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How does this oscillator work?

Discussion in 'Electronic Basics' started by greg, Aug 11, 2009.

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  1. greg

    greg Guest

    I have come across a design for an oscillator that, boiled
    down to its essentials, has a configuration like this:

    ,--- VCC
    |
    .-.
    | |
    330 | |
    '-'
    |
    |
    .----UUU----o
    | |
    | |
    --- |
    --- |
    | o--- Output
    | |
    | 22k |
    | ___ |
    o---|___|---o
    | |
    | |
    | |/
    '---------|
    |>
    |
    |
    '--- GND

    But I can't see how it's supposed to work. The series LC
    network from collector to base looks like it should be
    providing negative feedback rather than positive!

    (The full circuit is shown below, in case it makes any
    difference. The frequency-determining capacitor is actually
    a pair of varicap diodes used for frequency control.)

    ,--- VCC
    |
    |
    C
    C
    C
    |
    |
    |
    .-.
    | |
    330 | |
    390p '-'
    |
    || |
    ,-----o----||-----UUU----o
    | | || |
    | V |
    | - |
    47k | - |
    ___ | | o--- Output
    Control ---|___|----)-----o |
    | | 22k |
    | - ___ |
    | - ---|___|---o
    | ^ | |
    .-. | | |
    | | | | |/
    47k | | '------o---------|
    '-' |>
    | |
    | |
    '------------------------o--- GND
     
  2. It looks like a Pierce oscillator I can't recall ever seeing one
    with an LC circuit, but it sure is common as a crystal oscillator
    (the crystal has the capacitance and inductance in series). The
    crystal provies the 180degree phase shift.

    Michael
     
  3. Bob Eld

    Bob Eld Guest

    I don't think it is a Pierce circuit because in a Pierce, the resonator
    (xtal) is between the collector and base providing the required addional 180
    deg of phase shift. This circit has nothing but capacitance in that
    position. So, where does the required phase shift come from? It looks
    dubious to me, there must be more to it. Maybe the thing marked "UUU" is
    supposed to be an xtal?
     
  4. He said the UUU was a coil, which is where I got the idea that it's an
    inductor.

    But after I posted, I wondered if it was a real circuit, or to represent
    a circuit with a crystal. Like I said, I don't recall seeing a Pierce
    oscillator with an LC network, but if someone was trying to explain
    the circuit, they might represent the crystal as a coil and capacitor
    in series.

    Michael
     
  5. This looks similar to an adjustable crystal oscillator:
    http://www.patents.com/Fully-integrated-adjustable-oscillator-a-crystal/US4827226/en-US/

    But perhaps the component in series with the collector, marked CCC, is also
    a choke, and coupled to the one shown as UUU, which would make it something
    like an Armstrong oscillator.

    Paul
     
  6. greg

    greg Guest

    It's a real circuit, and there's definitely no crystal
    in it anywhere.

    The circuit is from a Mitel Semiconductor application note
    titled "A Low Cost 1.5 to 2.2GHz Voltage Controlled
    Oscillator":

    http://www.g1sle.com/files/downloads/sp5070.pdf

    My current theory is that it's actually a Colpitts, and
    the feedback is provided by the collector-emitter and
    base-emitter capacitances of the transistor (and maybe
    any other stray capacitances that happen to be floating
    around).

    At those frequencies, it wouldn't be surprising if
    such parasitic elements formed a vital part of the
    circuit. The "coil" is actually specified as 0.5cm
    of component lead!
     
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