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Radio and Aliasing

Discussion in 'Electronic Basics' started by Radium, Sep 8, 2006.

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  1. Radium

    Radium Guest


    Lets say there is an AM station with a carrier frequency of 150 KHz.
    What is the highest frequency of modulation that it can handle?

    In digital audio, the sample rate must be at least 2x the highest
    frequency. What is the equivalent in analog AM radio?


  2. Bob Myers

    Bob Myers Guest

    Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
    transmitter; I'm talking about the legal limits on radio
    stations in the AM broadcast band, 535 - 1705 kHz).

    In theory, you could get to 150 kHz, and no higher.
    This gets to your second question,
    Amplitude modulation can actually be viewed as very
    analogous to sampling, but the simple answer is that when
    you perform this sort of modulation, you wind up with
    "sidebands" which extend above and below the carrier
    frequency by the frequency of the modulating signal. In
    other words, if you modulate a carrier with audio in the
    0-10 kHz range, the sidebands will extend to 10 kHz
    above and below the carrier frequency. The complete
    AM signal, then, occupies a bandwidth twice that of the
    modulating signal.

    Bob M.
  3. John Larkin

    John Larkin Guest

    Define "handle."

  4. Radium

    Radium Guest

    Sorry. I meant to ask what is the highest audio frequency that can
    [physically] be broadcasted through a 150 Khz carrier.
  5. Eeyore

    Eeyore Guest

    Errr.......... 150kHz maybe ? Maybe SSB can do better ?

  6. Alan B

    Alan B Guest

    Perhaps you should have read just a wee bit further, in particular, just as
    far as the very next sentence:
  7. G. Schindler

    G. Schindler Guest

    Someone seems to be misreading the question (conceivably, it could be me).

    Because of the reference to sampling I believe the question is meant ask
    the highest modulation frequency that can be transmitted over a 150 kHz
    AM broadcast. The answer here is 75 kHz due to Nyquist Criteria.
    Nyquist Criteria is the reason for limiting the Digital Audio data to
    one half the sampling rate.

    One of the implications of this is that the highest frequency component
    (including distortion and/or harmonics) of the "data" must be less than
    one half the carrier frequency (or modulation frequency) to avoid
    aliasing effects.

    As someone said, however, there are legal limits that should keep one
    from approaching this.

  8. Radium

    Radium Guest

    Isn't Nyquist Criteria for digital data only? AM radio is analog.
    Can aliasing occur in analog AM radio?
  9. Alan B

    Alan B Guest

    The fist part of the question was: "Lets say there is an AM station with a
    carrier frequency of 150 KHz. What is the highest frequency of modulation
    that it can handle?"

    With an theoretically allotted bandwidth of 300kHz (lower and upper
    sidebands), I can modulate a tone of 150kHz. Now, this has no relationship
    to Nyquist, but simply to how high a tone can be transmitted inside an
    allotted bandwidth. Even so, the answer is still half the bandwidth, since
    the modulated tone will appear both above and below the carrier. Then
    eliminate the carrier and the upper sideband and we run into some physical
    constraints, such as: a 150kHz carrier with a lower sideband modulation of
    150kHz will be theoretically be broadcasting at zero Hz.

    This brings to mind a question: can upper-sideband modulation exceed the
    lower boundary of lower-sideband modulation?

    The second part of the question was: "In digital audio, the sample rate
    must be at least 2x the highest frequency. What is the equivalent in analog
    AM radio?" My answer, I don't think there is an equivalent. Nyquist is
    used for decoding information from an analog signal by taking samples at
    some fixed rate, and the upper bound of intelligible information is set at
    one-half the sampling rate. I don't believe this has anything to do with
    the capability of a broadcast transmitter to transmit a tone of some
    maximum frequency.

  10. Alan B

    Alan B Guest

  11. jasen

    jasen Guest

    no it's for sampling. and each peak (or valley) of the carrier is a
    sample of the transmitted signal. It has other analogue uses too.
    like switched capacitor filters.
    modulate a 150Khz signal at 200 Khz an there'll be a 50Khz alias generated.

  12. Radium

    Radium Guest

    PAM [Pulse Amplitude Modulation] is used for sampling. AM seems to be
    the "smooth" equivalent of PAM.
    So I take that the the highest pitch that can be transmitted on an
    analog AM 150 Khz station is 150 KHz. Or is it 150/2 khz?

    Does Nyquist apply to AM as well or just PAM?
  13. Don Bowey

    Don Bowey Guest

    Uh.. Nope.

    Regulations aside, the highest modulating frequency you should use with a
    150 kHz carrier is 60 kHz. Anything higher will be garbage.

  14. Don Bowey

    Don Bowey Guest

    In "AM radio" the modulation is not synchronized to the carrier, so 2:1
    isn't adequate. I would say 2.5:1 for carrier to modulation frequency will
    deliver a good output signal.
  15. Don Bowey

    Don Bowey Guest


    Nyquist does apply to AM modulation, and AM is considered a linear system
    where each sideband (when demodulated) is identical to the modulation

    Consider a 100 kHz carrier and a 500 kHz modulation signal.... Between
    successive samples (the carrier frequency), five cycles of the modulation
    signal will be available to be sampled. Since four will be unsampled, one
    cannot say the process is linear, failing the basic requirement for AM. Use
    40 kHz for the modulation, and the result will be linear.

  16. G. Schindler

    G. Schindler Guest

    Right ... Aliasing will happen anytime the audio signal exceeds 1/2 the
    carrier frequency so it would also happen for 100 kHz audio on a 150 kHz
  17. G. Schindler

    G. Schindler Guest

    Nyquist applies here too and you could visualize this by trying it on
    paper. Draw out a sine wave and try to modulate it with a signal of
    it's own frequency. Remember that AM impresses the information on the
    "envelope" of the carrier peaks.

    These two links do a pretty good job of helping you visualize the
    process but unfortunately neither allows you to visualize what happens
    when you violate Nyquist Criteria. I'll keep looking.
  18. John Fields

    John Fields Guest

  19. John Fields

    John Fields Guest

  20. G. Schindler

    G. Schindler Guest

    Yes, and no (I think) .... If we were sending a single frequency tone,
    anything greater than 2:1 should be adequate. The real rub comes when
    trying to send something like music or voice with multiple time varying
    frequencies. These signals are full of harmonics by their vary nature.

    We normally place a low pass filter in front of our modulator/sampling
    circuit circuit to eliminate alias causing frequencies. Unfortunately,
    in the real world filters are not perfect and we have to select our
    filter so that any alias causing signal is reduced to insignificance.
    Because of this we will have to choose a filter frequency that is
    substantially lower than the Nyquist frequency (Nyquist Frequency =
    Fc/2). But really, all of this is really a compromise of practicality
    to keep the real ratio above 2:1.

    I don't think synchronization in itself will have any effect except when
    it is indicative of higher frequency components in the modulation signal.
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