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Very basic crystal question

Discussion in 'Electronic Basics' started by Ecnerwal, Nov 2, 2007.

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  1. Ecnerwal

    Ecnerwal Guest

    When I go looking for information on crystal filters, everything I can
    find seems to be aimed at using a bunch of crystals and capacitors to
    widen the bandwidth, or else it's marketing for people who would be
    happy to sell me crystal filters of the above sort. I don't want to
    widen the bandwidth...

    What I'm presently interested in is an exceedingly narrow-band filter
    and/or amplifier. 60Khz for the monent, as I'm playing with WWVB - and
    while I can buy an off-the-shelf amplifier or even amplifier and digital
    buffer chip for $3-4 (CME6005 and CME8000) which uses an external
    crystal to get very narrow bandwidth, there's no real discussion of
    what, precisely, that's doing. As a practical person, I may end up
    buying one of these units as a major building block, but I want a better
    general understanding going into it, as I've really done very little
    with radio, and this is as much about learning what's what as the actual
    end result.

    All the DIY WWVB receivers I've found are using LC filters, which is
    classic, but seems to be more prone to noise than a very narrow
    bandwidth filter would be, given that in this case the frequency of the
    signal is _very_ well defined - in fact, given that I can get "the
    correct time" rather easily from any network attached computer these
    days, the frequency reference is as much or more of a driving reason as
    the time codes are. I don't have any logical reason to spend money
    sending my elderly ovenized crystal reference out for calibration, but
    I'd still like to be able to maintain it on spec as much as possible -
    which is, in theory, possible from the WWVB signal. The units mentioned
    above claim to get about a 10HZ bandwidth, though I'd call the datasheet
    "sketchy" at best.

    Is it as simple as "slap a 60KHz crystal in line with the input (or the
    feedback path of an amplifier), and only 60KHZ will get through"
    (perhaps with some bypassing to block any multiples: 120KHZ, 180KHz,
    etc.)? That seems a bit too simplistic, but other than a reference to
    Walter Cady's 1922 paper (which I have not tried to go track down in
    person) using single crystals for very narrow bandwidth coupling (in
    this article: http://www.ieee-uffc.org/freqcontrol/crystal.html ), I've
    somehow managed to miss any discussion of very narrowband crystal
    filters at the most basic (or too basic to be discussed) level.

    I need to clear the paper and clutter off my bench and get to playing
    with this, having finally gotten a sweep generator.
     
  2. Don Bowey

    Don Bowey Guest

    I'm not familiar with WWVB, but a word of caution is advised: If you wish
    to recover modulation, the filter bandpass must be wide enough to at least
    match the highest modulating frequency; an L/C circuit or a multiple crystal
    network will do well. If all you want is to recover the carrier, then a
    single crystal will do an admirable job.

    Here are a couple links to look at:

    http://www.k8iqy.com/testequipment/pvxo/Atlanticon2002V1R5.pdf

    The Bode Plot of figure 2 is valuable in showing that the pass and reject
    frequencies are different, and you need to deal with that by tuning the
    network.

    And


    http://darleys.pwp.blueyonder.co.uk/radio_07/advanced/techasp12.htm

    This has an assortment of filter design insight, but mainly, it has a
    simple, generalized schematic for a single crystal filter network, that
    might be helpful.

    Assuming you will build a TRF receiver, the filter would be at 60 kHz., and
    the transformer would be fairly simple to build or buy.
     
  3. Rich Grise

    Rich Grise Guest

    Pretty much, yeah, with a couple of caveats. The crystal has to have a
    series resonance at that peak, and you will get a VERY narrow bandwidth;
    and there are phase considerations either side (which I guess there are
    with any resonant circuit). And the passband will be _exactly_ at the
    freq. of the crystal; I don't want to guess what the bandwidth or
    damping factor might be, but I guess the Q can be in the thousands.

