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Crystal Serie VS Parallele

Discussion in 'Electronic Design' started by Jacques St-Pierre, Mar 20, 2008.

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  1. Hi,

    Anyone can explain to me the difference in configuration require for a serie
    or parallel crystal pierce oscillator. I wish to use a SB74LVC1404 chip to
    build a oscillator using a 3.000Mhz crystal. The application sheet use a
    parallel load crystal, but the only 3Mhz crystal I can found is serie. I did
    try it, it look to work properly, but I am not sure it will work all the
    time. In the past, I observe that using a serie crystal often result in
    instable oscillator. So what can I do to make sure the oscillator will alway
    start at proper frequency. I wish I can find a proper crystal, but I can not
    at this point, and further more, the right frequency for me should be
    1.5mhz.

    Bye
    Jacques
     
  2. Phil Hobbs

    Phil Hobbs Guest

    There are two issues here. One is the series/parallel distinction. All
    crystals looks like a capacitor below its series resonance and also
    above its parallel resonance. In the very narrow region between, it
    looks like an inductor. So if you make an ordinary LC oscillator, and
    replace the inductor with a crystal, you'll get a crystal oscillator
    running at a frequency somewhere between the two resonances. (This
    assumes that the capacitors are in the right range of values--20 to 50
    pF usually.)

    The frequency marked on a 'parallel resonant' crystal is the resonant
    frequency of the tank circuit built from the crystal and the specified
    capacitance. (Parallel resonant crystals always have a capacitance spec
    as well as a frequency spec, for this reason.)

    The second issue is startup. It's very possible for a poorly-designed
    crystal oscillator not to start reliably, or to start up at the wrong
    frequency. There are two classes of wrong frequencies: (a) overtones,
    and (b) LC resonances.

    Crystals, like guitar strings, have more than one resonance. Generally
    the higher overtones are harder to use, because the parallel capacitance
    of the electrodes in the crystal tends to swamp the inductive reactance
    more and more at higher and higher overtones. So if you design your
    oscillator so that it needs a decent amount of inductance, and doesn't
    have too much gain, it'll usually work reliably at the fundamental.

    But you have to really design it. Calculate how inductive your crystal
    gets (minimum and maximum Q specs apply); choose the capacitor values
    accordingly; and calculate the losses, so you can choose the right
    amount of gain in the active device: enough to start reliably from zero
    signal, but not much more than that.

    If you don't do the design carefully, you're liable to find that your
    gizmo can oscillate at many frequencies--a few crystal resonances, the
    LC resonance of a poor layout, or the (much higher) frequency where the
    propagation delay and phase shift due to the capacitive load add up to
    180 degrees. (Low and stable gain is your friend.) Crystal oscillator
    startup is one place where SPICE isn't that much help, so do the
    algebra. If you don't know where to begin, find the crystal parameters
    from the data sheet and post them. One of us will probably be able to help.

    If you have a 3 MHz crystal, the easiest way to get 1.5 MHz is to use a
    flipflop as a divide-by-2.

    Cheers,

    Phil Hobbs
     
  3. Thanks for the explaination.
    I used the ECS-30-S-1X
    Here a link for the crystal:

    http://www.ecsxtal.com/store/pdf/hc49ux.pdf

    I am not sure I am familiar with the computation require to match the
    crystal to the 74LVC1404

    http://focus.ti.com/lit/ds/symlink/sn74lvc1404.pdf

    For test, I plan to use:

    Rs= 2.2m
    Rf = 1K
    C1 & C2 = 22pf

    I was planing to use a simple flipflop to divide the 3Mhz by 2.

    Bye
    Jacques
     
  4. Phil Hobbs

    Phil Hobbs Guest

    Well, they don't seem to give much info about the circuit's gain, do
    they? They do say that the gain is a function of frequency, so for a
    one-off, you could try changing the supply voltage and adjust Rs to get
    reliable starting and frequency keeping over the whole available range.
    A hair dryer and some freeze spray will help ensure that it works
    over the temperature range you care about. (Don't blast it too hard
    with the freeze spray, or you're liable to cause cracks in some of the
    components.)

