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VCO Design

Discussion in 'Electronic Design' started by redhat, Jun 23, 2005.

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  1. redhat

    redhat Guest

    In VCO design using negative resitance amplifier, i want to design the
    resonator first,should it designed to resonate at the wanted center
    frequency or this frequency will chage when connecting the negative
    resistance amplifier,so, what should i do first? what is the steps of
    design?

    Regards
     
  2. Andrew Holme

    Andrew Holme Guest

    All circuits contain stray capacitance and inductance. The higher you
    go in frequency, the more significant these strays become. Transistors
    have junction capacitance. These strays and parasitic reactances can
    be hard to predict, but they can be minimised, and they should be
    small. It's common practice to include a small trimmer capacitor to
    correct for them. The circuit you posted recently in s.e.b included a
    trimmer. Be aware though: it's hard to make a stable, accurate
    LC-controlled oscillator at frequencies much above 10 MHz.

    An empirical approach is often taken to RF design. Why not make a
    prototype? It could save you a lot of time and you will learn from it.
    Don't try to work out every last detail before soldering two
    components together.
     
  3. redhat

    redhat Guest

    in designing vco with negative resistance analysis , the output
    reactance of the resonator should be equal to the input reactance of
    the reflection amplifier, so the resonant frequency will be the same as
    the resonant frequency of the resonator, am i right? what about the
    value of Q, i have no specific phase noise requirement, what value of Q
    should i take?
     
  4. Andrew Holme

    Andrew Holme Guest

    The magnitude of the negative resistance produced by the amplifier
    needs to exceed the loss resistance of the inductor.

    Any reactance seen looking into the amplifier will appear in series
    with the tuned circuit and will slightly alter its resonant frequency,
    but this should not be a significant proportion of the total reactance.
    Resonant frequency should primarily be governed by the tuned circuit
    components.

    Inductor Q is the ratio of its reactance to loss resistance. You want
    this to be as high as possible. Plucking figures out of the air, for
    an oscillator, with no specific application, I would say 20 is rubbish;
    50 is OK; 100 is good.
     
  5. Andrew Holme

    Andrew Holme Guest

    [snip]

    Colpitts and Clapp circuits can be analysed as negative resistance
    oscillators, and the capacitive divider is shown as part of the
    negative resistance amplifer, even though it is part of the tuned
    circuit. If the dotted line - where tuned circuit meets "reflection
    amplifier" - is drawn to the left of the capacitive divider, there
    would be equal and opposite reactances either side of the line. But
    the oscillator frequency is not the same as the resonant frequency of
    the series LC components to the left alone, because the capacitive
    divider contributes significant reactance.
     
  6. Mark

    Mark Guest

    [snip]
    I am glad to hear you say that. I have always had a problem with the
    concept of the negative resistance connected to a tank circuit. My
    problem is that the transistor by itself cannot create the negative
    resistance, it needs the impedance transformation created by the tank
    circuit to create the negative resistance. So it really is all about
    where you "put the dotted line" as you say. The negative resistance
    may be one concept for the thought process but there is no actual
    physical way that you can disconnect the tank circuit from the
    transistor, and connect a network analyzer, and measure a negative
    resistance. The tank circuit must also be attached to create the
    negative resistance because it creates the impedance transformation.
    So the tank circuit and transistor TOGETHER create the negative
    resistance, but this is just another way of saying that you connect an
    amplifier to a tank circuit and the amplifier makes up for the losses
    of the tank and for the output power so it oscillates. To me, the
    amplifier concept is much more straight forward compared to the
    negative resistance concept.

    Mark
     
  7. redhat

    redhat Guest

    i am going to use this resonator:
    http://www.geocities.com/aezzat3/resonator.jpg
    i have a coil that has Q of about 48 at the specified center frequency
    950MHz, should i find another coil with higher Q to have a high
    effective Q, why does the Q of the coil affects Qe that much? Qe=-92.3
    for QL=100 , Qu=48
     
  8. redhat

    redhat Guest

    i searched for another inductor but the maximum Q that i found for an
    inductor at this frequency is about 70, may be i didn't search well
    ,but if i want to have higher Q what should i do? is there anyway to
    increase the Q of the vco?
     
  9. redhat

    redhat Guest

    i searched for another inductor but the maximum Q that i found for an
    inductor at this frequency is about 70, may be i didn't search well
    ,but if i want to have higher Q is there anyway to increase it? what is
    the meaning of negative Q
     
  10. doug dwyer

    doug dwyer Guest

    A broad band amplifier can be shown as a negative resistance. Start with
    the assumption that there is zero phase across the ampliier i/p to o/p
    if it had a negative resistance of -51r then it would just oscillate a
    series LC with a 50r loss.
    The stability of the oscillator will be the ratio of the Qs ie the
    resonant Q and the rate of change of phase through the amplifier.
     
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