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Understanding modulation

Discussion in 'Radio and Communications' started by camerart, Aug 30, 2016.

  1. camerart

    camerart Active Member

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    Hi,
    I learned about radio signal modulation many years ago. I thought I understood it, but it is becoming apparent that I don't properly. I watched a one hour long modulation video lecture, and even though it was mostly above my head, learned the first building block to understanding how modulation works: The lecturer asked the students " how long would an aerial/antenna need to be to transmit a human voice?" There was a multi choice answer: 1mm, 1cm, 1mtr, 100mtrs, 10km, >100km. When I heard the answer is >100Km, along with the explanation, a penny dropped. Then he went on to explain different modulations.
    I now need to start again and find a less 'above my head' video (I learn best by video) that starts at the human voice and slowly works though the subject. Does anyone know of a video like this please?

    Cheers, Camerart.
     
  2. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    Which do you want to start with AM or FM??

    The youtube channels are the best place to look
     
  3. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Modulation.gif
    The "Signal" (for instance, a low frequency, audible tone) is used to modify (modulate) either the "AM" or "FM" carrier signal that is then transmitted at a much higher frequency. At the receiver end, the signal is "stripped" from the carrier, leaving the original signal for you to hear.

    Thus, the antenna length required for radio transmission of this "signal", using these modulation techniques, is much, much shorter.
     
    Last edited: Aug 30, 2016
  4. dave

    Dave New Member

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  5. camerart

    camerart Active Member

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    Hi I and B,
    I want to understand all.
    The visual is very useful, and I can see much of what I want. I can see that the signal on it's own would need anaerial as long as the sign wave, and mostly understand how AM is modulated. I think the animation needs more work though to fully do it's job. I'm sure it's quite obvious to anyone who fully follows, just as I can mostly understand mechanical things.
    Thanks, C.
     
  6. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    Just to cause a tiny bit more confusion.. Normally a half wave aerial or 1/4 wave aerial is used... Most transmissions can be recovered with the smaller aerials..

    AM, as the animation shows, is the easiest to follow The amplitude of the signal controls the amplitude of the carrier... The FM is a bit more involved as the frequency can only shift a bit... I believe up to 90° up or down... If you lock on to the frequency, the phase shift from the norm is the signal amplitude.. A quadrature decoder is used to detect the phase shift.. This isn't my area of expertise and I'm sure others will help explain..
     

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  7. JimB

    JimB Super Moderator Most Helpful Member

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    That to me is a very strange way of approaching the subject of modulation.

    We had a discussion here on ETO some time ago which concerned this very subject, where it was postulated that the purpose of modulation was to reduce the size of the antenna.
    See here:
    http://www.electro-tech-online.com/threads/digital-modulation.136155/#post-1213112

    To me it makes far more sense to ask the question:
    "How can we use radio waves to convey information?"
    Answer:
    "Modulate the radio wave".

    JimB
     
  8. JimB

    JimB Super Moderator Most Helpful Member

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    Actually when a carrier is FM modulated, the deviation can be very wide. The deviation is how far (in frequency) the carrier shifts and is directly proportional to the amplitude of the modulating signal.

    How much deviation is used depends on the application and the system parameters.
    Examples:
    FM broadcasts in the 88 to 108MHz band have a system deviation of 75kHz and a maximum modulating frequency of 15kHz.
    Using Carsons Rule, the bandwidth occupied will be 2 x (75 +15) = 160kHz.

    FM transmitters for business use (taxis etc, 160 to 170 MHz ish) have a system channel spacing of 12.5kHz.
    To avoid adjacent channel interference, the deviation is 2.5kHz and the maximum modulating frequency is 3kHz.
    So, using Carsons rule the bandwidth will be 2 x (2.5 + 3) = 11kHz, which fits nicely within the system channel spacing.

    Sounds like a form of Phase Shift Keying (PSK) for digital signals. Not a strong area of my knowledge.

    JimB
     
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  9. nsaspook

    nsaspook Well-Known Member

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    A couple of good videos on I/Q signals and modulation.


