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need help with fm circuit study

Discussion in 'Electronic Basics' started by Bruno, Apr 3, 2004.

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  1. Bruno

    Bruno Guest

    The circuit in question is

    http://www.geocities.com/myelectronicsrevision/fmcct1.jpg

    (you may need to copy and paste this link in a new browser
    if you don't get anything)

    The left portion is just an audio amplifier which is easy. The right
    portion, circled in blue, is an fm transimitter. Is this a Hartley
    oscillator. I do understand the modulation bit, which is performed
    by the variable capacitor/diode D, but if we forget the fm
    modulation for a moment, how does the oscillator work, are there
    any books I can find an explanation about such a configuration?
     
  2. Hi,
    The reason, I guess, that you haven't yet received a reply to your
    posting, is that the explanation of how that particular oscillator
    works (in the time domain) is notoriously long-winded. In my relative
    youth, we used to have oral theory exams where you had to stand up and
    explain circuits to the instructor without any aids (not even a piece
    of chalk). The erstwhile multivibrator (of which this circuit is a
    variant) was a favourite of their's and so I don't intend to try here.

    Call me chicken if you like but, unlike Marty in "Back to the
    Future", I don't care. The problem is that you can get a rough-and-
    ready "This is how it works" but the finer detail does get somewhat
    involved. Sufficient to say, look-up multivibrator and push-pull
    circuits, put them both in a poke (take the pig out first though),
    shake and there you are.

    As for books, try an older copy (cheaper and probably better for
    you) of the "ARRL Radio Amateur's Handbook" which is updated every
    year. It isn't filled with FM circuits but should be one of the first
    books on any RF start-up's bookshelf. In the general electronic sense,
    go for Horowitz & Hill's "Art of Electronics", not cheap but worth it.
    Not only that you get to praise, or complain to one, of the writers,
    Winfield Hill, who is a denizen of sci.electronics.design. We are all
    waiting for the 3rd edition but may not live that long :)

    Finally, no it isn't a Hartley. All oscillators that have a tapped
    tuned circuit are not by Mr. Hartley. Notice that C8 decouples the
    centre-tap to ground which cannot then take part in the feedback
    process. The trimmer capacitor TR2 incidently is there to tune out the
    leakage inductance of the antenna coupling coil. The antennas on these
    things are usually short with respect to a quarter wavelength and
    present a capacitive reactance, so TR2 helps resonate that coil with
    the antenna.


    Cheers - Joe
     
  3. Bruno

    Bruno Guest

    The reason, I guess, that you haven't yet received a reply to your
    Normally oscillators are simply amplifiers with an added feedback such
    that A*beta = -1. Is it possible to idetify the input and output in this
    case, and the feedbach path? Is it a differential amplifier (long tailed
    pair?) ?
    The hartley oscillator usually is two coils and in the middle there is
    earth, which is why I was asking. C8 for RF is short circuit, i.e. we
    have earth between the coils, which, anyway, seems a bit redundant
    as the center tap is also connected to the battery, which is also earth
    for ac, but I am sure there is some reason for it...
     
  4. Bruno

    Bruno Guest

    I can see though that this is not a Hartley due to the symmetry of the
    circuit. The split coils should connect output to input, but here I cannot
    see that this is happening.
     
  5. Rich Grise

    Rich Grise Guest

    [Joe McElvenney?]
    You didn't finish reading the instructions. Throw in an astable
    multivibrator coincident with two mirror-imaged Hartley oscillators.

    Does this help?

    Good Luck,
    Rich
     
  6. camediaman

    camediaman Guest

    You didn't finish reading the instructions. Throw in an astable
    which instructions are that ? no I haven't got a clue about mirror
    Hartleys, though I wouldn't mind if you wanted to expand, it does
    sound interesting. (or if you can say from which book you found
    this information)
     
  7. Neil Koozer

    Neil Koozer Guest

    This is a Hartley oscillator.

    The cross-coupled capacitors makes it look like a multivibrator, and the
    push-pull topology tends to obscure the classic Hartley circuit at first
    glance. You can recognize the Hartley if you erase one of the transistors
    and move the AC ground from the emitter to the collector of the remaining
    transistor. Then you can move the coil so that the bottom end is grounded,
    the tap is at the emitter, and the top end goes through the coupling
    capacitor to the base.

    I recommend the ARRL handbook, especially if you can find a vintage one from
    the 60's to augment the current ones.

    Neil
     
  8. The circuit looked familiar, and as something more than a multivibrato
    with a tuned circuit.

    Checking the RSGB "VHF-UHF Manual" from 1972, it sure looks like
    the oscillator in there referred to as the "Kaliatron" oscillator.
    It doesn't really say much about it, though, even though there is
    a sample circuit, tube-based, and an FET-based "GDO".

    I'm sure I've seen a similar circuit, either a variation or just
    under a different name. There was a 1964 issue of QST that had
    an article about phasing SSB ringt on 144MHz, and I have a vague
    feeling that they suggested an oscillator like this (albeit crystal
    controlled), because of stability. But I'm not certain of that and
    can't easily dig out the magazine. It was a less than common oscillator
    circuit.

    Michael
     
  9. Rich Grise

    Rich Grise Guest

    Heh - I just made it up. :) What I meant was two Hartley
    oscillators, with the schematics drawn such that they're
    mirror images of each other, and you put one on top of
    the other so that they share the coil. Coincidentally,
    when you do that, the circuit comes out looking almost
    exactly like an ordinary astable multivibrator, where
    the coil(s) is(are) the collector load(s).

    HTH!
    Rich
     
  10. phil

    phil Guest

    What ?!"£$%$£~~## How can you say that tapped tuned circuits are
    not by Hartley, this is absolute ignorance, open this winfield book you
    are recommending and read about Hartley & Colppitz, this is exactly
    what they are, i.e. tapped tuned circuits.
     
  11. Dbowey

    Dbowey Guest

    Phil posted:

    << What ?!"£$%$£~~## How can you say that tapped tuned circuits are
    not by Hartley, this is absolute ignorance, open this winfield book you
    are recommending and read about Hartley & Colppitz, this is exactly
    what they are, i.e. tapped tuned circuits.
    Don't be such a !!?!"£$%$£~~##. Not ALL circuits that have a tapped coil are
    Hartley oscillators.

    Go read the book again.

    Don
     
  12. phil

    phil Guest

    Don't be such a !!?!"£$%$£~~##. Not ALL circuits that have a tapped coil
    are
    I am not anything mister. The poster said no circuit with tapped coil is by
    Hartley. Hartley is a tapped coil isn't it!
     
  13. Dbowey

    Dbowey Guest

    phil posted (that dbowey posted):



    I am not anything mister. The poster said no circuit with tapped coil is by
    Hartley. Hartley is a tapped coil isn't it! >>

    It has become clear to me that American english is not your native language, so
    I understand your occasional comprehension problem.
    If you will go back and read the poster's comments more carefully, you will see
    that he did not say what it is you have attributed to him.

    It would help if you would post a quote of other poster's comments rather than
    post a paraphrase.

    Just as a guess, is THIS the comment with which you disagree? "...Finally, no
    it isn't a Hartley. All oscillators that have a tapped tuned circuit are not by
    Mr. Hartley...."

    As an aside, it is interesting to note that in your language and in english
    (American and U.K., I believe) "!!?!"£$%$£~~##" has the same meaning, so I hope
    there is no confusion over it.

    Don
     
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