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Multisim simulation problem with RF

Discussion in 'Electronic Basics' started by Blue News, Feb 16, 2004.

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  1. Blue News

    Blue News Guest

    Hi All,
    Wonder if anyone could explain to me why I am unable to simulate a simple
    (!) RF circuit using Multisim pro 2001. in fact it must be a fundamental
    thing as I have tried it with another simulator and had the same results.
    here is the link to the circuit:
    http://www.geocities.com/x_file_space/fmt-1.gif
    I have also tried the version with a resistor from the base of Q2 to ground
    without joy - does not seem to want to oscillate. some one once mentioned
    something about having to give it an initial condition first...maybe an
    initial pulse or something?
    Would that also apply to a crystal osc similar to the above?

    Thanks very much
    Sean
     
  2. You may have to kick start the circuit. The simplest way to do this is
    to make the power supply voltage a pulse with a long on time. You also
    may need to make sure that the minimum time step is small enough. Most
    oscillators, in my experience can usually start on their own. Al the
    ones in my Demo SuperSpice don't need a kick start.

    Try playing a bit with the values. Off hand, that 0.1uh looks a little
    low. What frequency is it supposed to oscillate at? The bias is also a
    bit suspect. There is approximately OV across the collector emitter of
    Q2. Try putting a 10k from the base of Q2 to gnd.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"
    http://www.anasoft.co.uk/replicators/index.html

    "quotes with no meaning, are meaningless" - Kevin Aylward.
     
  3. I knocked up the oscillator bit, as is, in SuperSpice and it oscillated
    first time without a kick start, although quite a long start up time (t
    max step=1n, Ctune=10p). With a pulse turn on (tr=1ns, ton=large)it
    started to settle down after about 500ns.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"
    http://www.anasoft.co.uk/replicators/index.html

    "quotes with no meaning, are meaningless" - Kevin Aylward.
     
  4. How about letting us see the circuit please Kevin?
     
  5. http://www.anasoft.co.uk/osc.gif - circuit
    http://www.anasoft.co.uk/oscwave.gif - Waveform
    http://www.anasoft.co.uk/osc.zip - SuperSpice schematic file


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"
    http://www.anasoft.co.uk/replicators/index.html

    "quotes with no meaning, are meaningless" - Kevin Aylward.
     
  6. Jamie

    Jamie Guest

    i think i got into this before with some one else.
    that circuit is alittle flawed.
    i am sure that it will osc once you get it going.
    from what i can see the type of osc should be a
    colpitts how ever. there is a slight error if that is the
    case.
    in order for an osc to start there must be some miss balance to
    take place, this can be done by having a T section of capacitors coupling
    the inductor. the center point is then looped back to the emitter.
    most circuits usually have 1 cap aprox twice the value in the coupled
    net work.. but still can be done with equal values.

    http://www.mitedu.freeserve.co.uk/Analysis/colfr.htm

    go there , you can see the second stage of what your trying to do.
    you will notice the fee back into the emitter and biasing from the
    audio pre amp to shift the frequency.
    P.S.
    also consider the transistor your using, it may not be sufficient for
    the frequency your trying for.
     
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