AM transmitter

Discussion in 'Wireless & RF Design' started by cpleng7, Sep 8, 2009.

  1. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
    120
    0
    Can some one tell me how to prove the advantage of the advantage and disadvantage of the AM transmitter?
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    The advantage of AM is that it gets you on the air :p. Disadvantages may include that it has a wider bandwidth than other types of modulation and that you have both the upper portion and the lower portion. What I mean by this: Single Side Band (SSB) is basically AM with the negative portion chopped off. Have you ever seen a sine wave? You can kind of get the idea of what this would be like. Thus, AM has both the negative and positive portions used while SSB only has the positive portion. I would suggest that you pick up a ARRL Handbook from your local library. Inside, they have most of the modulation techniques used and also what they look like on a oscilloscope.
     
  3. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    Some thoughts:

    Pros:
    AM transmitters (tx) are rather simple to build, you need a simple receiver (rx) as well and setting the receiver and transmitter's frequencies are not all that touchy. If your tx and rx are tuned to within, oh, even a kHz or so, things will sound alright.

    Cons:
    Am transmitters transmit a carrier and both sidebands. This takes up a lot of RF spectrum AND consumes more electrical energy than is used by an SSB transmitter.


    And....
    SSB transmitters (and receivers) are much more complicated, but SSB is more spectrum efficient. However, the transmitter and receiver MUST be tuned to within 100 hertz or so of one another, otherwise the audio at the reciever end will sound poor. ALSO, since they must be so close in frequency, stability of both tx and rx is very important in SSB.

    Again, SSB conserves spectrum since AM transmits a carrier AND both sidebands but SSB transmits only one sideband and no carrier.

    Those are the highlights!
     
    Last edited: Sep 9, 2009
  4. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    f.c.c. rules to build ?
     
  5. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    One disadvantage of AM is that you have to tune the magnifacation up/down on the reciever depending on whether you are moving away from, or towards the sender.
     
  6. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    loosewire,

    I dont think he wants to build, I think he was asking more to gain a general understanding.
     
  7. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    Um, HUH? :confused: (Or was that an attempt at sarcasm? :cool: )
     
  8. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    Yeah thats what i'm told. As signal strength goes down you need to magnify the signal more since the information is stored in it's amplitude. So if you are driving away from the transmitter you need to constantly re-adjust the volume level as the signal level reduces. At least if we're talking about AM radio.
     
  9. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    Oh, ok. The "magnification" thing confused me -thought you were talking about the Doppler effect - as I re-read it, I have NO IDEA why I got that idea.

    Yea, you ARE right, I mean, AGC SHOULD compensate somewhat for what you are talking about - but it is somewhat limited in it's capabilities.

    Of course, same goes for SSB. FM does not suffer from this effect.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    It is not!
    An AM signal has an upper RF sideband and a lower RF sideband plus the carrier frequency.
    The positive and negative voltage swing of the RF signal still occcurs with SSB but one RF sideband is filtered out. Sometimes the carrier is also removed.
     
  11. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Note that the first picture is SSB and the second is AM. That is what I mean, that
    I apologize for not being "technical" enough.
    [​IMG]

    [​IMG]
     
  12. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
    120
    0
    thank you for all giving me the suggestion.

    if the wide bandwitdh is the disadvantage of the AM transmitter, then can some one tell me in which area they use need this AM bandwidth?
     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    A.M. is the simplest modulation system to implement (at least until you get to really high power), and requires little more than a diode to detect it on the receiving end. The disadvantage of A.M. is that receivers are subject to electrical noise to a much greater extent than FM.....you can't use a limiter in an A.M. receiver (though PULSE type limiters are relatively effective).

    A.M. has no "capture effect" unlike FM, which will only respond to the strongest signal on any channe. The capture effect can be really nice, or really disastrous, depending on the application. ALL aircraft radios, even at VHF, use AM for solely this reason. Pilots and controllers need to hear EVERY signal on any channel...not just the strongest ones!


    Eric
     
  14. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    To clarify SSB, which is difficult to understand from many conventional explanations..

    The resulting transmitted signal, after all the filtering etc., is the original Audio / speech signal or waveform, but upshifted in frequency.

    When you receive SSB, it is demodulated by mixing with a 'carrier insertion oscillator'.

    Eg. a SSB transmission on 10MHz, mixed with an oscillator around 10MHz, produces sum and difference products, one very low frequency (ie around audio) and one at around 20MHz (ignored).

    This is why you can get the 'growling' and 'donald duck' effects when tuning an SSB signal, you are directly controlling the frequency shift from the on-air signal back down to audio.

    (There are minor frequency offsets for convenience of filtering, also it can be 'inverted' frequency-wise depending on upper or lower sideband, but the 'shifted audio' principle still holds.)
     
  15. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    A touch of the past,how many guys served hard time for C.B. rule
    violation for high powered out put,then they backed off like booze
    leaving guys with records. A lot of good experience like the bread
    boarder of today. Bad langage and a record for life. C.B.'s never
    made standard equipment on new cars.Triangelatation was the
    F.T.A. of the day.Some of the new guys want have a clue. C.B.'s
    was there pot and L.S.D. of the past. I just saw that hugh
    amplitude,he would have got time for that power If caught.
    THEY DO VERY LITTLE ABOUT PIRATE F.M. STATIONS TODAY.
     
    Last edited: Sep 12, 2009
  16. cpleng7

    Thread Starter Active Member

    Dec 18, 2008
    120
    0
    Is that possible AM power spectrum get more than double side band?
     
  17. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    If you mean can the bandwidth be wider than the double sideband signal, the answer is yes - if an AM transmitter is overmodulated then additional out-of-band frequencies can be generated.
     
  18. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    And if you have ever listened to 27.555 mHz, you have probably heard it yourself. Hard to say for sure, because it COULD be all the LOUSY (non-linear, over-driven or improperly tuned) amplifiers on there.
     
Loading...