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Crystals For Odd Frequencies?

Discussion in 'Electronic Basics' started by Ron Hubbard, Sep 11, 2003.

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  1. Ron Hubbard

    Ron Hubbard Guest

    I need a stable clock oscillator for a digital filter. Does
    anyone know if it's possible to make a crystal oscillator to
    produce square waves at odd frequencies like 400 Hz, 700, Hz,
    830 Hz, and 2kHz?
  2. If you need all those at once, you can build a single crystal that
    runs at the product of all their prime factors.

    830=2*5*83 (yikes!)

    So 2*2*2*2*5*5*5*7*83=1,162,000 Hz

    If you can find a crystal that runs at any integer multiple of that
    frequency, you can use counters doing integer frequency division to
    create all those frequencies.
  3. You usually divide a higher frequency down to these frequencies.
    Look at the CMOS 4040 or 4060 chips.
    Natural frequency quartz for these low frequencies mean VERY BIG crystals.
  4. Steve

    Steve Guest

    An interesting question.

    A couple of those would be theoretically possible using frequency

    A small technicality, but I would think you'd be after a frequency
    synthesiser rather than an oscillator.

    Maybe a micro (PIC) that's crystal locked and suitably programmed
    would have the accuracy you're after?
  5. Ron Hubbard

    Ron Hubbard Guest

    Hmmm, Mouser makes custom crytal oscillators for about $15 and
    it shouldn't be hard to get one made for 1.162 MHz.. But I
    don't see how that one frequency can be divided in so many
    ways using only the usual divide by 2, divide by 5 or 6,
    divide by by 10, and divide by 12 counters.
  6. 1,162,000/83/7/5=400

    This takes more divisions than necessary.


    For value less than 10, a 10 bit decoded counter like a CD4017 can be
    hooked up to roll over on any count below 9.

    The only hard to construct counter is the divide by the prime number,
    83. This could be built with a preloadable counter that counts 82
    down to zero, and then reloads. These often come in 4 bit packages,
    so two of those would hold the preset value of 82.


    I have done this.

    I used a 10 MHz crystal oscillator and divided it with an Intel 82C54
    programmable counter. It has a "square wave" mode where it divides the clock
    by a number and produces a square wave output. If you wanted 440 Hz, for
    example, you would give the counter the number

    10,000,000 / 440 = 22,727

    This would produce an extremely accurate 440Hz square wave.

    You have to set up and feed the counter with some sort of processor though. I
    use a Basic Stamp. It's pretty simple.

    Hope this helps.

  8. Charles Jean

    Charles Jean Guest

    Check out page 11 of the data sheet for the Phillips 74HCT40103 8-bit
    down counter at:

    F(out) = F(in)/(N-1), where 0<N<256, N an integer

    Depending on the value of N wired into the device, you can get one of
    255 lower output frequencies.

    They can also be cascaded to get even lower frequencies.

    If God hadn't intended us to eat animals,
    He wouldn't have made them out of MEAT! - John Cleese
  9. Ron Hubbard

    Ron Hubbard Guest

    And if I understand you right, John, an awful lot of counter
    chips; at least fifteen, and more if I need to add additional
    clock frequencies. That makes what starts out as a simple
    project immensely complicated with, ironically, the clock for
    the filter becoming far more complicated than the rest of the

    I think, while it's more expensive to purchase and use several
    custom crystals, it would be easier to use crystals like 1.000
    MHz, 1.400 MHz, 1.660 MHz, etc and divide those frequencies by
    10/10/10/2— thus requiring overall only four chips. Three
    chips (minus the oscillator), if I use double decade counters.
    I believe in that basic engineering tenet, "keep it simple,
    [stupid]." It started as a simple project and I really want to
    keep it that way.

    However, thanks the advice.

  10. I did not make a judgment that this method was a good solution to your
    problem. I answered your question about how multiple frequencies
    could be derived from a single crystal oscillator. If the technique
    is some use to you, that is good.

    I don't even know that you need the stability of crystal oscillators
    for your frequencies. For all I know, a few 555 timers might be a
    better solution to your problem.
  11. Ron Hubbard

    Ron Hubbard Guest

    John, you gave me a few options and food for thought that has
    led to a solution. Not an "ideal" one, but what is? :)

    I want to build a cheap (less than $100) but rather special
    type of EEG biofeedback unit designed to allow one to learn
    how to produce 4.0 Hz theta, 7.0 Hz theta, 8.3 Hz alpha, and
    20.00 Hz beta waves; and possibly two or three other
    frequencies if the need arises. commercial units usually run
    around a thousand bucks!

