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Above 108MHz with FM radio (or other)?

Discussion in 'Electronic Basics' started by Michael J., Dec 10, 2004.

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  1. Clarence

    Clarence Guest

     
  2. John Fields

    John Fields Guest

     
  3. Bob Myers

    Bob Myers Guest

    A slight nit to pick here, John; there's "mixing" in the sense
    that you use it here, and then there's modulation based on
    multiplication, which may be something else entirely. A
    "mixer", meaning the specific sort of circuit that this term is
    generally applied to, does preserve the original frequencies.
    However, a "balanced mixer" does not, and the difference is
    that the latter actually DOES perform an operation that is
    solely a multiplication. In mathematical terms:

    sin(A) * sin(B) = 1/2[cos(A-B) - cos(A+B)]

    which clearly doesn't have the original frequencies in the output.
    So pure multiplication results in DSBSC, which is then often
    converted to SSB by filtering out one of the sidebands.
    To get the original carrier back, a constant term has to be introduced,
    i.e.,

    sin(A) * [K + sin(B)]

    Here, the K term corresponds to the DC offset which exists
    when , for instance, the modulating signal (here, sin(B)) is applied
    to the carrier (sin (A)) via a modulation transformer (classically,
    in the "plate" circuit of a class C output stage), as is the case very
    commonly in commercial AM installations. The result is
    conventional "AM", with a "full" carrier.

    Just to clarify that for those who may be trying to follow the
    terminology, since this IS sci.electronics.basics after all. I
    know YOU know this stuff already...

    Bob M.
     
  4. Steve Evans

    Steve Evans Guest

    whkatever.. i'm so happy for you.. it still doesn't alther the fact
    that the majoritiy of his remarks I found on Google were simply
    disparaging one-liners often made without the slightest provokation.
     
  5. Steve Evans

    Steve Evans Guest

    sorry, roger, but I think Bob Myers is the only person here to have
    got it absolutely right.
     

  6. Even a double balanced mixer cannot completely remove the two
    frequencies we start with, only lower their level a lot.

    In this case there is no need for that level of detail. The OP just
    wanted a simple way to tune in the air band on a common FM radio.

    If he had asked for a very high quality communications receiver class
    solution there would have been a need to talk about double balanced
    mixers.
     
  7. It's the "disparaging" and "without the slightest provocation" parts that I'd
    probably not see eye to eye with you, about. When I first wrote my question
    (and you can check google) on these phone indicators that I wanted to operate
    without a battery, his reply wasn't exactly sugar and honey -- but it was
    exactly the kind of information I didn't have and needed to hear about:
    He made the job really hard for me to properly achieve. But then, learning to
    get the job done well and right, even for a hobbyist, isn't a bad thing. One
    grows from it and learns better why the standards *are* standards, too. In the
    end, I developed a circuit that presents 20M continuous (it doesn't present
    different momentary loads) to the phone line and still provides me with an LED
    indicator. Don's challenge to me was from someone who knew the material and I
    worked to meet it.

    I think Don is brusk when people start asking about pirate broadcasting or else
    receiving broadcasts on bands where they aren't supposed to (for experimental
    purposes when you are licensed it is one thing, but for general tapping in it is
    another), when it looks even something like that's what the question is about.
    He pushes those considering such ideas to find another avenue, just as I'm
    certain he was pressing me to not improperly hook up equipment to my phone lines
    that was ignorant of the intended use and might otherwise interfere with it.

    I can respect that.

    In any case, he's helped at least one person I know of. Me.

    Jon
     
  8. Steve Evans

    Steve Evans Guest

    thats fair enouugh, Johathan. You gotta speak as you find.
     
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