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Am transmitter?

  1. Jun 18, 2005 #1
    can anyone tell me how AM transmitter works???thanks
     
  2. jcsd
  3. Jun 18, 2005 #2
    I believe they work by creating a rapidly changing electric current, for example, attaching a deattaching a battery from a circuit will produce radio waves.

    -NewScientist
     
  4. Jun 18, 2005 #3

    dlgoff

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  5. Jun 18, 2005 #4
    thks New Scientist & dlgoff!any idea how to make a powerful AM transmitter??is it depends on the inductor and transistor?
     
  6. Jun 18, 2005 #5
    Well just a note of warning - are you actually going to make one?

    If you are, I cannot legally tell you how to do so, as broadcasting on AM wavelengths in most countries is illegal

    -NewScientist
     
  7. Jun 18, 2005 #6
    AM has nothing to do with wavelengths. AM is a type of modulation. Although there are countless places it is not legal to transmit.
     
  8. Jun 19, 2005 #7
    Average Supernova- where is it legal?! And the legality of transmitting has a lot to do with waelengths.

    Part 15 of Title 47 of the Federal Code of Regulations addresses the construction of homemade AM band transmitters. The three most germane paragraphs follow:

    § 15.5 (General conditions of operation)

    (a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or for power line carrier systems, on the basis of prior notification of use pursuant to § 90- 63(g) of this chapter.

    (b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by an other intentional or unintentional radiators by industrial, scientific and medical

    (ISM) equipment, or by an incidental radiator.

    (c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

    (d) Intentional radiators that produce Class B emissions (damped wave) are prohibited.

    § 15.23 Home-built devices.
    (a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.
    (b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equipment.

    § 15.219 Operation in the band 510-1705 kHz.
    (a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100\milliwatts.
    (b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
    (c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

    -newScientist
     
    Last edited: Jun 19, 2005
  9. Jun 19, 2005 #8
    The point ASN was making was that AM is a type of broadcast and as such is not tied to the band one broadcasts within. You can legally use AM transmitters in the ISM bands if you choose to. The legal code you state above says as much BTW. The only time one cannot use an ISM band is when use of said band endangers a life or interferes with pre-existing licensed device operating in the same band.

    In fact, a lot of low powered low BOM transmitters(Nordic, Atmel and Microchip make these as do other manufacturers) give a designer the option of using ASK(AM) or FSK(FM) transmitting modes.

    There are legal barriers to the power level and the frequencies at which one may broadcast without a license, but there are no legal walls preventing someone from using AM at 915Mhz or 2.45GHz.

    That was the point ASN was trying to make. AM is not a wavelength. AM is a way of transmitting data. FM is not a wavelength. FM is a way of transmitting data. The same goes for PM, ASK, FSK, BPSK, etc.
     
  10. Jun 19, 2005 #9
    Thanks faust. I also wonder where in my post I said it was legal to transmit AM? Reread my post newscientist. I think you missed a word or 2.
     
  11. Jun 19, 2005 #10
    Ye - speed reading doesn't work well does it! :P sorry for any problems!

    -NewScientist
     
  12. Jun 26, 2005 #11
    Amplitude Modulation works like this:

    First you have a radio frequency, and you run it through a circuit called a modulator, which is really just an electronic volume control. This electronic volume control, (which obviously has a variable amplitude amplifier as part of its circuit) is then controlled directly by the audio signal (in a typical broadcast).

    AM transmitting (click on this link for diagram)

    The changes in the (much lower frequency) audio signal turn the volume up and down on the radio (carrier) frequency. There is no *real* audio component in the final signal, just a radio signal that is varying (relatively) slowly in volume as it copies the audio signal. Later, another radio (receiver circuit) takes the radio signal and pumps it through a capacitor, which "shorts out" all the radio frequency harmonics, and re-connects the 'dots' to reconstruct the audio signal on the fly. This is then amplified by an audio circuit and sent to a speaker.

    AM receiving (click on this link for diagram)
     
    Last edited: Jun 26, 2005
  13. Jun 26, 2005 #12
    You're a bit off rogue. You make no mention of sidebands, the fact that the carrier does NOT change in amplitude when modulation is applied, various other tidbits...
     
  14. Jun 26, 2005 #13
    aren't you thinking of FREQUENCY modulation?
     
    Last edited: Jun 26, 2005
  15. Jun 26, 2005 #14
    No I am not.
     
  16. Jun 26, 2005 #15
    Okay you've really lost me:
    Amplitude Modulation is EXACTLY about having the carrier change in amplitude by applying modulation. Whatever can you mean, if you are not referring to some other method of transmission, like 'single-sideband', or 'Frequency Modulation', or 'Pulse Modulation'?

    Please explain!?! You're starting to remind me of the song, "I remember it well" from the movie "Gigi".
     
    Last edited: Jun 26, 2005
  17. Jun 26, 2005 #16
    No it isn't. That is the perceived result as well as the perceived method of obtaining a result.
     
  18. Jun 26, 2005 #17

    dlgoff

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    Say what? Here's a good definition of Amplitude Modulation.
    Regards
     
  19. Jun 26, 2005 #18

    Integral

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    AM can be produced in severl different ways the far and away most common in modren radios is a Superheterodyne circiut.

    Here is what wikipedia has to say about Super Superheterodyne

    As you can see from the block diagram, the key work is done with mixers.
     
  20. Jun 26, 2005 #19
    There was a discussion about modulation several months back in this forum which I took part in. I would suggest reading it. I finally lost interest.
     
  21. Jun 27, 2005 #20
    What's all the snobbery about? Amplitude Modulation is about modulating amplitude. It's not just 'perceived' action, its the principle upon which the audio signal is encoded onto the radio carrier.

    I still don't see what was wrong with my simple easy to understand explanation, and the accompanying improved diagrams made from the Wikipedia ones.
    I added simple passive component circuits which are actually used in building a crystal radio....

    So what is wrong with being clear and concise in a form that anyone with an education extending into high-school can understand? Isn't that what helping people is all about, as opposed to showing off your 'knowledge' about dipole expansions and advanced radio transmission techniques?
     
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