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generating sine wave

Discussion in 'Electronic Design' started by pinku, Aug 9, 2006.

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  1. pinku

    pinku Guest

    Is there any simple method to generate a sine wave of frequency 1MHZ by
    using LM324
    op-amp/by using filters /by using oscillators (please mention the
    specifications and schematic diagrams).
    please reply me with in 24 hours(urgent).
     
  2. Eeyore

    Eeyore Guest

    Homework or business ?

    1 MHz is a bit fast for an LM324 btw ! Would you like to consider another device
    ?

    My hourly rate is $80 btw.

    Graham
     
  3. And payment term 24 hours ;)
     
  4. Guest

    The answer is no, whch isn't going to help get your homework done.

    If you look at the data sheet for the LM324

    http://www.national.com/ds/LM/LM124.pdf

    and check out page 7, you will see that amplfier runs out of small
    signal gain at about 1MHz (top of page, left hand side) while the large
    signal output swing falls below 1V at 100kHz.

    If your instructor was silly enough to specify an LM324 as the op amp
    to use for this job you've got an easy answer to the question.
    Otherwise you are going to have to start out by finding a faster
    amplifier.

    How much distortion in the sine wave can you accept? What sort of
    amplitude do you need? Does the frequency have to be exactly 1MHz? If
    so, tell us what sort of range frequencies would count as "exact" -
    +/-10Hz?

    How stable do amplittude and frequency have to be?

    In the meantime, googling on Wien bridge and phase-shift oscillators
    might give you some ideas.

    http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/oscphas.html

    http://www.ecircuitcenter.com/Circuits/opwien/opwien.htm

    The circuit given above for the Wien bridge will produce a rather
    distorted sine wave - there are better versions of the Wien bridge, See
    Jim Williams Linear Technology application note AN43 on bridge circuits
    - the first half of the application notes talks about Wheatstone
    bridges, but the second half gets onto Wien bridges.
     
  5. If you have around >12V supply a quick and dirty way of
    getting a sinewave in the low MHz-region is with a
    tuned cross-coupled long tail pair.

    Vs-------------------+
    |
    +--/////---+---/////--+
    | --------L------- |
    | C |
    +----------||---------+
    | |
    +--------/\/\/--------+
    | RLoad |
    +---------, ,--------+
    | \/ |
    \| /\ |/
    npn|--/\/\-' '-/\/\--|npn
    e/| R R |\e
    | |
    +----------+-----------+
    |
    \
    /Rtail
    \
    |
    0v------------------+

    L is the centre-tapped primary of a transformer and can be
    wound on about a 10mm ferrite toroid. Rload can be coupled
    directly as shown, or via a secondary winding.

    L+C form a tuned circuit, resonant at 1MHz.

    R should be about 10k.

    The load on L+C is Rload and 2R in parallel (more or less).

    Make the impedance of the C about 1/10 of the effective
    collector-collector load then calculate the value of L for
    resonance.

    Assume a voltage across Rtail of about 1/3 to 1/2 of Vs.
    Start off with a tail current of around 1-2mA dc.

    A bit quick and dirty, but it oscillates quite readily
    and is simple to breadboard.
     
  6. budgie

    budgie Guest

    at least that roll-off helps subdue harmonics.
     
  7. Phil Hobbs

    Phil Hobbs Guest

    I don't know that that's true. Given that you can make a Colpitts
    oscillator out of an emitter follower plus an impedance transforming
    network, it might be sort of fun to do the same with a 324. I bet you
    could make a 1 MHz oscillator out of one by using a pi network between
    the output and input. Not a very _good_ sine wave oscillator, but all
    you need is power gain, not voltage gain.

    Cheers,

    Phil Hobbs
     
  8. TuT

    TuT Guest

    The big problem with the 324 is its slew rate, (about 0.5V/us). At 1MHz,
    this would limit the output amplitude to about 160mV p-p for a sinusoidal
    waveform.
     
  9. One might as well go to a single transistor, which of course works
    fine for the purpose.

    This is once again an example of someone stuck in low frequency
    mentality trying to extend it to radio frequencies. Time after
    time, we see people needing an oscillator at 1MHz or so, and want
    to know why their op-amp or function generator IC doesn't work very
    well up there. Of course, it's because they are hitting the top
    of the frequency range for the device, and while the range may
    include 1MHz, that doesn't mean the IC designer every intended good
    operation up there.

    Yet, they are trained in op-amp and IC solutions, so they can't see
    that long before op-amps came along, such an oscillator would be
    made with a single active element, tubes in the past, a transistor
    in recent decades. The 324 is a stretch at 1MHz, but it's close to
    DC for building an LC oscillator with a single active element. It's
    all perspective.

    Michael
     
  10. Rich Grise

    Rich Grise Guest

    With an LM324, no. According to the data sheet:
    http://www.national.com/ds/LM/LM124.pdf ,
    At 1 MHZ, its gain is unity, so it will not oscillate.
    (well, maybe just a little, but it'd be a nightmare trying to get it
    stable.)

    Sorry,
    Rich
     
  11. Ken Smith

    Ken Smith Guest

    I haven't tried this but here's an idea to get around that.

    Use only one output of the LM324 and make a Colpitts common collector
    oscillator by placing an impedance in the ground line of the package.
    IIRC the compenation of the LM324 controls the slew rate of the output vs
    the ground pin.
     
  12. Fred Bartoli

    Fred Bartoli Guest


    Hehe...
    I thought about boostrapping the supplies, but this one is a nice weird
    idea.
     
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