Connect with us

crystal series resonant circuit

Discussion in 'Electronic Design' started by Johnny Chang, Nov 25, 2007.

Scroll to continue with content
  1. Johnny Chang

    Johnny Chang Guest

    i'm trying to create a standalone crystal circuit to create a square
    wave but all I can find are parallel resonance circuits. the series
    resonant circuits don't seem to have a resistor and it is my
    understanding that you change the frequency of the crystal by having
    different series resistances, listed here: Data/C-2-TYPE,C-TYPE.pdf

    does anyone have an example circuit I can use with this crystal? i
    dont think i can use a parallel resonance circuit with it because of
    the 6pF to infinity statement.

    this is the only place i've seen with series resonance:

    but i don't understand how to select my frequency of oscillation or
    the purpose of the op amps at the end.
  2. Tom Bruhns

    Tom Bruhns Guest

    So why, with those crystals, do you think you want to use series
    resonance? They are specified for a particular load capacitance,
    which to me implies parallel resonance. It's usual to order a crystal
    for a particular load capacitance, and if you want to adjust the
    frequency, provide a variable capacitor in the load circuit.
    Actually, it's not too different for a circuit that uses series
    resonance, since a capacitor in series with the crystal will also
    adjust the frequency of oscillation, as the net resonant circuit
    equivalent capacitance is the series combination of the motional
    capacitance and the external capacitance...though you must also
    account for the holder capacitance. And be aware that the frequency
    of oscillation will depend to some extent on all the external parts as
    well as on the crystal itself.

    In your second link, I don't see any "op amps at the end." I see only
    logic gates. Their purpose is to buffer the crystal oscillator output
    so that the oscillator is isolated from changes in load impedance.

  3. Tam/WB2TT

    Tam/WB2TT Guest

    Regardless of what your links may say, you do not change the frequency of a
    crystal oscillator by changing the resistance. You change it by varying the
    capacitor. You can only change the frequency of a crystal about +/-.01% or
    so. Also, no crystal oscillator inherently has square wave output. You get a
    square wave by having enough gain in the system so that the sine wave gets
    squared up.

  4. Johnny Chang

    Johnny Chang Guest

    i'm confused, the link i gave provides series resistance for frequency
    ranges while load capacitance is 6pF to infinity, which doesnt give
    any information for getting the right frequency between 20 - 165 KHz.
    am i supposed to buy a bunch and experiment? i did not know there were
    variable capacitors? for series, they list series resistance, so i
    assumed i need some kind of resistor to vary the frequency. but i
    suppose it could just be the amount of resistance it looks like.

    well, if i can use that crystal in parallel mode, then i will. i was
    just confused by the lack of data on the sheet as i was only familiar
    with parallel resistance and specific #'s.

    as for the circuits, im referring to the other 74LS04 inverters.

    are there specific specs i should be looking for inverters based on
    frequency? output current?
  5. Johnny Chang

    Johnny Chang Guest

    thanks for the information, i actually want a sine wave output and was
    considering running it through a low pass filter afterwards, but with
    what you're saying I just need to find a low gain inverter?
  6. Johnny Chang

    Johnny Chang Guest

    thanks for the information, i actually want a sine wave output and was
    considering running it through a low pass filter afterwards, but with
    what you're saying I just need to make the circuit low gain ?
  7. Johnny Chang

    Johnny Chang Guest

    thanks for the information, i actually want a sine wave output and was
    considering running it through a low pass filter afterwards, but with
    what you're saying I just need to make the circuit low gain ?
  8. Guest

    If you are wanting a frequency range of between 20 - 165 KHz then you
    don't want to use crystals at all unless like you say you'll need a
    bunch of $them$.

    What would be so wrong with using a $34 Elenco function generator?

    What IS your time worth?

    "op amps at the end." hold the secret to the desired square wave
    output. Their gain is so high that on and off ramps become more or
    less vertical. But those aren't op amps, they are gates - they are
    just drawn like op amps some times are drawn. I'm not bad, I'm just
    drawn that way...
  9. John Fields

    John Fields Guest

    No, that's the maximum series resistance _of the crystal_ when it's
    oscillating in parallel resonance mode in the various frequency
    ranges indicated.

    View in Courier:


    | \
    +---| >O--+
    | | / |
    | |
    | |
    | |
    [2Cl] [2Cl]
    | |

    Where 2Cl is twice the value of the load capacitance specified by
    the crystal mfg.

