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vcxo bandwidth

Discussion in 'Electronic Design' started by Stefan Simion, Aug 21, 2004.

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  1. Hello,

    I would some help to understand why the VCXOs "in general" have a relatively
    low
    modulation bandwidth, 10 to 30 kHz at most? I would like to know if there
    is
    a fundamental reason for this, or is it that for some practical reason, the
    BW is
    limited on purpose?

    Thanks,
    S. Simion
     
  2. Rich Grise

    Rich Grise Guest

    It's because of the nature of the crystal oscillator, so, I guess you could
    say there's a "fundamental" reason for it, nyuk nyuk. :) The crystal has a
    natural resonant frequency. You can change the frequency by adding some
    kind of load that will literally slow down its vibration, but not by
    much, electrically.

    Think of adding a weight out at the end of a tuning fork. Adding capacitance
    to change the resonant frequency is like adding a 1g weight to a 100g fork,
    so to speak.

    Hope This Helps!
    Rich
     
  3. Ken Smith

    Ken Smith Guest

    I think you'll find that this is a case of both being true.

    If you attempt to change the frequency of oscillations too quickly the
    amplitude of oscillation changes and the frequency doesn't follow the
    intended change. This is because the Q of the crystal is very high and
    the amplifier's coupling to that high Q system is loose.

    If high frequencies were allowed to go into the variactor diode, the
    oscillator would tend to lock to harmonics of signals on that input.
     
  4. OK, thanks. I think I really meant "small signal" BW, i.e. small
    frequency changes. It is probably better to say "phase modulation"
    of the XO? Is it possible that, in addition, the modulation BW
    which can be achieved may also have to do with the proximity of
    other (unwanted) modes of the xtal?
     
  5. Andrew Holme

    Andrew Holme Guest

    It is fundamental. You simply cannot pull a crystal very far. If you
    get your oscillator working way off frequency, it is because the
    external LC elements are now controlling the frequency instead of the
    crystal. The stability and Q will be poor. The crystal may be
    contributing some reactance to the circuit but it will not be
    resonating.
     
  6. "Stefan Simion" ...
    The crystal is a resonator with very high Q.
    Bandwidth is Fosc / Q, thus with a very high Q you get low bandwidth...

    Example: 30 MHz / 10000 = 3 kHz

    Regards,
    Arie de Muynck
     
  7. Ken Smith

    Ken Smith Guest

    In general the further you pull a crystal the lower the Q gets. At some
    point the crystal may as well be left out of the circuit.

    If there is a nearby mode, it may hope to it or it may get very noisy.
    The noisy case is partly because the phase vs frequency characteristic
    gets flattened, partly because noise at the other mode gets through and
    partly because the system can have a multivalued responce.
     
  8. ddwyer

    ddwyer Guest

    A cooperative crystal manufacturer can design with reduced spurious
    modes.
    Such crystals are necessary for filter design.
    Fundamental crystals pull 9 times better than 3rd overtone.
    Large diameter fundamental crystals pull further than small.
    therefore large diameter with smaller than normal electrodes are
    required.
    Some suggest that paralleling crystals reduces motional inductance and
    increases pulling.
    SAW resonators pull further can be run at higher power and can be
    divided down.
    ..
     
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