Connect with us

Crystal load?

Discussion in 'Electrical Engineering' started by DaveC, Feb 12, 2013.

Scroll to continue with content
  1. DaveC

    DaveC Guest

  2. Tauno Voipio

    Tauno Voipio Guest

    I'm pretty sure that the circuit does not function
    as a crystal oscillator. Is it an own invention?

    For a crystal, you should decide if the circuit
    needs the series or parallel resonance. There are
    both, pretty near each other. For details, get
    e.g. the ARRL Handbook and read about crystal
    oscillator basics.
  3. DaveC

    DaveC Guest

    I'm pretty sure that the circuit does not function
    I double-checked the board layout; it is as I describe.

    The circuit is part of an existing video driver board that outputs ASCII text
    to a monochrome CRT monitor. I have substituted a sine wave generator in
    place of the crystal and the board works.

    The original crystal is gone (missing when I obtained the equipment) else I
    would have looked up the numbers on the original.

    I'm not asking for evaluation of the circuit's function -- that is confirmed
    -- only to help calculate the load on the crystal.

  4. DaveC

    DaveC Guest

  5. Jamie

    Jamie Guest

    I see you're having difficulties in determining the load..

    If you are looking for an exact figure, I think you need to use
    your signal generator via 100 Ohm R for example and a scope
    to measure the drop in the circuit.

    First, test the scope probe by measuring the drop after the
    R only, to make sure you know the exact cap value in your probe.

    Feed the circuit with this signal via the 100 ohm R, measure the
    drop. Calculate the load and remove the scope probe load from the

    The net results should give you a load that is going to be close
    enough.. You may want to operate the circuit for a bit before taking
    final values. The logic chips are going to shift a little.

    I'm guessing you'll end up with an approximate value that equals 8 pf.

    In the capacitor manufacture world, the common practice was to zero
    beat a tuned circuit with a fixed frequency. You attach a test subject
    to the post which were part of this tuned circuit. You then moved the
    calibrated dial which was nothing more than a capacitor, to make it
    zero beat again. A scope was used with the X,Y inputs for that nice
    lissajous circle or spiral curves.

    In any case, this dial would give you the exact capacitance load.

  6. DaveC

    DaveC Guest

    The request was to help by looking at the circuit and provide a load value.

    How to Google? I've done that. The search results are too non-specific to
    risk ordering a $50 crystal on.
  7. DaveC

    DaveC Guest

    Why not hack in an oscillator? It would be dead-on frequency, without buying
    If I can find a 4-pin DIP oscillator at 8.86723 M I'd be tempted. But don't
    see such an animal...
  8. DaveC

    DaveC Guest

  9. Andrew Holme

    Andrew Holme Guest

    "DaveC" wrote in message
    Load capacitance applies when the crystal is operated at its parallel
    resonance. Your circuit operates at the series resonant frequency. Tell
    the crystal manufacturer you want it to be series-resonant at 8.86723 MHz.
  10. rickman

    rickman Guest

    I haven't looked at standard frequencies, but you might have better luck
    finding that frequency if you drop a significant digit or two. It is
    unlikely the original part was specified to 1 ppm, 100 ppm would likely
    do the job just fine. Didn't you say this was for a display? It will
    probably work fine with just five digits or even four digits of
    frequency specified. Is either 8.867 MHz or 8.868 MHz a common value
    perhaps? Really anything near 8.87 MHz should do the job.
  11. Guest

    I just looked at digikey, they list 17.734475MHz and 35.46895MHz
    oscillators, but expensive and not in stock

  12. DaveC

    DaveC Guest

    Not if it is a video clock, to a fixed frequency monitor.

    I learned a little more about this circuit.

    It's a proprietary video card card from a piece of German offset printing

    I used a sig. gen. in place of the crystal. The circuit generated a video
    signal without any data (just sync pulses). The video card is separate from
    the system processor, so being on the bench the card has no data to display.

    Viewed on a scope, varying the 8.867 MHz frequency doesn't change the video
    signal at all.

