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Frequency standard

Discussion in 'Hobby Electronics' started by Suzy, Feb 24, 2008.

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  1. Suzy

    Suzy Guest

    I am needing to generate about 1v p-to-p at 50Hz sine wave as accurately as
    I can. Please don't suggest syncing to mains as that's the application -- to
    calibrate an instrument to measure the actual variance of the mains from
    time to time from 50 Hz precisely.

    My current (ouch) thoughts are to get a 2 MHz crystal and divide down to
    50Hz. Any comments on this idea or an alternative?
  2. I wouldn't trust a 'plain' crystal on its own, it's not good enough to
    compare against mains.

    There are several options.

    One "problem" that comes to mind is temperature stability.

    To work around that, there are temperature compensated crystal oscillators,
    that are pretty good (and cost more), and oven temperature regulated crystal
    oscillators that are better (and cost lots more). I'm sure both are available
    as modules, to make it easy to integrate into your design, but I can't think
    of any off the top of my head.

    I also vaguely recall hearing of "pre-aged" crystals too, so they're less
    likely to drift as far due to ageing than new crystals. Basically, this means
    it drifts less between each calibration interval (well, the first few anyway).

    Would be best for you to come down to numbers, that is, find out what
    stability you *DO* need, and go shopping from there.
    You would need short term stability for this, for mains monitoring work,
    this is vital. Long term stability would be important if you need to go long
    periods of time between calibrations.
  3. Suzy

    Suzy Guest

    Thanks for that. I've been looking at 2meg crystals with accuracy of
    30herz. Wouldn't that give me plus/minus .06 Hz @ 50Hz?
  4. Phil Allison

    Phil Allison Guest


    ** Huh ??

    What sort of math is that ?

    30 / 2exp6 times 50 = 0.00075

    ....... Phil
  5. Mr.T

    Mr.T Guest

    0.075Hz, at the specified temperature.

  6. Phil Allison

    Phil Allison Guest


    ** Get real.

    You have quoted a percentage as a simple ratio.

    ........ Phil
  7. 2MHz divided down to 50Hz would give a possible inaccuracy drift of 0.0025%,
    surely close enough???
    What is the "real" purpose of this exercise in relationship to 50Hz? Prove
    the electricity provider is 'off' frequency?
    If so, what is the point? Sue them for being 0.000025Hz off? Your mains
    synchronised alarm clock was out so you missed your bus or train by a few
    microseconds? ROTFLMAO
  8. Mr.T

    Mr.T Guest

    No point in doing calculations when you just can pick figures out of the air

  9. Crystals has an inverse parabolic frequency variation with temperature.
    If you know or can measure the parameters of that parabola, you can
    measure the temperature and compensate the timings down to the order
    of a couple of seconds per month.

    That might be useful for a 0.000001Hz standard, though not as a 50Hz
    frequency standard. Depending on exactly what you want, you might be
    able to get away with counting 50Hz cycles over a longer (accurate)
    time, to determine variance from the nominal frequency.

    Clifford Heath.
  10. What is your real application here?
    If it's just to get an occasional spot frequency check then your
    crystal should do just fine.
    But the easiest way to do that is to simply use a frequency counter
    that has reciprocal measurement (to get a lot of resolution at low
    frequencies like 50Hz).

    If you are talking long term drift accuracy and/or logging then that
    is a different ball game.

  11. David

    David Guest

    Hi Suzy,

    We have used these temperature compensated oscillators from Hy-Q.
    Significantly better than a normal crystal.

  12. Phil Allison

    Phil Allison Guest

    "Alan Peake"

    ** Huh ???

    That is way more than it actually varies.

    You method must have been flawed.

    ....... Phil
  13. Suzy

    Suzy Guest

    I think you've missed the point Alan! The exact frequency of the mains gives
    some indication of the demand/supply balance. It is imposible to maintain
    exactly 50 Hz unless supply and demand are perfectly balanced, which is a a
    rare event. There are in fact legal limits and overall there is an attempt
    by the managers of the grid to compensate so that clocks stay pretty well in
    time over the long term. However, it is still informative to see
    fluctuations as demand peak and more generators are brought onto line, and
    vice versa.
  14. Suzy

    Suzy Guest

    Agreed Phil. And for all those interested, see this site for the UK grid.
  15. Bob Parker

    Bob Parker Guest

    Remember the old DATUM microprocessor training board which was in EA
    around 1982?
    I wrote some code for mine to watch the mains frequency and it never
    deviated from 50Hz by more than a tiny part of 1Hz.
    That was 25 years ago and I don't recall the exact figures but it
    was far less than 1.5Hz.

  16. Elmo

    Elmo Guest

    Hi Suzy

    The grid may be slow by two or three seconds during the day and they
    usually catch up at night. We've just upgraded to a Siemens T3000
    control system and we use a GPS for time keeping. I think Silicon Chip
    published a project recently using a GPS as a frequency standard.

  17. Suzy

    Suzy Guest

    Yes, Elmo, that's right. But it's not the overall slowness during a typical
    high load day that interests me (which as you say is made up during the
    night). It is the constant fluctuations that illustrate changes in the
    supply/demand balance, which promise to be more dramatic as consumption
    grows and various political (state, national and international) effects are
    felt. As Phil intimates, such variations as exist are (currently anyway)
    very small and fractions of a Herz, but they are there nonetheless. The link
    I posted gives a most interesting monitoring of the UK grid. Significant
    loads *do* drag down grid speed, and surplus supply *does* ramp it up,
    albeit by very small amounts. In fact there are legal limits for the
    variation (someone here will no doubt know them) but legal limits are not
    brick walls... What I have in mind is somewhat of a "Grid stress" indicator.
    Some time in the future, it may serve to give early warning of rolling

    I'll look up that Silicon Chip project. Certainly to make an accurate unit
    would require a very good calibration sources.
  18. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** That web sites created by complete amateurs - greenie loopy ones too.

    The supply frequency is not so simply related to demand as they naively

    IOW - it's bollocks.

    ........ Phil
  19. Suzy

    Suzy Guest

    But isn't it true, Phil, that more demand than supply will drag down
    alternator shaft speed and thus frequency, and vice-versa? Apart from the
    clock "make-up" events in times of slow load, what other factors would you
    offer that influence mains frequency?
  20. Suzy

    Suzy Guest

    I meant "low" of course. BTW, that site looks quite good to me, and the
    meter is natty.
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