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Can anyone please tell me more about local oscillator?

  1. May 10, 2012 #1
    Local oscillators that is used in RF transmitters for AM modulation.Any help is appreciated. Thanks !
     
  2. jcsd
  3. May 11, 2012 #2
    Look up oscillator on Wikipedia. 99% of the oscillators used for AM modulation would be the "relaxation oscillator" type.
     
  4. May 11, 2012 #3
    A local oscillator is usually mixed (through a multiply circuit) with the baseband signal. This creates an intermediate frequency (IF) that is easier to work with.
     
  5. May 11, 2012 #4
    Thank you !
     
  6. May 11, 2012 #5
    Thank you !!
     
  7. May 11, 2012 #6

    vk6kro

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    Relaxation oscillators do not produce continuous sinewaves.

    Typical sinewave oscillators would be Hartley and Colpitts oscillators. These use tuned circuits with varying methods of connection to a transistor or FET.

    Here is a Hartley oscillator. Ignore all the capacitors at left. These are just to get fine tuning of the oscillator.
    Only one (say "Ct" ) is needed.

    osc1fig4.gif

    You can read the whole article that this circuit came from here:
    http://www.hobbyprojects.com/H/hartley_oscillators.html

    There is a link to this site:
    http://mysite.du.edu/~etuttle/electron/elect16.htm
    which gives a summary of the different kinds of sinewave oscillators.
     
  8. May 11, 2012 #7

    davenn

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    The term LOCAL OSCILLATOR is more correctly used in a receiver not a transmitter

    its the freq of the LO that is mixed with the incoming signal from the antenna to produce an IF signal which depending onthe received frequency there may be more than one LO and several IF (Intermediate Frequencies) for example

    a received freq of 550 MHz mixing with the first LO of 505MHz gives the first IF of 45 MHz,
    it then may be mixed again with a second LO of 494.3MHz to give a second IF of 10.7MHz
    and finally mixed again with another LO of 10.245 MHz to give a 3rd IF of 455kHz from which the audio is demodulated.

    thats a common arrangement for VHF and UHF and up received frequencies
    lower freqs such as AM broadcast bands 550 - 1620 kHz more commonly just used a single IF down to 455 kHz ( there are a number of variations around 455 kHz used + - a few kHz)

    altho that does happen for the transmit side its not overly common. Its more common to use an oscillator either operating at the freq of interest or multiplying up from a lower freq
    say a crystal based Colpitts or Hartly osc as VK6KRO indicated.

    Also to the OP --- in an AM transmitter, the audio modulation is normally applied to the last amplifier stage of the transmitter. No Local Oscillators involved in that process. This is quite opposite to a FM transmitter, where the oscillator is modulated with the audio signal.

    cheers
    Dave
     
    Last edited: May 11, 2012
  9. May 16, 2012 #8

    davenn

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    to correct a typo pointed out to me :redface:

    " a received freq of 550 MHz mixing with the first LO of 505MHz gives the first IF of 45 MHz,
    it then may be mixed again with a second LO of 34.4MHz to give a second IF of 10.7MHz
    and finally mixed again with another LO of 10.245 MHz to give a 3rd IF of 455kHz from which the audio is demodulated.

    was just having a "senior moment"

    Dave
     
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