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Crystal oscillator start-up times

Discussion in 'General Electronics Discussion' started by zirissjk, Feb 12, 2013.

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  1. zirissjk

    zirissjk

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    Feb 12, 2013
    Hi,

    I'm new to electronics forums and only a beginner circuit builder. I'm looking to find the start-up times of various crystal oscillators primarily OCXO stratum class. I know it is difficult information to find due to the random start nature of crystal oscillators but still need to find min/max start times and was wondering if anyone here can point me in the right direction. I've spent hours and hours google searching with no luck. If anyone can help it would be greatly appreciated.
     
  2. john monks

    john monks

    693
    2
    Mar 9, 2012
    This is a typical diferential equation problem. It is a damped harmonic problem and I refuse to work it out because I have already rightfully accused of doing peoples homework for them.

    I can tell you in short that it is a function of the inverse of the band width. Generally crystals have a narrow band width or Q so generally the turn on and turn off times are much longer than a LCR circuit.
     
    Last edited: Feb 12, 2013
  3. zirissjk

    zirissjk

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    0
    Feb 12, 2013
    Thank you for the quick reply. I'm more of a hobbyist new to the field. I'm playing around with time of flight circuity systems and am trying to broaden my understanding on oscillator functionally. Taking no courses on the subject my knowledge base is limited to search engine results limited library access and a few free online courses. The part that keeps me from understanding how to even begin to structure a formula is from what I understand:

    "An oscillator crystal has two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. During startup, the circuit around the crystal applies a random noise AC signal to it, and purely by chance, a tiny fraction of the noise will be at the resonant frequency of the crystal"

    For my desired application I need a relativity stable and repeatable start-up times for comparative purposes. I think of hitting a tuning fork when trying to picture a random noise AC signal starting up the oscillator.

    My question is how random is the start-up? Does is cause the start-up time to defer by a matter of ms or closer to the ps range? I'm sorry Im just having trouble understanding this very basic concept. Thank you for your input and much appreciation for any additional help you can give. also I apologize for my lack of technical terminology, I am trying my best as I am only a beginner in the field.
     
  4. john monks

    john monks

    693
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    Mar 9, 2012
    Now I understand. Basically what you are describing is a gated oscillator. That is an oscillator that can be started very fast and shut down very fast. So an oscillator that uses an inductor and a capacitor can be started very fast by placing direct current through the coil and then releasing it when you want the oscillator to start. So this type of oscillator can be started very quickly like in terms of nanoseconds.
    The crystal oscillator is different because it is very difficult to ring or impact the crystal to get it to start oscillating rapidly. Therefore industry typically does not use crystal oscillators in a gated fashion like you describe. And when you start a crystal oscillator you have little control of the timing relationship of your signal out and your start signal. For practical purposes this timing or phasing is a random event, partly because of the length of time it takes to start a crystal oscillator and the unstable nature of the circuitry surrounding a crystal oscillator.l
    Bottom line is: crystal oscillators are typically very high Q circuits, that is you cannot deviate much from the resonant frequency of the crystal before the oscillator stops oscillating. For that reason crystal oscillators take much longer time to start than most other types of oscillators. Therefore, for example, crystal oscillators are not typically amplitude modulated with audio because of the type required to change the oscillating amplitude of a crystal. And crystal oscillators are generally not frequency modulated because it is hard to shift the frequency very much without causing the oscillator to quit oscillating. To start a crystal oscillator by hitting it would be very difficult because of the strange ways the crystal bends. Sometime it just bends down the middle. Sometimes it stretches lengthwise and sometimes it twists. And if a crystal is oscillating a one million times a second it is very difficult to just hit it just hard enough and fast enough to start the normal resonant oscillation. But maybe you can figure out how to do it and if you do post it so we can all do it.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Another issue (since you mentions OCXO's) is the time to get to a working temperature. This will be much longer than the time the oscillator takes to start, possibly in the order of many minutes, and equilibrium may not be reached completely for some time after that.

    If you can keep the oscillator running and gate the signal, then you can achieve "startup" in between zero and half the period of the signal. Of course this will be longer if you need to prevent the initial pulse from being short.

    An OXCO requires far more power for the oven than it does for the oscillator. If you're using such a device then presumably you want/need the stability it offers. As such, you don't want to be turning it on and off all the time.
     
  6. woodchips

    woodchips

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    Feb 8, 2013
    To add support to comments above. A low power microcontroller tends not to use a crystal because it takes so long, 10s of milliseconds, to start. Have to use an RC oscillator. Not a problem when just powered up and used, but if woken up every 10-20-50 or whatever milliseconds then just doesn't work. Bob
     
  7. zirissjk

    zirissjk

    3
    0
    Feb 12, 2013
    Hey, I can not thank you enough for your help all of you, I have scoured the net looking for a good explanation of the xtals start-up sequence and John Monk yours by far surpassed anything I could find. I think I might have been misleading about what Im looking for, more then the speed of start-up I'm looking for stability of start-up, from powering on of the oscillator to wave form positive edge. I believe I can get that from a simple LRC circuit but want stable oscillation as well. I don't have any issues with the slow speed of the crystals start-up but more that it has such random start times. That's why I was hoping when reading "random noise" was used to begin the oscillation It only accounted for a few picoseconds of variability.
     
  8. john monks

    john monks

    693
    2
    Mar 9, 2012
    Thak you for your kind words.
    You asked the ten million dollar question, how do you make a coherent gated crystal controlled oscillator. There are many ways of doing this. One way is to use a variable width one shot RC delay circuit to shift the phase angle of the crystal oscillator from 0 to 360 degrees and gate only the signal coming from the oscillator and electrically forcing the signal to be in phase with your trigger signal. One other approach is to have an LC oscillator that is phase locked to a crystal oscillator and gating the LC oscillator and starting the phase relationship by not locking the LC oscillator until the phase of the crystal comes in sync. The same approach can be done with a delay line oscillator. Another approach is to electrically multiply the crystal frequency by a factor of a hundred and locking a gated and much lower frequency LC oscillator. And there are still many other approaches.
    Things like these are done in pulse generators all the time.
    Thanks for the compliments and I hope this helps.
     
    Last edited: Feb 16, 2013
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