     
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  10. camerart

    camerart Active Member

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    Jimb, Your answers are too advanced for me to go back and understand the basic concept. The point the lecturer made about the length of an aerial to transmit a voice would be >100K was a light bulb moment for me.
    Nsaspook, I have an animation on IQ modulation, and I'll look again later.
    Thanks to both.

    I don't tend to learn as well from lots of words and calculations. One thing I've learnt is that after a lot of reading, I don't remember much of it till later, if at all. I have to make shorthand notes and pictures to prompt me quickly when I'm a on a subject. As I walked into the amateur radio exam in 1985, my lecturer reminded me about oscillator mixing. I passed:), but I didn't fully 'get it'

    Going back to POST No 3 the frequency animation! Tell me if I'm correct. When transmitting the voice, the top sign wave is one tone of many to be transmitted, and the AM and FM frequencies (chosen for more practical aerial length) that are mixed with it are fixed? Is the FM part correct, in that one 'top' fixed tone is varying the fixed FM frequency?
    C.
     
    Last edited: Aug 31, 2016
  11. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    Ditto!! I have studied PSK at Uni but had no interest whatsoever!! People make transmitters and receivers, so I'll buy theirs...
     
  12. camerart

    camerart Active Member

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    I think I remember this bit from the lecture I watched. PSK switches the phase of the carrier frequency 180Deg. EDIT: At each 1 or 0.
    C.
     
    Last edited: Sep 1, 2016
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Basically.
    The "fixed" FM carrier frequency is, of course, varying around the carrier center frequency.

    The relative deviation for commercial FM broadcasts in the 100MHz band is much less then shown in the animation.
    If you look at it with an oscilloscope it would be difficult to see the deviation (about 0.075% maximum of the carrier frequency).
     
  14. camerart

    camerart Active Member

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    Hi,
    Is this correct: The frequency of the FM carrier wave is modulated by the amplitude and frequency of the signal? I hope this is true, as I have been thinking that the FM is only modulated by the frequency of the signal.

    EDIT: Just found this animation, that explains the animation in #3 a bit better. I think this answers my question, as I assume that if the amplitude of the signal varies, then so does the amplitude of the modulation.
    C.
     

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    Last edited: Sep 1, 2016
  15. JimB

    JimB Super Moderator Most Helpful Member

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    Yes.
    The amplitude of the modulating signal determines the deviation ie how far the carrier frequency changes,
    and the frequency of the modulating signal determines how quickly the carrier frequency changes.

    JimB
     
  16. camerart

    camerart Active Member

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    Hi J,
    A penny has just dropped!! Simple really, I don't see what the problem was:rolleyes:
    Thanks to all for your patience.
    Camerart.
     
  17. JimB

    JimB Super Moderator Most Helpful Member

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    Don't you just love those "Ah, that's what it means!" moments.

    JimB
     
  18. camerart

    camerart Active Member

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    Hi J,
    Yes, I do, especially as they unlock doors.
    C.
     
  19. Mosaic

    Mosaic Well-Known Member

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    I suppose the main purposes of modulation is the efficient use of bandwidth and hopefully reduced signal strength for maximum information transfer fidelity. By applying multiple modulation formats to a single CW you can multiply the information delivered within the same occupied bandwidth.
    The challenge we face now going into 5G cellular is while the higher frequencies can carry more information they have less ability to penetrate obstacles and thus less range. That means more cellular sites or some other kind of repeater/hand off system.
     
  20. camerart

    camerart Active Member

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    Hi M,
    Recalling the aerial/antenna length analogy, where a phone would now be able to use a small one, as the frequency goes higher so the aerial gets shorter. With the shorter aerials accuracy is key.
    My last phone had (I think it was called) PTT, so that each phone switched to being a walky talky, talking the short distance to the next phone, instead of using masts, I think this is the way phones will go, only each phone will be a relay for other people's messages, in a similar way to the internet.
    C.
     
  21. crutschow

    crutschow Well-Known Member Most Helpful Member

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    I think most people will be very unhappy if there phone battery is being drained and battery life shorted because their phone is being use to relay someone else's signal. o_O
    Transmitting takes a lot more battery energy than just receiving.
     

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