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FM transmitter oscillator question

Discussion in 'Electronic Design' started by shehry, Mar 19, 2007.

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  1. shehry

    shehry Guest

    Hi,

    I was trying to design an FM transmitter and there is one thing that
    is puzzling me somewhat.

    I know that I will have to build an oscillator running at a certain
    frequency. I want to make a BJT based hartley oscillator. I will have
    to give an audio signal as an input to the RF oscillator. Meaning that
    the oscillator will change its frequency. However, the tank circuit in
    the hartley would remain at more or less the same resonating
    frequency: 1/(2*pi*sqrt(LC)). So why:

    1. would the oscillating frequency of the oscillator change in the
    first place.

    2. if it would change then there ought to be a loss in efficiency for
    input frequencies that make the oscillator shift too far from its
    resonant frequency. Am I right in assuming that this loss would be
    negligible since we are RF frequency would be much larger than the
    audio frequency?.

    Also, I have been trying to find some site which would explain hartley
    and colpitts oscillators in detail but I havent been able to...could
    somebody help in this regard.

    thanks...
     
  2. As it turns out, you're in luck. The base-collector capacitance of
    your typical transistor, it varies quite a bit with bias.

    So you can simply feed some audio to the base and you'll get plenty of
    FM.

    The typical circuit circa 1968 for this was a 2N706, LC circuit in the
    collector, a 5pf capacitor from collector to emitter, a 270ohm emitter
    resistor, and a 100K resistor from Base to +. This could be easily
    tucked into a plastic olive
     
  3. The site you seek is called a library. They are made up of "webpages"
    called books, which cover this territory well.

    Michael
     
  4. Andrew Holme

    Andrew Holme Guest

    FM transmitters work by vaying the capacitance, and therefore the resonant
    frequency, with the audio signal. You can do this using a varicap diode, or
    rely on the variation of transistor junction capacitance with bias point, as
    suggested by another poster.
    No, as I've already stated the resonant frequency changes; but you should
    also be aware of the difference between deviation and modulating frequency
    in FM. Deviation of the carrier from its mean frequency depends on the
    amplitude of the modulating audio. The frequency of the audio determines
    how fast the carrier frequency changes NOT the amount of change.
     
  5. There are sevral ways.
    One is to use a varicap, a capacitance diode, that is a diode
    whos capaciance depends on the reverse voltage, all diodes have this more
    or less, as part of your tuned circuit.
    But with many BJTs modulating Uc wil lchange Ccb enough to FM modulate your
    transistor oscillator.
    It can even be as simple as putting a dynamic mike in series:

    L Dynamic mike, about 200 Ohm.
    ___________ =
    | | )________ O_____ +9V
    ===C1 |C2 )
    | === )
    |--R1 ---|-----
    | |
    | c
    --------b BC107
    e
    |------------------- 0V
    ///

    C1 = 10pF
    C2 = tol trimmer
    L is 7 turns on 8mm diameter tube,
    tapped somewhere so your feedback is in the right phase.
    R1 is 470k or so.

    Design from end of fifties....
    Worked every time, connect short wire to collector as antenne.
     
  6. Rich Grise

    Rich Grise Guest

    Better ask the teacher.

    Do you know what gives this away as homework? Well, I'll tell you:

    Why do you "want to make a BJT based hartley oscillator", if you don't
    even know what a hartley oscillator is yet? ?:-/

    Good Luck!
    Rich
     
  7. I don't know. Lots of times people just know they have the solution and
    need a little help, when the reality is they don't have a clue and if
    they started from that premise they'd get better answers.

    Just like that question in sci.electronics.basics the other day, when
    someone wanted to know about germanium transistors, without any real
    clue that their time is long in the past.

    Yes, the poster is a beginner, because they'd not make the leap to knowing
    they need a hartley without knowing anything about that type of oscillator.
    But that doesn't make it a homework question.

    Even if it is, you do have to give them credit, since most homework questions
    are immediately recognizable because someone just types in the actual
    question. This one is (if it's homework) masked by someone at least putting
    the question into their own words.

    Michael
     
  8. Jamie

    Jamie Guest

    the old Capacitance between the Base and emitter trick.

    capacitance in a transistor plays a roll in the resonant
    circuit.
    By altering the current flow in the transistor (especially the Ibe)
    the capacitance changes.
     
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