    I'm guessing (with my limited understanding of how video works) that the
    8.867 runs the dot clock, basically how fast the dots are shoved out of the
    data bus into the video generator IC.

    So, how critical *is* this frequency?

    Dave (not an EE)
  13. DaveC

    DaveC Guest

    Recently you mentioned a source in Ireland that had crystals of this
    It's a monochrome system (even more forgiving?).

    So, it's series resonance I want?

    This is the one in Ireland:


    Should this one work in my circuit?

    The only reason I was going with the manufacturer is that they can provide
    some expertise re. the application, whereas the Irish are only sellers.

    I'm no engineer so cannot determine for myself the appropriateness of this
    crystal for my application (hence my posting here).

  14. Jasen Betts

    Jasen Betts Guest

    it's hard to say. it depends on the monitor.
    try a 9MHz crystal it's within 2% of the target , you might need to tweak the
    vertical and/or horizontal hold adjustemnt on the monitor.
  15. DaveC

    DaveC Guest

    So, how critical *is* this frequency?
    When I vary the sig. gen. I used to simulate the crystal in this circuit, the
    output video signal doesn't vary at all (sync pulse timing stays the same).

    Therefore I presume that video timing is determined by the other crystal (6
    MHz) that is running the 8085 uP on this antique board.

    So I also presume that this 8.867 MHz crystal drives the dot clock (shifting
    bus data into the 8275 CRT controller IC).

    Is the 8.867 frequency critical at all? Since it doesn't control the video
    sync pulses, etc., what effect will it have on the (ASCII text only) video

  16. DaveC

    DaveC Guest

    So I also presume that this 8.867 MHz crystal drives the dot clock (shifting
    Or rather, loading 8-bit bus data into a parallel-to-serial shift register
    and then into the 8275 CRT controller.
  17. Jasen Betts

    Jasen Betts Guest

    usually the CRTC manges the raster size and does its own
    data fetches... I'm not familiar with the 8275, but all
    the CRTCs i've encountered had internal counters to
    determine the sync rates and sync pulse widths. from the
    pixel clock that crystal may be doing something else,
    colour-burst perhaps.

    Is this circuit used with some sort of genlock - text over video ?
    if so then the crystal frequency may be critical.

    perhps feed an FM signal into where the crystal goes and
    probe around to see where it's going.
  18. rickman

    rickman Guest

    Did you miss the fact that he has specified a common crystal to nearly 1
    ppm? They don't make crystals that will hold a frequency to 1 ppm. You
    can get units specified to some 10's of ppm along with aging of similar
    ranges. To do better you need to temperature compensate and perform
    other "magic" in the oscillator.

    Why would you think the monitor gives a durn? It may not take the
    signal from an 8 MHz crystal, but 1000 ppm off should be no trouble.
  19. rickman

    rickman Guest

    It is hard to say just how critical that value is. How did you arrive
    at this value anyway? A 1 ppm value would be hard to measure.

    Anyway, much of it depends on the device that will be displaying the
    data. Is this a TV or a computer monitor? A monitor will likely
    tolerate a good range of values relative to the number you have
    specified. The electronics in the display generator may or may not be
    pushed to its timing limits. It would be hard to imagine that upping
    the frequency 1000 ppm would cause a problem, but certainly lowering it
    is not likely to disturb a good design. Even a 10,000 ppm deviation
    shouldn't cause any real problems.

    Here are links for parts available from Mouser and Digikey in stock.
    Low cost and you can get them within the week.

    The Mouser unit is 8.5 MHz and should work unless there is something
    "particular" about the monitor. The Digikey device is 8.912, closer to
    the requested frequency, but is high, so there is a small chance it will
    over clock some part of the design. It is also a surface mount unit
    that might be harder to solder in.

    No promises, but this is certainly an easy thing to try.

    BTW, the one reason to get a part that is within 100 ppm is because in
    the "old" days monitors would have some noticeable affects from power
    line noise. If the display is closely synchronized to the power line
    frequency this is not such a problem. Today's monitors don't have this
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day