it by a frequency the same as the frequency of its carrier(oscillator frequency) so we can extract the data modulated by this frequency by a mean of filters.

so simple mathimatical analysis you will find that any signal with frequency not equal the carrier frequeny when reach the reciever will be multiplied also by the oscillator frequency giving high frequency components thta will be suppressed by the filter then the problem will be with signals with the same frequency of the carrier frequency which when reach the reciever and multiplied by the oscillator frequency it will give a dc or very low frequency component which won't be suppressed by the filter andcompine the wanted signal.

we conclude that no interference in communication is a fictious expression meaning that signal with different frequencies than the carrier of the wanted signal won't affect my wanted signal when they reach the reciever with it,although, they interfere it in the medium

Re: a stupid question

Dear,
Im replying in a silly way, but i think it will work.

Suppose u have one glass fill with water, another glass half filled with water, and the other half filled with sugar.
Now,
1. If you mix the two glasses of water in a jug, what you think will it not be difficult to extract same water as it were initially??
2. If you mix one glass of water with half glass of sugar; it will be easy to separate water and sugar.

apply this to concept of frequency, u will become to know that same frequencies when interfere, are difficult to separate; whereas different frequencies are easy separate after interference..


I think, u will understand the answer.

Regards,
Naveed

Re: a stupid question

As I am not an expert on frequency domain, I ask for the apology of the better than me.
I believe that the issue is not a qulaity study on the frequency interference rather than to explain with the mathematical models what and why is happening in a medium that different frequencies are applied.

I will start my thinking and I hope someone can fill up the gaps:

Say the medium is the air. All possible frequencies, artificial transmissions to natural E/M signals are there. Do they interefere? Yes. How we overcome this problem and do our transmission with the less possible mistakes? By coding the signal in our frequency.
If a arbitary signal is added or multiplied with our signal ( a frequency same or other) won't that be a problem? If coding is following a specific pattern, if the power of our signal is higher than the arbitary one, etc etc etc then we will not have any interference.

Electronic Counter Measures in the Army are based on this: You have a frequency source transmiting, then you find the frequency and you transmit bulshit on 10 times higher power.That's it.! You destroyed the transmition. But if you transmit with different polarity ( differeent aerial ) then you do nothing.

I use this example to point out that there are a great lot of factors that you have to take care before you assume that interference will destroy your signal. I.E. how we tranmit DSL via normal voice lines? Different frequencies one will say. Ok. But what about interference? So, you have one carrier and you supperimpose these different frequencies. So, you lose no info, and on Demux on the other side you split the frequencies...

I think maths have the answer on your question...

And I also hope that someone will fill the gaps on this post (xe,xe).

D.

Re: a stupid question

hi
i dont know if this explanation would be useful,but anyway, when one talks of interference, it must be understood that if two waves are traveling in the same medium, they will interfere, no matter what their frequencies are. Thus in communication, the problem is not to remove interference( as it cant be possibly done), but to reduce the loss of information due to interference

to illustrate,consider 2 people receiving different signals thru the same channel. For simplicity assume the two signals are sin(Ωt) and sin(Ψt). As they are transmitted over the same channel both persons will receive the interfered signal , (sinΩt + sinΨt)

Now the 1st person needs only the Ω frequency. So he will be using a BP filter tuned to Ω, similarly the 2nd person will be using a filter tuned to Ψ.
Now suppose Ω = Ψ. Then all the incoming signal(ie, interfered signal) will pass through both the filters hence both of them will lose information.

To prevent it, it is necessary that Ψ lies way beyond the pass band of the 1st person's filter tuned to Ω and vice versa. This can be achieved by keeping the frequencies far apart.
Thus by keeping frequencies far apart while transmitting thru a single medium, we are not preventing interference, but the loss of information due to it.
Hope i am clear [/i]

Re: a stupid question

who says you that signal of different frequencies not interfere ,there is interference but still to transfer signals in different frequencies is not only that because of interference ,there are other reasons also....

a stupid question

In communication there could be cases when 2 signal with same frequencies do not interfere (let say tdma cdma, uwb .. ) or 2 signals with distant frequencies do interfere (when distant frequency signal is so powerful that it results in intermodulation or exceeds receiver selectivity).

The answer is dependent on what meaning and in what context the interfere term is used.

Are we about physics or communication ?

this page is about interference term usage in physics :
https://en.wikipedia.org/wiki/Interference

Added after 13 minutes:

and this one is about communication :
http://en.wikipedia.org/wiki/Interference_(communication)

Re: a stupid question

I have a very enlightening communication example from real life:

In a mil airport decided to put up a new antenna for use in mil frequencies. So they did. They tried to communicate with their jets but they couldn't. On the other side, the fighter pilots where picking up a local radio station!!!

Why this happened? The distance between this antenna and the one existing in the same place worked as a frequency mixer that resulted in a frequency which was the difference between the two main frequencies! Is it true? It is because it created a hell lot of problems to the technicians of the radio station before they figure it out. What happened with the modulation of the signals, didn't it work? I think that the carrirer frequency is the one responsible for this "accident" and as the difference became dominant then FM could be propagated through the net.

It is tricky, but I think can help understand how interference occurs in any media...

D.

Re: a stupid question

Signals of different frequencies do add up and make a wave that contains all of the frequencies transmitted, but as long as they are different in frequency they can be separated out by filtering at the receiver,

this is how all communication systems work.

if two signals are sent at the same frequency they will be tangled even after the filtering and hence we will not be able to differentiate them...

Re: a stupid question

Dear friends,

let me try to answer. hope our good friend will understand it.
Basically u can share a N number of frequencies in a shared medium. Receiver will accept only the specified frequency window. Other frequencies will be rejected. let me take an example. Suppose in a lengthy employment Queue, UG are not allowed to post their resume where PG only allowed and vice-versa.

Rajesh

Re: a stupid question

They do interfere one to each other. You need a filter to separate them. Is at the receptor side that you think that there is not interference, but in the chanel (the space or in a cable) they are positively interfering one to each other