    Many years ago, I saw some crystal filter designs, and they used sets
    of crystals that were different in frequency by about the bandwidth you
    wanted - they were used in SSB transmission. Also mechanical and ceramic
    filters; I've even seen mechanical filters centered at 455KHz, so 60KHz
    should be no problem, if you could find one or design it. :)

    Good Luck!
    Rich
     
  4. Tom2000

    Tom2000 Guest

    There's a pretty nice freeware filter design program at
    http://www.aade.com . I used it to design an SSB filter recently.
    After I'd characterized my crystals, the results I measured from the
    completed filter compared surprisingly well to the numbers the AADE
    program generated.

    Tom
     
  5. It's far easier to build a filter that is narrow than one that is
    wide.

    For a long time, the only crystal filter in receivers was a single
    crystal, with a phasing capacitor to knock out the capacitance of
    the crystal's holder. That was when narrow selectivity first hit
    shortwave receivers, and even when other things came along, that sort
    of filter continued. It was really selective, though of course the
    skirt wasn't so great (ie it was nice and narrow at 3db down, but
    further down the curve it would broaden out). Later, when they
    wanted to use it for voice bandwidth, they'd load down the crystal to
    broaden the peak.

    A lot of later work is about getting a wider bandwidth, and maybe
    more important, a controlled passband. So early wider crystal filters
    were of a lattice nature, using a pair of crystals (or more if you
    wanted to improve the skirt) on different frequencies. The spacing
    set the bandwidth.

    Later, people started playing with ladder filters, where the crystals
    were all on the same frequency, and the associated capacitors and loading
    resistors affected the bandwidth. The greater the number of crystals,
    the greater the skirt selectivity.

    So for a narrow filter, a single crystal in the signal path may be fine,
    if the requirements aren't too heavy. More crystals won't narrow the
    passband, because it's already narrow enough, but it will improve the
    skirts.

    You may find it is too narrow, and hence need some sort of loading,
    and you may find a need to null out the capacitance of the crystal
    holder.

    What you will find is that it's likely a very narrow filter, which
    won't matter for WWVB but would likely mess things up if you were
    trying to receive traditionally modulated signals.

    One neat trick, that seemed to circulate when ceramic resonators
    came along, was to simply used them instead of cathode/emitter bypass
    capacitors, so they'd show low impedance at the resonator's frequency
    but high impedance elsewhere, so the gain of the stage was sharply
    peaked at the frequency of the resonator. Cascade a few stages
    of that, and you get your improved skirt selectivity. Nothing was
    needed beyond the crystal, so it made for a very easy method
    of improving selectivity.

    The same thing can be done using crystals.

    Michael
     
  6. Tim Wescott

    Tim Wescott Guest

    Try searching for "crystal ladder filter". If that doesn't work, go to
    the ARRL site (http://www.arrl.com) and get this year's Handbook or one of
    their radio circuits design books. They may even have a book specifically
    on crystal filters.

    At any rate, it can be done. When you're building a cheap crystal ladder
    filter you often find that the center frequency isn't exactly the marked
    crystal frequency -- this would be a problem if you want a 60kHz filter,
    'cause you'd have to buy custom-made, low-frequency crystals.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  7. Ecnerwal

    Ecnerwal Guest

    They certainly seem to available as mass-produced, $1-2 parts from
    Mouser and Newark, most specified to + or - 30PPM (about 1.8 hz, if I
    didn't slip a digit). Mouser has a + or - 20 PPM for a buck. Same for
    100KHz (if one was going after Loran-C, another possible frequency
    reference.)
     
  8. Tim Wescott

    Tim Wescott Guest

    If you just build a ladder filter from the ARRL references you'll get a
    center frequency that's a bit off from the crystal's marked frequency.
    Hence the "custom made" in my post.

    However, thinking about this in a new day, it occurs to me that you could
    probably load each crystal in the ladder with an inductance or capacitance
    that would bring the filter center frequency into line, maybe.

    Some SPICE simulations or just computations on paper should tell the truth.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
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