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Crystal oscillator frequency stability?

Discussion in 'Electronic Design' started by [email protected], Feb 19, 2007.

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  1. Guest

    Hi guys,

    I'm using a cheap, can-type clock oscillator in a project as a
    frequency standard. It's 10 MHz. Here's the specs:

    http://www.abracon.com/Oscillators/ach.pdf

    What I don't understand is that in the literature they say that the
    frequency stability is only +/-100 ppm. If I'm figuring right, that
    means that the clock can vary per second by as much as +/-1000 clock
    ticks. But when I connect the output to a frequency counter it reads
    10,000,060 Hz and only fluctuates maybe one or two ticks over time,
    which is orders of magnitude better than what they claim.

    Did I just get lucky, or am I misunderstanding the specs? Is this an
    average error over a batch of many parts? Are they taking the entire
    temperature range into account? What gives?

    Thanks.

    Sid
     
  2. colin

    colin Guest

    youl find it varies with many things :-
    temperature at a certain rate,
    supply voltage at a certain rate
    and ageing over long time at a certain maximum rate,
    plus the initial accuracy,
    all this in total will probably be less than +-100ppm.
    (and also acceleration too ! but this probably isnt included)
    so dont expect to see any such wild variations in the space of a second or
    two.

    Colin =^.^=
     
  3. Phil Allison

    Phil Allison Guest

    ** Groper alert !


    ** That last option is the one.

    Crystal stability is normally quoted over the specified operating temp
    range.

    So, +/- 100 ppm over the range from 0 to 70 C equates to roughly +/- 3
    ppm per degree C. This is an average figure and it may well be that it is
    much better at a particular temperature - like around 20C.

    Don't forget the initial accuracy figure too and the fact you can usually
    tweak a crystal onto its marked frequency with a little capacitive loading.

    It follows, that if you can keep the temp of the crystal stable, its
    frequency is very stable.



    ........ Phil
     
  4. John  Larkin

    John Larkin Guest

    That's typical for a cheap can oscillator; they're usually within 10
    ppm of dead-on. But they only guarantee +-100 PPM.

    But from that initial value, expect a few PPM drift per year and a
    good fraction of a PPM per degree C (much less if you're lucky) and
    some more power supply voltage influence. But short-term, stability
    should be better than 0.1 PPM, even for a cheap rock.

    John
     
  5. mpm

    mpm Guest

    On Feb 18, 8:33?pm, wrote:
    But when I connect the output to a frequency counter it reads
    Do not forget, your counter also has a timebase.
    You must consider the drift and age of that timebase (probably crystal
    derived?) on your quoted measurement. It could be that the crystal is
    dead-on, and the instrument is 60 Hz "off".

    In reality, it is highly likely that they are both "off", at least a
    little...

    Convert these numbers to percentages and you will discover that for
    most circuits, the accuracy is plenty sufficient.

    -mpm
     
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