    Also, for your perusal, I've posted RCA's ICAN-6539 to

    It's a good read from the good old days, and just might help to
    smart you about CMOS and Xtal oscillators. :)
    That statement indicates that the crystal can be special-ordered
    with a _customer-specified_ load capacitance of 6pF upwards.

    But, if you want to buy an off-the-shelf crystal, what you'll need
    to do is accept whatever load capacitance is specified, by the
    manufacturer, for that crystal and make sure your circuit presents
    that load capacitance to the crystal.

    Each of the load capacitors should be twice the specified load
    capacitance since they're series-connected. In other words, if the
    load capacitance is specified as 16pF, the load capacitors should
    each be 32pF.
    You select the desired frequency of oscillation by ordering a
    crystal which, when properly loaded, will be ground to operate at
    the frequency you want.

    Series resonance you don't want, and the opamps at the end are, as
    someone else posted earlier, not opamps, but buffers which will
    isolate the eventual load from the oscillator's output and keep it
    from pulling or pushing the crystal.
  10. Eeyore

    Eeyore Guest

    How did you get that idea ? I see nothing in the data sheet you linked to that
    even remotely suggests such a thing. And indeed, it's not the case.

  11. Eeyore

    Eeyore Guest

    No it doesn't.

  12. Eeyore

    Eeyore Guest

    Why an *INVERTER* ? You just need a buffer (NOT a logic gate). Mind you, the
    crystal circuits I've come across don't produce a very good sinewave.

  13. Guest

    There is a Phillips chip I ran into when I needed to pull a crystal
    without a varactor. The Philips design involved changing the
    transconductance of a long tail pair by varying the tail current.

    Since you're building a discrete design, look around for the varactor
    based designs.
  14. Tam/WB2TT

    Tam/WB2TT Guest

    You may not need it. The signal at the output side of the crystal will be a
    sine wave. Capacitively couple that to an emitter follower. Bias the base of
    the EF at half of VCC.

  15. Tam/WB2TT

    Tam/WB2TT Guest

    I have seen TV 3.58 MHz PLLs that work something like that. Changing the
    current causes a change in the phase shift of the oscillator.

  16. Fred Bloggs

    Fred Bloggs Guest

    Actually it is the xystal ESR at resonance and is a minimum. 50K is
    pretty much the standard for that 32KHz thing. Off resonance ESR goes to
    infinity and is the main cause of start-up failure. That ESR value only
    holds for a small band of frequencies about Fresonant, and the same goes
    for the ESC and ESL.
  17. Tom Bruhns

    Tom Bruhns Guest

    You need to separate the crystal parameters from the circuit
    parameters. The "load capacitance" is a circuit parameter, and is the
    capacitance which the crystal "sees" from your circuit. You can
    adjust it (the capacitance) to adjust the frequency of oscillation.
    You (or the crystal manufacturer) specify the desired capacitance, and
    the manufacturer guarantees oscillation within the stated tolerance
    when the crystal is operated in a circuit in which it "sees" that load
    capacitance. The parameters in the data sheet you linked to
    immediately below that, from "series resistance" to "shock
    resistance," are all parameters of the crystal and its holder. Series
    resistance is the electrical equivalent circuit resistance for the
    crystal. You should be able to find info through a Google search on
    the generally-accepted equivalent circuit of a quartz crystal; a first-
    order equivalent is a series R-L-C, shunted by the C of the holder.
    In actuality, there are other series RLC's resonant at other
    frequencies that correspond to minor resonances, but you probably
    don't have to worry about those in your application.

    I'm QUITE sure you can find a LOT more info on crystal oscillators on
    the web, through a Google search. You can also find a lot of info in
    various ham radio publications like the Radio Amateur's Handbook.
    From a combination of thinking about the circuit, looking at the
    available info, and experimenting with some circuits, you should be
    able to learn a lot and become much more comfortable with how crystals
    and crystal oscillators behave, and what the important parameters are.
  18. Guest

    Bingo. A very clever design.

    Most people go with the varactor if you can use one of the cheap types
    used in radios.

    I don't know why I didn't reference VXCO in the first place when the
    topic came up.
    The M E Ferking reference is quite a book. You need an IEEE membership
    to read it.
  19. Fred Bloggs

    Fred Bloggs Guest

    Guess I should add that the ESR specification is at resonance and at a
    given power level, like uWatts. The highly nonlinear ESR is also a
    function of power level, recall seeing a graph showing that 32KHz 50K
    going asymptotic to 500K at small powers like nanowatts, even at the
    resonant frequency. That is the start-up failure mechanism.
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