    I started with analog filters but finding precise resistor
    values and matching capacitors to be a major drag. I replaced
    the analog filters with the tunable LTC-1164-8a bandpass
    filter. It's not good for anything high frequency (read that
    as greater than 5 kHz) but it should be perfect for this
    application. However, the bandpass frequency is determined by
    the clock frequency: 500 Hz in equals 5 Hz out in a 100:1

    I intended to use the VCO in a CD4046 as a clock, but it
    wasn't stable; drifting about 1 or 2 Hz every few seconds. For
    most other applications, a slight deviation in clock frequency
    wouldn't matter, but here a one or two Hz drift would mean the
    filter might cycle between 5Hz and 5.01 Hz sporadically. For
    this biofeedback purpose, that wouldn't be acceptable in the
    least. Hence the need for a stable square wave clock source.
    And to further complicate things a bit, the LTC1164-8a
    requires that square wave to have a 50% duty cycle.

    There are a lot of oscillators out there, but so far I can't
    find any that's highly stable without using crystals. I hear
    he old tube-based Franklin oscillator was very frequency
    stable but I don't know about the newer FET-based version.
    Going with microprocessors would add to the complexity, time,
    work, and cost— But at least crystals are a relatively cheap
    sure thing. :)


    I did not make a judgment that this method was a good solution
    to your
    problem. I answered your question about how multiple
    could be derived from a single crystal oscillator. If the
    is some use to you, that is good.

    I don't even know that you need the stability of crystal
    for your frequencies. For all I know, a few 555 timers might
    be a
    better solution to your problem.
  12. I've written some PIC code to take a crystal and produce a given
    set of squarewaves. The squarewaves are selected based on digital
    input to two of the pins. The output is preprogrammed internally,
    and can be set up for 4 different frequencies. You need a 20Mhz
    parallel crystal, a resistor, and a couple of 22pF caps to drive it.

    Its quite stable. Are you interested? email me at rcmonsen at

    Bob Monsen
  13. Ron Hubbard

    Ron Hubbard Guest

    Thanks, Bob, but as a design objective I wanted to stay away
    from PICs, microprocessors, and anything even remotely related
    to computers..Back in the "old days" (the 70s) people could
    buy small biofeedback units that had little more but a couple
    of op-amps and some transistors. Were it not for me sticking
    in a digital filter, I would've been finished with this thing
    in a couple of spare evenings with no hassles (well, few
    hassles). Now it's already taken on a complexity I find
    appalling. For all the exotic chips being made, you'd think
    someone would make a dedicated clock chip...

    Oops, somebody did: Intersil used to make the ICL7209 clock
    oscillator that would drive any crystal, had a divide by eight
    output, and an inhibit pin for digital control. Too bad, those
    are as extinct as the dodo these days.

    But thanks anyway.

    Robert Monsen wrote in message ...
  14. Bob Masta

    Bob Masta Guest

    Just out of curiosity, why wouldn't 5.01 Hz be acceptable?
    Aren't the standard alpha, beta, etc designations for fairly
    broad bands that vary between individuals? Or is there
    some advantage in biofeedback training to hit a very
    precise value? I would have guessed that 10% would
    be accurate enough for this.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Shareware from Interstellar Research
  15. Ron Hubbard

    Ron Hubbard Guest

    Hi, Bob;

    It's true that there is a bit of debate in the EEG community
    about the boundaries of the various brainwave boundaries:
    delta, theta, alpha, beta, and gamma. But generally its:

    0.5 Hz-3 Hz delta
    4.0 Hz-7 Hz theta
    8.0 Hz-12 Hz alpha
    13 Hz-40 Hz beta
    40Hz-- gamma Or 30 Hz-500 Hz high beta

    There are frequencies within each band that are beneficial if
    you can learn to produce them at will: 3 Hz delta is good for
    headaches (if you can learn to stay awake); 30 minutes of 5Hz
    theta is equal to 8 hours of deep sleep, etc. Biofeedback when
    you know what frequency to shoot for can help with ADD/ADHD,
    smoking & alcoholism, improved memory and improved creativity,
    plus a fairly wide assortment of other things— some of those
    that would amaze you.

    But learning how to change your brainwaves is not like
    learning to tie your shoes; it's bloody difficult. With my
    original design that used analog filters, I could be sure that
    a specific frequency would be passed and others rejected.

