Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Radio Carrier Signal Trouble Understanding

  1. Jun 3, 2017 #1
    I seem to have a lot of trouble understanding radio carrier signals I have read a bunch of stuff online and seen videos trying to explain it but I just can't understand it!

    Could some of you who understand try and give me some analogies or anything that might help me out?

    Thanks alot!
     
  2. jcsd
  3. Jun 3, 2017 #2

    Drakkith

    User Avatar

    Staff: Mentor

    It's difficult to help you out if you've already read a lot about the subject and have already watched videos explaining it. Perhaps you could give us some specifics that you're confused about? What is it about a carrier signal that confuses you?
     
  4. Jun 3, 2017 #3
    Ok lets take this explanation for example http://kids.britannica.com/students/assembly/view/53879

    I think I understand the first part correctly where audio sound waves are converted into electrical signals by the function of electromagnetism but after that I have no idea what is happening.

    "In sound transmission, the initial sound is first picked up by a microphone. The microphone generates a pulsating direct current called a black wave. Meanwhile an oscillator supplies a carrier wave. Electrical circuits combine the black wave and carrier wave into a modulated carrier pulse of alternating current. This pulse is amplified and used to radiate a carrier wave."

    I have problem understanding these parts "Meanwhile an oscillator supplies a carrier wave." and "Electrical circuits combine the black wave and carrier wave into a modulated carrier."

    Also why do they always say "Modulated" instead of Modified? Isn't it the same thing?
     
  5. Jun 3, 2017 #4

    Drakkith

    User Avatar

    Staff: Mentor

    Okay. Can you answer a few questions? These will help narrow things down a bit more.

    What do you know about each of those (the oscillator supplying a carrier wave and combining the black wave and carrier wave)?
    Do you understand the basics about how electrical circuits work?
    Do you know what voltage and current are?
    Do you understand how the signals are mixed together?
     
  6. Jun 3, 2017 #5

    jtbell

    User Avatar

    Staff: Mentor

    The page that you linked to is about amplitude modulation. Here's a diagram from the Wikipedia article about amplitude modulation that shows the concept more explicitly.

    Illustration_of_Amplitude_Modulation.png

    "Modify" means to make any change. "Modulate" has a more specific meaning in this context: to modify some property of the carrier signal so that it varies in step with the information signal. Here, the property of the carrier signal that's being modulated is its amplitude.

    The other two fundamental properties are a wave are its frequency and its phase. Each of these can also be modulated to carry information. Google for "frequency modulation" and "phase modulation" for more information on Wikipedia and elsewhere.
     
    Last edited: Jun 3, 2017
  7. Jun 3, 2017 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    What have yo actually read about it so far that doesn't make sense to you?
    If you think of a simple Morse Code signal. A continuous radio frequency signal is switched on and off, on and off to 'carry' the da di da da signals. The RF wave is a Carrier and it is 'Modulated' using the Morse Key switch. Other modulation systems are more complex and carry more information faster. Read on . . . . . .
    You need to do more than glance through "a bunch of stuff" if you want to learn about this. It may take some effort to get over hurdles. Identify an actual hurdle and PF can probably help you over.
     
  8. Jun 3, 2017 #7

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Hi revv
    welcome :)

    upload_2017-6-4_14-33-43.png


    That's got to be about the worse description I have ever seen and bears little resemblance to reality

    no wonder you are confused

    Black Wave ??? really ? I have been doing radio transmission stuff for 40 yrs and have never heard that term .... IT IS BAD and incorrect
    It is more correctly called an audio signal

    There is No Carrier Wave till the transmitter stage. The transmitter stage produces a carrier wave that is MODULATED by the audio signal
    That modulated carrier wave is then sent to the antenna for transmission

    @jtbell gave a good response in post #5


    Dave
     
  9. Jun 4, 2017 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I think it should be a general principle to avoid 'Encyclopaedia' articles about anything but pure facts (dates, names etc). Electrical Engineering (and any of the Sciences) will not be well supported by the small number of staff who are involved in getting an A toZ compendium of info. I used to look at Britannica in our Department library, occasionally and it was often in subtle (or even total) disagreement with text books and research papers.
    I am afraid that your OP contains far to many implied questions to be dealt with on one PF thread. The answers you get can easily confuse you because PF contributions to this sort of thread are often too specific and lead you to imply general rules when those rules only apply to the specific context of each answer.
    Bottom line is that the whole topic of Information Transmission is VAST and requires a lot of reading round (reputable sources). Strangely enough, I could suggest reading Radio Ham publications. Not top notch for theory, perhaps, but describing working systems in friendly and practical terms that will make the topic approachable. (You always have Google to call on for meanings of the jargon terms that amateurs tend to use)
    My teenage intro into electronics involved reading through Practical Wireless Magazine again and again - as boys do. (No such thing as an Internet at the time)
     
  10. Jun 4, 2017 #9

    Drakkith

    User Avatar

    Staff: Mentor

    I agree. The best thing to do is probably to find a textbook on the subject. A quick look on amazon for "principles of radio transmission" yielded a huge number of results.
     
  11. Jun 4, 2017 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I just looked at the Britannica diagram. It's total garbage. It shows an amplitude modulated carrier wave coming out of the microphone. The so called "black wave" refers to the black envelope of the AM signal in the diagram I think. Shocking!
     
