Overmodulation

Discussion in 'Homework Help' started by kee23, Oct 3, 2012.

  1. kee23

    kee23 Thread Starter New Member

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    Hello,

    I was wondering what happens to the signal quality of the demodulated audio signal when a carrier envelope is overmodulated?
  2. JoeJester

    JoeJester AAC Fanatic!

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    take a look at these ... 100 percent modulation, 105 percent and 120 percent.

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  3. kee23

    kee23 Thread Starter New Member

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    umm what about the sine wave that comes out from demodulator? amplitude of a sine wave clips?
  4. vk6zgo

    vk6zgo Active Member

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    The distortion,both harmonic & intermodulation,will increase,but strangely enough,on speech and/or program material on a normal AM system,it is not immediately obvious.

    On one occasion,we had a Modulation Monitor sent in for repair & recalibration,because it was reading too high.
    We sent off an exchange unit,& the local guys complained that it was faulty,too.

    The Boss bumped the routine visit forward on the schedule,& on arrival there,we stuck an Oscilloscope on the RF probe,& were
    confronted by massive overmodulation,with carrier cutting.
    We quickly wound the input attenuator down,roughly setting it on time pips,& waited till closedown.

    We found that the local Telephone Techs,who were used to +8 dBm lineup levels,had set the Transmitter input attenuators for 90% mod at this level.
    Program material hits+16dBm!!:eek:
    The funny thing was that this gross overmodulation was not noticeable on casual listening.
  5. mlog

    mlog Member

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    Let me guess. Heavy metal music station.
  6. vk6zgo

    vk6zgo Active Member

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    Nope! Easy listening!:D
  7. mlog

    mlog Member

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    Isn't that elevator music?
  8. JoeJester

    JoeJester AAC Fanatic!

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    no....easy listening is not elevator music, more middle of the road.
    Last edited: Oct 4, 2012
  9. t_n_k

    t_n_k AAC Fanatic!

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    You may or may not observe any distortion - depends on the demodulation method.
  10. crutschow

    crutschow AAC Fanatic!

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    Over-modulation may cause higher harmonics to appear in the RF spectrum. That would be largely rejected by the bandpass filter in the receiver but would cause "splatter" noise in any adjacent channels. See this for a demo.
  11. vk6zgo

    vk6zgo Active Member

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    On a similar subject,Newbie "Hams" sometimes buy cheap CB-type "linear" RF amps on eBay.

    More knowledgeable Amateur Operators advise them:-

    "Dont use them,as they are horribly non linear!"

    Their answer is usually:-

    "But I used it already on 10m & got good reception reports!"

    Other spectrum users are far less happy!:D

    Of course,this isn't anything to do with overmodulation,but it demonstrates how difficult it is for a Receiver operator to discern quite severe distortion by ear.
    Last edited: Oct 5, 2012

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