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Modulate HF onto LF?

Discussion in 'Electronic Design' started by Klaus Jensen, Nov 30, 2006.

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  1. Klaus Jensen

    Klaus Jensen Guest

    I found an obscure reference which reads as follows. There is no
    further information given.

    "One person with whom I discussed this said you can use a lower
    frequency carrier than the modulation. You can view the resulting
    signal via a 'constellation diagram' such as used in quadrature
    amplitude modulation and other techniques associated with modems, FSK
    and PSk radio."

    Is this bunk, or would someone care to venture how this might be

    I don't think they are referring to something as simple as
    superimposing a HF ripple on the LF. It seems to equate to full AM of
    lower frequency by a higher one, instead of the normal way around.

    Awaiting your invocations.

    Klaus Jensen
  2. Guest

    This is might confusing verbage. Are you questioning if you can use a
    low side mixer instead of a high side mixer? If that is the question,
    then the answer is yes, but the spectrum is reversed. The effects for
    data can be compensated.
  3. Modulation of two signals is like multiplication. If C= AxB then C=BxA

    The result will be identical, main signal components at B +/- C which
    also = C +/- A (overly simplified). But the higher frequency carrier
    will have sidebands of the lower carrier just the same.

  4. J.A. Legris

    J.A. Legris Guest

    See "Product-to-Sum Formulas" at

    For example, take a couple of sine waves that are functions of time and
    angular frequencies u & v. If we multiply them:

    sin(ut) sin(vt) = 1/2 [(cos((u-v)t) - cos((u+v)t)]

    The sum and difference frequencies are immediately apparent.

    In the special case of AM, where the modulating signal, u, never goes
    negative, we get:

    (1/2 sin (ut) +1) sin(vt) = 1/4 [(cos((u-v)t) - cos((u+v)t)] + sin(vt)

    The first term is the sum and difference frequencies and the second
    term is the carrier. For example, if the carrier frequency is 100 and
    the modulation frquency is 10 we get the usual situation where the
    powerful carrier at 100 is flanked by a pair of weaker sidebands at 90
    and 110.

    But if the carrier frequency is 10 and the modulation frquency is 100
    we get a powerful carrier at 10 while the weaker "sidebands" are still
    at 90 and 110.
  5. It sorta sounds like he was asking if you could put a wide bandwidth
    signal onto a narrower one by downconverting. Thus reducing the channel
    width for a given amount of transferred info.

    The answer is that yes you can, but you totally demolish the info
    content in the process by unavoidable aliasing. There is no free lunch.

    Alternately, he might have been talking about plain old trellis modulation.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
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