    The crystal wants a 32-pf load capacitance, so your capacitor values are
    probably too low. The load capacitance seen by the crystal is basically
    C1 || (C2+Cin), where Cin is the input capacitance of the oscillator
    chip, so you probably want something more like 56 pF for C1 and 68 pF
    for C2, which works out just about exactly with Cin=6 pF (typical value
    from the datasheet). You can adjust the gain by trading off the C1/C2
    ratio.

    The datasheet says to try the circuit with the crystal replaced by the
    equivalent series resistance (200-300 ohms in your case), which is a
    good idea.

    You might very well need to increase the value of Rs since you're
    running at a low frequency, where the chip's gain is high.

    If you need to adjust the frequency to be exactly right, you can reduce
    the tank capacitor values a little (say 56 and 56 pF) and put a 1-5 pF
    trimmer cap in parallel with the crystal. Watch out for the capacitance
    of the pads and traces, which also have to be factored in, and don't use
    a trimmer that's anywhere near the value of the tank capacitors, or
    you'll have problems.

    Cheers,

    Phil Hobbs
     
  5. Fred Bloggs

    Fred Bloggs Guest

    That doesn't make any sense, the parallel mode crystals are far more
    popular than series. Brush up on your search skills. The two types are
    identical in construction but tuned differently. Series mode crystals
    are designed for circuits that resonate at the series frequency, where
    the crystal appears as a pure resistance of low value and is loaded by a
    low resistance load at its output. There is 0o phase shift through the
    crystal at this frequency, meaning the buffer is non-inverting. The
    parallel mode crystal is intended to be loaded by a high impedance, it
    is combined with other reactances to produce 180o phase shift and
    requires an inverting buffer. You can go ahead and stick the series
    resonant crystal in the parallel mode circuit, but the frequency will be
    off. Otherwise, stability and immunity to spurious oscillations should
    be the same, if you're having these problems with a series mode crystal,
    you will also have them with the parallel mode type, in the same
    circuit. The usual method of eliminating the spurious and overtone
    response is to RC filter the gate drive into the crystal, hence the
    popularity of the Pierce configuration.
     
  6. Fred Bloggs

    Fred Bloggs Guest


    Apparently you have a knowledge gap here, those are parallel resonant
    mode crytals, this is inferred from the "load capacitance" specification.
     
  7. Joop

    Joop Guest

    There is technically no difference between a parallel and series
    crystal. Each crystal operates at or near its (series) resonance
    point. If the oscillator circuit is designed to look capacitive to the
    crystal, it will operate on the high, inductive side of it resonance
    point.
    The (bad) terms parallel mode crystal or parallel mode oscillator
    refer to this capacitive load aspect. The crystal manufacturer
    incorporates this by manufacturing the crystal a bit lower in
    frequency so it will operate at the rated frequency using a certain
    capacitive load. The proper load is specified in the data sheet.

    The popular Pierce circuit is such an oscillator. Like Phil said,
    start with about double the specified load capacitance as values for
    C1,C2 and tune (one of) them if you really need to be exactly on
    frequency. Your Rs of 1K is a good starting point to keep the crystal
    power below the 1mW of the data sheet.

    Joop
     
  8. Joop

    Joop Guest

    If it works, I would not worry too much. The crystal types are
    basically the same. The only difference is perhaps that the frequency
    of a series crystal might be a bit high in a Pierce circuit. How much
    accuracy do you need?
    About your problems in the past, this might have been with cheap
    computer crystals that may have had a bit high series resistance.
    Increasing the drive level could probably have fixed that. In general
    keeping the drive level low enough makes sure the crystal survives
    forever and does not drift much. Making it too low might give trouble
    starting or make the oscillator a bit noisier. What is your
    application?

    Joop
     
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