    With this digital filter being clock dependent, if there are
    any slight frequency variations it makes it doubly hard to
    learn how to reach whatever center frequency I may want to
    learn. If I wanted to learn how to produce 5 Hz at will, a
    slight deviation of .01 Hz isn't going to be constant with
    anything but a crystal-based clock oscillator; using my
    original CD4046 as an clock it would shift up and down maybe 1
    Hz or maybe 2 Hz sporadically; other oscillators were even

    For most applications a 1 or 2 Hz shift might not be a
    problem, but in this case it would be shifting the center
    frequency up and down— can you drive when sunlight is
    flickering in your face unpredictably? Such a frequency shift
    would amount to the same thing in this application.


    Bob Masta wrote in message
  16. [brainwave information]

    So what kind of sensor do you use to detect the brainwaves? Where are they

    Bob Monsen
  17. Ron Hubbard

    Ron Hubbard Guest

    The unit uses three silver electrodes of the kind used in most
    biomedical devices (EEGs, EKGs, etc): one on the back of the
    head, one on the forehead, and the ground is clipped to an
    ear. These electrodes are connected to an instrumentation
    amplifier with an incredibly high impedance provided by a pair
    of TL081 JFET op-amps.

    The signal is then amplified quite a few times by a couple of
    LM301 externally compensated op-amps, and then bandpass
    filtered since the brain at any point in time cane be
    producing multiple frequencies at once and in different parts
    of the brain. Most people when awake and active produce beta,
    but other parts of the brain could be producing alpha or theta
    at the same time.

    The desired frequency leaves the filters and is used to
    frequency modulate a square wave oscillator to let you know
    that a particular frequency is being produced. As biofeedback
    units go it's pretty simple, but it does the job without
    needing a computer and software, and considerably cheaper than
    commercial EEG units that rely on computer technology. :)


    Robert Monsen wrote in message ...
  18. You cannot first amplify and then filter, you need active filters
    which amplify only the frequencies which are of interest.
    Too much noise if you don't.

    After these active bandpass filters all you need to do is to rectify
    and indicate the activity within these pass bands.
    There is no need for crystals, which have 5-7 digits precision, in
    this project. Plus minus 30% is a more useful precision for these

    I read about an EKG (heart monitor) project once, it used an active
    filter with very high amplification within a pass band where you can
    expect the heart rate to be.

    Something like that will be needed for this project. Active filters
    which use op-amps.

    I think you could find schematics for such EEG units if you search
    thoroughly for them on the web. Try EKG too, it is the same basic
  19. Bob Masta

    Bob Masta Guest

    On Mon, 15 Sep 2003 16:22:22 -0700, "Ron Hubbard"


    It seems to me that if you are trying to produce 5.00
    Hz and succeed in producing 5.01 Hz, that is a definite
    hit! Also, consider that the analog filters probably had drifts on
    this order of magnitude due to component drift with temperature.
    Since you are only changing the center frequency of a fairly
    broad band (even with digital filters), a small shift is not going
    to send a valid signal down by very many dB; it just might move
    out from the passband onto the start of the cutoff slope.
    My guess is that you can probably tolerate 5 or even 10%
    drift in the center frequency before you will get a perceptible

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Shareware from Interstellar Research
  20. Ron Hubbard

    Ron Hubbard Guest

    Bob Masta wrote in message

    In this one particular application, neither a 5 nor 10 per
    cent drift is acceptable. If you just want to learn how to
    produce alpha or theta-- yeah, it don't matter. But I have
    reasons for very precise, very specific frequencies +/- no
    more than 1% and that's being generous. There's a list of
    brainwave frequencies available on the "Net and that list is
    quite clear where a frequency like 3.6 Hz may do one thing,
    but 3.84 Hz does something entirely different. That's less
    than a 7% difference, but there are other frequencies even
    closer together than that that do very different things. So
    there are no allowances for drift of any kind.

    But the LTC1164-8 is a 8th order, ultra-selective digital
    bandpass filter,

    whose center frequency depends only upon it's clock frequency,
    which, as I said elsewhere, is in a 100:1 ratio: 500 Hz in to
    get 5 Hz bandpass. With a few custom crystal oscillators and
    few counters, I can get even fractional frequencies with a
    high degree of accuracy out of the LTC1164-8 that I could
    never achieve with analog filters even using 0.1% resistors
    and matching caps to three places.

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