1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How come no interferance takes place

  1. Nov 14, 2007 #1
    If 2 signals of different frequencies are sent through the same medium... lets say 2 rf signals are sent through the air at the same time of different frequencies, how is one able to receive the signal at the other end? I mean even though the receivers are tuned to the frequencies at the other end, won't the 2 signals interfere with each other in air?? I am quite confused about this whole concept...:grumpy:

    The way I understand it, frequency actually depicts the time period at which the signals are sent, like after such and such time only the signals are sent, so is is this understanding correct? I just want to confirm what I think I have understood...
  2. jcsd
  3. Nov 14, 2007 #2
    Electromagnetic signals do not interfere (at least not in classical physics, anyway, and even in QED it takes very powerful photons to generate interference). Air has nothing to do with it, by the way, as EM signals do not need air to propagate.

    Receivers are usually tuned in to pick up specific EM frequencies by means of resonance, and can therefore tune-out other signals with other frequencies.

    I find this statement hard to understand. Perhaps you could clarify?

    http://www.physicallyincorrect.com/" [Broken]
    Last edited by a moderator: May 3, 2017
  4. Nov 14, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper

    Doesn't even apply just to radio waves - you can have 2 different frequency vibrations on a string at the same time.
    The trick is making them interfere, that's what you need to build a receiver.
  5. Nov 14, 2007 #4
    ah... okay... right, so Em waves don't interfere much? I did not know that... that clears up a lot...

    Okay forget EM waves, lets say you take a transmission of signals through a link between 2 computers... In like a DSL connection... I have read that it cuts up normal telephone line into different frequencies, and uses the lower frequencies to transmit telephone signals and the rest of the wide spectrum of frequencies for internet connections... Now... what I don't understand is how this is managed... I mean it is not possible to send multiple signals of multiple frequencies at the same time through the transmission line and decode them at the other end right? or am I missing something?

    Now, the way I understand it is, the telephone line signals are sampled at a 2w rate and sent through the link... now, the signals that are being sent occupy only a small part of a set transmission time, like lets say we take an interval of 1 second, then the telephone signal needs only like 1/4 of the time to send its sampled signals, for the rest amount of time... the transmission channel is free to transmit other signals through the line... The time intervals between the signals being sent is interpreted as its frequency...

    What I meant by frequency is only the time period between which the signals are sent applies here... since f=1/t, since essentially thats what the frequency depicts anyway... I hope I am being clear here not just confusing you, which is possible since im pretty confused about the whole idea myself...

    Is what I think I have understood correct? That is my question...

    thx for prompt reply by the way...
  6. Nov 14, 2007 #5
    The problem is that you're using too simple a definition of frequency. Usually, when people are talking about "frequency," they are implicitly referring to a signal's Fourier components, and not to some overall periodicity (which is the definition that you're using).

    If you've not yet come across Fourier analysis in your studies, the basic idea is simply from the mathematical fact that many functions can be expressed as a linear superposition of sinusoidal components (i.e., a sum of sine waves). In fact, practically all physical signals (which are functions of time) can be conveniently expressed in this way, through either Fourier series or Fourier transforms. (Fourier series for periodic functions, Fourier transforms for normalizable functions.) As it turns out, sinusoids are immensely convenient for solving many physical systems, because they are the solutions to the simplest linear differential equations.

    So, the key thing to note is that when transmitting information through the air on different frequencies, the signal is not chopped up in the time domain, as you seem to think. Rather, what this means is that the two signals are literally imposed on top of each other, all at the same time. (Mathematically, think about a sum of sine waves.) The receiving circuit, however, is fully capable of resolving only the desired frequency from the overall signal.
  7. Nov 14, 2007 #6


    User Avatar
    Science Advisor
    Homework Helper

    You can hear music with more than one note, or people talking rather than singing a pure note! It's the same thing.
  8. Nov 15, 2007 #7
    Okay... I think I get it, its like using superimposing a carrier signal onto a message signal, when you superimpose a lot of frequencies together, you get the resultant output signal, but does this apply to even digital signals? The way I understand it, does this superimposing apply only to analog signals? And I understand there are ways to interpret the signal.

    But during transmission, in digital one supplies only pulses of signals right?

    does the use of the term frequency vary for a digital signal and an analog signal or do the both have the same meaning for it? or... am i just blabbering?
  9. Nov 15, 2007 #8


    User Avatar
    Science Advisor
    Homework Helper

    That gets into the whole new topic of carriers and encoding.
    If you simply make the line 5V for 1 and 0v for 0 then you can't really mix two signals, because a 5V for whatever length of time means 5V at all frequencies.

    You can endcode the signal as difference frequenies using the same technique as FM radio with different data streams as analog levels at different frequencies.

    You can also add a second data stream by altering the width of the pulses, so the main data still sees 5v/0V but it can also measure narrower/wider pulse to extract the second set of data. Usually you can't send a high data rate this way but it's useful for adding a new data layer onto an existing standard - older equipement just ignores the slight width variation.
    It's used to send monitor resolutions or channel id over video signals or to put time/date info on top of timing pulses.

    But typically no, you don't send multiple different frequncies on a digital wire. You packet up different data streams into blocks send them one at a time and unpack them at the other end, or for optical fibres you send each stream as a different wavelength ( wavelength division multiplexing).
  10. Nov 16, 2007 #9
    ah... okay... thats what I was wondering... it clears things up...
    But nowadays they don't use analog signals anymore right? And yeah... I know about PWM So...

    What did you mean by...
    Code (Text):
    Usually you can't send a high data rate this way but it's useful for adding a new data layer onto an existing standard - older equipement just ignores the slight width variation.
    I mean Isn't digital faster?? Thats the idea behind DSL and ADSL technologies... yes?
  11. Nov 16, 2007 #10


    User Avatar
    Science Advisor
    Homework Helper

    Sorry wasn't clear.
    The main data stream is fast (limited only by cable and speed of recevier/transmitter)
    The 'extra' data stream is limited because it is piggy-backing on an existing data stream that it has no control over. You can only change the width of the pulses very slightly without compromising the main data stream. Often you need to set a wider pulse on say 100 of the main data stream clocks to be sure it has got through.

    This arrangement isn't very common - it's just a clever way of sticking some new data over the top of an existing system.
    It's not really PWM it's sort of 'PWM ontop of existing data'.
    Last edited: Nov 16, 2007
  12. Nov 17, 2007 #11


    Staff: Mentor

    Not necessarily. Digital is useful because it allows a signal to be replicated and transmitted without any loss. It is not necessarily faster, but it is less susceptible to degradation.
  13. Nov 18, 2007 #12
    ah... so, in for say very short distances with little interference in the transmission line your telling me that analog signals can be actually faster?

    hmmm... could you tell what type of modulation scheme allows this?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook