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AM/FM on one carrier

Discussion in 'Electronic Design' started by [email protected], Jun 13, 2005.

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  1. Guest

    Hi All,

    Given that I have a carrier which I can AM modulate and FM modulate, AM
    is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
    are different signals BW limited to 2.5KHz.

    On the other end is a AM only receiver with 10KHz front-end
    and an narrowband FM only receiver tuned to the carrier frequency.

    Will the receivers be able to demodulate with the other modulation
    riding the carrier?

    Any comments?


  2. More interesting questions would be "How much of the
    AM signal will the FM demodulator output have?" and
    vice-versa. The FM demodulator should do well if you
    have enough signal to produce hard limiting before the
    detector. The AM demodulator may have some trouble
    depending on how flat the AM frequency response is.
    By "10KHz front-end", do you mean 10 KHz of AM
    response, or a 10 KHz RF bandwidth? If it's the latter,
    plan on seeing FM signal from your AM demodulator.
    You're welcome.
  3. John  Larkin

    John Larkin Guest

    Not very well.

  4. Guest

    Andy writes:

    I suggest you use a simulator such as PSPICE to superimpose
    an AM signal and FM signal with different modulation
    frequencies and see for youself how the amplitude envelope
    of the composite signal changes.... That will give you an indication
    of how well the AM demodulator will respond to only the
    AM signal.....

    Then build a jperfect limiter and simple HI-Q tank to use as an FM
    (easy to do since you can specify damn near infinite Qs) and
    see how the amplitude output of the tank changes, which will be
    an indication of the independence of the FM signal.

    This , in my opinion, is one of the ways in which PSPICE (and
    other simulators) can shine. You can model almost any modulation
    and demodulation, and, using a transient simulation, see what

    My gut feel is that the two signals will not be demodulated
    independently.... The superposition of two independent signals will
    probably result in a mutually generated amplitude composite as
    well as a mutually generated phase composite..... It is easier to
    visualize if both modulating signals are exactly the same.... However,
    my gut feel has failed me before, and , if I were really curious, I
    would use the simulator.....

    Good luck... when you find the answer, why not post it back here?

  5. Joerg

    Joerg Guest

    Hello John,
    True. Especially since real-world IF filters will have a finite slope
    and the passband won't be level. Unless you spend big bucks on a crystal

    A much better solution would be FM with a subcarrier. This used to be
    done on FM radio for distributing background music for stores. I don't
    know if they still do that. Another option is dual sideband or multiplex.

    Regards, Joerg
  6. Joerg

    Joerg Guest

    Hello Andy,
    But resist the temptation to place an ideal filter. That's the danger of
    Spice. It can show you a rose garden and then on the breadboard it's all

    Regards, Joerg
  7. Thomas Magma

    Thomas Magma Guest

    Is this digital or analog modulation? 100% AM modulation would imply that no
    power is being emitted for a certain percentage of the time. Therefore you
    would lose FM information. FM data could get lost if it was a digital system
    or FM analog could sound crappy if it was audio. But it real depends on the
    type of information that is being transmitted and how you are mixing the two

  8. John Fields

    John Fields Guest

    Since the amplitude of the carrier will be varying because of the AM
    and there will be a constant amplitude offset due to the FM, I think
    the envelope detection used to demodulate the AM signal will be

    However, since the AM will generate sidebands which will vary in
    frequency and mix (?) with the deviating FM, I think demodulating the
    FM will be difficult to impossible.
  9. colin

    colin Guest

    I beleive I heard of something like this to do with stero AM.

    you can sometimes pick up AM transmissions on FM receivers but quite
    faintly, either the broadcast isnt pure AM or the receiver isnt so good at
    rejecting AM.

    Colin =^.^=
  10. John Fields

    John Fields Guest

    The broadcast _can't_ be pure AM because the process of modulation
    creates sidebands which deviate from the carrier center frequency by
    the frequency of the modulating waveform.

    That is, for a 1MHz carrier being modulated by 1000Hz, three signals
    will be broadcast: f1, the 1MHz carrier, f1 + f2, the carrier plus the
    1000Hz modulating signal (the 1.001MHz upper sideband) and f1 - f2,
    the carrier minus the 1000Hz modulating signal (the 0.999MHz lower
  11. Single sideband is effectively simultaneous amplitude and phase
    modulation. Frequency is the derivative of phase. Amplitude and phase
    represent two degrees of freedom. In theory, one should be able to
    resolve the two degrees with the appropriate method of synchronous
    detection. However, practical implementation may be more difficult.

    The familiar cartesian (rectangular) form of "I" and "Q" implemented in
    many RX'ers contains all the information, but in the wrong form. You
    are asking for the polar form of "A/theta" where A is the amplitude
    portion and theta is the integrated frequency. These have the obvious
    mathematical transformation and relationship. You need to implement
    "A/theta" rather than IQ, although A/theta can be derived from IQ in
    principle. Also, the large carrier in "regular" AM represents a DC
    offset. You'll need to get rid of this DC if you transmit the large

    I doubt envelope detection will work for extracting only the AM portion
    (I suspect some form of synchronous detection is required), although
    limiting and ratio detecting for the FM portion may work to some extent.

    I think this is possible but tricky. The RX'er will have some
    complexity, so what is the advantage over something like SSB, which can
    carry two channels in the same total bandwidth?
  12. John  Larkin

    John Larkin Guest

    I was thinking that, in the frequency domain, the AM has lots of
    sidebands, and the FM has, well, lots of sidebands, and they're going
    to get a lot more tangled than they simplistically appear.

    Consider the case where the AM is at 100% modulation... there's no FM
    signal at the negative peaks of the AM mod! That can't be good.
    Similarly, modulated FM adds sidebands that the AM detector will


  13. Not exactly fitting your example but-

    a) IIRC VOR navigation systems use a signal that has both AM & FM

    b0 Some M/W radio relay links use a QAM modulated carrier which is
    FM'd to (cheaply & easily) implement the site to site orderwire voice
    & data streams. The QAM will be in the order of megabits/second
    while the ordedwire only has a modulation bandwith of a few 10s of
    kilohertz, it's so "slow" it doesn't affect the QAM to any major

  14. Andy

    Andy Guest

    Andy replies:

    Absolutely ! One has to use realistic models and
    simulations to correctly model real world circuits. That
    comes with experience. And mistakes....

    But my understanding of a LOT of theoretical stuff and
    mathematical stuff exploded when I was able to feed a
    perfect waveform thru a perfect circuit and see exactly
    what happened, as the purely mathematical treatment which
    was taught to all of us back in school predicted...

    I also found that a lot of stuff that I thought I
    understood just wasn't so..... and became the better
    man for it....

    As time went by, I started doing this more and more and
    simulators and models got better and better...... I still
    don't trust predictors of things like third order intermod
    or compression points, tho some people do. I guess they have
    either done a better modelling job or have better transistors
    to use (grin). But it is still fun as hell to do....

    When I retired, I made damn sure I had my student copy
    of PSPICE running at home......

  15. Andy

    Andy Guest

    Andy writes:

    That's a good point, Howard, --- I had forgotton about the VOR

    That system has a reference signal which is a 9960 AM 30% subcarrier .
    9960 is then frequency modulated at 30 hertz with a deviation of 480

    The "variable" signal is produced with a rotating antenna. The
    VOR signal is sent to a cardiod radiation pattern that rotates at 30
    A receiver located in space would pick up a 30% AM component of 30 hz
    the radiation pattern passes round....

    So, the receiver gets a 30 hz AM signal which is a function of it's
    relative to the transmitting station, and a reference signal which,
    demodulated in an FM demodulator, goes thru zero when the antenna
    is pointed North ( or east, .. I forget which..)

    The phase of the ref 30 hz is then compared to the phase of the
    variable 30 hz
    to determine the bearing to, or from, the receiver to the

    And, this stuff passes thru the receiver and the AM and FM are
    without crosstalk. So, maybe pdru's idea has a chance after all......

    I hope he tries it out and feeds it back to us.....


    PS In my youth, I designed VOR/ILS nav systems for Bendix Avionics in
    Ft Lauderdale...... It was fun as hell.....
  16. Joerg

    Joerg Guest

    Hello Andy,
    Try Linear's LT Spice. It is quite good and free.

    Regards, Joerg
  17. John Perry

    John Perry Guest

    In fact, FM stereo is a subcarrier scheme. The FM modulated channel
    carrier consists of a single (Left + Right) signal; the AM modulated
    38Khz subcarrier consists of (Left - Right).

    If you're in FM mono mode, you get only the demodulated FM sum (Left +
    Right) signal (this is rather uncommon now, except when the channel
    signal is too weak to get a good subcarrier signal). This was invented
    for compatibility with existing mono FM channels, to avoid obsoleting
    all existing mono FM radios, and promote acceptance of stereo by
    broadcasters, who were understandably nervous about adopting a new
    signal no existing listeners could use :).

    If you're in FM stereo mode, both signals are demodulated, and the
    difference signal is split and one copy inverted. The left channel is
    (Left + Right) + (Left - Right), and the right channel is (Left + Right)
    - (Left - Right).

    There's also a subcarrier above 38KHz (57KHz?) for distributing canned
    elevator style music. (Wasn't this MUZAK?)

    John Perry
  18. Don Bowey

    Don Bowey Guest

    This is not correct. In all AM modes that do not use a form of suppressed
    carrier (such as ssb), the carrier is constant. !00% modulation only says
    that the combined power of the sidebands is 50% as much as the power of the

  19. Don Bowey

    Don Bowey Guest

    Tjhat is not correct. The carrier amplitude is constant with modulation.


  20. If the carrier amplitude is constant, it is not being amplitude modulated.
    This is so obvious as to be tautological. "AM" = "amplitude modulation".
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