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FM question

Discussion in 'Electronics Homework Help' started by jdouglasusn, Aug 6, 2012.

  1. jdouglasusn

    jdouglasusn

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    May 29, 2012
    My instructor gave a problem and it is not making much sense to me.

    (This is word for word how he wrote it)

    "FM has a deviation of 7kHz & intelligence frequency of 2kHz. Si/Ni = 20 dB.
    Calculate So/No at the output."

    I know mf = deviation/intelligence frequency which is 3.5

    What I am stumped on is Si/Ni = 20dB and how that exactly factors in to calculate the answer.
     
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Suppose that the modulator/demodulator was perfect and no noise was added, then the signal (including noise) at the demod output would be a perfect reproduction of the signal at the modulator input; Si/Ni=So/No. Is there any modulation technology that can recognize the noise component of an input signal and remove it? If not, then how would modulation noise be added to the noise already in the input signal? Are the two noise components uncorrelated?
     
  3. jdouglasusn

    jdouglasusn

    29
    0
    May 29, 2012

    That is my hitch. This question he wrote made no sense. It doesn't matter anyways. I will ask him to clarify tomorrow. Thanks anyways
     
  4. foTONICS

    foTONICS

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    Sep 30, 2011
    I thought FM had a deviation of +- 75kHz
     
  5. davenn

    davenn Moderator

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    1,744
    Sep 5, 2009
    depends on the use you are referring to
    for broadcast in the 88 - 108 MHz band it can be up to 150kHz

    for FM on land mobile on other bands it is commonly 7.5, 12.5 or 25 kHz

    Dave
     
  6. foTONICS

    foTONICS

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    Sep 30, 2011
    your referring to narrow band?
     
  7. davenn

    davenn Moderator

    13,244
    1,744
    Sep 5, 2009
    well the OP referred to 7kHz which is definately narrow band

    you referred to + - 75kHz which is wide band broadcast

    I was making you aware that FM deviation can vary depending on the radio/TV service that its used for :)

    Dave
     
  8. john monks

    john monks

    693
    1
    Mar 9, 2012
    FM deviation of 7kHz means that the carrier frequency is being deviated +7kHz and -7kHz.
    Intelligence frequency means the modulating frequency or what?
    Does the question relate to the affect the modulating has on the signal to noise ration or the deviation or what?

    I,m very confused by the nature of the question.
    Please help me understand the question.
     
  9. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    intelligence frequency and modulating frequency are the same thing. if I had to take a stab at what he meant I would say the teacher wants to know what affect the mf has on the signal to noise ratio at the output
     
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