  12. Jun 4, 2017 #11

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yeah, as I said way up the thread where I reproduced the diagram
     
  13. Jun 11, 2017 #12
    What do you know about each of those (the oscillator supplying a carrier wave and combining the black wave and carrier wave)? I don't think I understand this.
    Do you understand the basics about how electrical circuits work? A little.
    Do you know what voltage and current are? I believe the voltage is the "force" and current is the flow of electrons?
    Do you understand how the signals are mixed together? I don't.

    I understand when you speak into a microphone it converts the audio into electrical current and then this current is amplified at some stage but what I don't get is how for example I can tune my radio to 100 MHz FM but isn't that 100 MHz a constant frequency or a constant amplitude in the case of AM? With no varied "height" and the same "frequency" all along the wave?

    Wouldn't an audio signal current from someones voice be varied with with a changing amplitude or frequency wave?

    I might be a hopeless case and never understand these kind of things but I might buy some books that some of you have recommended.

    Trust me Sophie I have read a lot and I think I watched all the youtube videos that try and explain it but I just can't wrap my stupid head around it like I said I might just never understand :[

    I really appreciate you all taking the time to help me and I hope some day I will be able to understand and explain these things. Thank you all again.
     
  14. Jun 11, 2017 #13
    let me help if I can, in the case of amplitude modulation imagine it this way, you have a microphone or a record player or any other audio frequency (20-20khz) source that outputs a low amplitude (signal strength) signal. Normally in your home you take that audio source whether an mp3 player or an iphone or whatever and plug it into an amplifier which is nothing more than a signal driven power supply, so it takes your weak signal and amplifies it saving the signal frequency and characteristic and then you put that into your speakers and drive them.
    The reasojn why you can't do this with a radio transmitter is because at such low frequencies the transmitter would have to be very large, also the receiver and it would require lots of power, it is easier to propagate a signal if it has higher frequency.

    So they take an audio signal and then it is sent to the transmitter amplifiers since basically a radio transmitter is nothing more than an antenna which is driven by an amplifier much like a speaker the only difference is that this amplifier is a so called RF amplifier as it amplifies high frequency which then being drive through an antenna radiates in all directions and can be picked up by antennas around.
    The tricky part here is the way in which the simple audio signal is coupled with the high frequency high strength signal going out from the amplifiers into the antenna.
    I'm not a big pro myself int his field but in the case of amplitude modulation which seems the easiest part I take it happens like this, they take the incoming audio signal and they use that low strength signal to vary the amplitude of the high frequency high strength signal which is then sent into the antenna, imagine like this,
    normally if you just needed to send a high frequency signal into the antenna you could just take a certain frequency steady signal and simply amplify it and send it into antenna but that signal would contain no information so would be of no use but you need a high frequency signal for the efficiency so you take it and make it's amplitude vary with the frequency of the incoming audio signal, so the high frequency signal always is at the same frequency but its amplitude is increased and decreased with time, so now whnen you stick out your AM radio antenna it does pick up the high frequency signal but its circuit is designed in such a way that instead of responding to the high frequency itself it rather responds to the increase and decrease in signal strength which then creates the very waveform which was put in or modulated with the audio signal back at the transmitter stage and so you can now hear your audio signal.

    the high frequency for the audio signal is simply like a car for a human which helps him get faster and more effectively to his target nothing more



    You see the radio antenna picks up the high frequency signal but you cant hear that with your ears but the changing amplitude creates a correspondingly changing current strength in the antenna and this second thing happens to be at the audio frequency so the circuit then amplifies this changing current and voula your speaker now outputs the very voice that was sent into air miles away, rather clever, isn't it?
     
    Last edited: Jun 11, 2017
  15. Jun 11, 2017 #14

    Drakkith

    User Avatar

    Staff: Mentor

    When you tune your radio to a specific frequency, the radio actually receives signals within a band of frequencies centered around the one you selected. Here in the US, the bandwidth is ±75 kHz with 25 kHz "bandguards". If you tune to 100 MHz, the radio receives signals at 100 MHz ±75kHz.

    Yes. The carrier frequency is modulated with the voice signal in different ways depending on the modulation type.
     
  16. Jun 13, 2017 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It is possible that you have read a lot of stuff that is too advanced for you. You need to have the basics first. Rome was not built in a day.
     
  17. Jun 14, 2017 #16

    Tom.G

    User Avatar
    Science Advisor

    Hi @revv,

    girts did a pretty good job of covering it, but I thought I'd try a slightly different approach. As such, part of girts post is repeated here with a minor addition in blue.

    With a bit of plagiarism from @girts :
    Now for Amplitude Modulation(AM), the voice signal from the microphone is used as an **electronic volume control** for the RF amplifier. A detail here is that without any voice signal, the RF amplifier volume is at the half way point. When a voice signal arrives, the low-voltage part of the voice waveform decreases the "volume control" of the RF amplifier, and the high-voltage part of the voice waveform increases the "RF volume control."

    A radio receiver for AM signals responds to the power of the RF signal it receives and sends the changing power signal to the speaker.
     
  18. Jun 14, 2017 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That's the best way to describe an Amplitude Modulator. But
    It isn't just the high voltage values of the audio signal- it's every instantaneous value of the audio signal voltage that controls the carrier amplitude.
     
  19. Jun 14, 2017 #18

    Tom.G

    User Avatar
    Science Advisor

    Yes. Thanks for expanding that and making it clearer. Maybe between the 5 or 6 of us we can get it sorted out for revv.
     
  20. Jun 14, 2017 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    We can only go so far with this. He needs to put in a lot of effort, reading 'proper' sources and joining the dots himself.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Radio Carrier Signal Trouble Understanding
  1. Radio signal (Replies: 2)

Loading...