Support / Product Q&A

Question
  • Q:What are the differences between AT-cuts and AT-strip Cuts?
    Both AT and BT indicate singularly rotated "Y" axis cuts through the quartz crystal. The BT is cut at an angle approximately opposite that of the AT. 
     
    BT-cut blanks are thicker than AT cuts at the same frequency; therefore, higher fundamental mode frequencies are possible with the BT before the blank becomes too thin to process. 
     
    BT cuts have a parabolic (∩) frequency vs. temperature response while AT cuts have a cubic (~) response. Thus, over a given temperature range, the BT will exhibit a greater frequency shift than the AT. In addition, BT-cuts tend to have a lower capacitance ratio (Co/C1) and, as a result, less pullabillity.
  • Q:What Is the difference between a "crystal" end a "shrip resonator"?
    The difference is primarily one of geometry. A "crystal" is usually thought of as a device using a disk shaped quartz plate while a "slip resonator" uses a rectangular quartz plate. The operating characteristics of the two may differ significantly.
  • Q:What happens if I operate a crystal outside of its specified temperature range?
    We do not recommend that you operate a crystal outside of its specified temp range. The most notable change will be a greater frequency drift due to the increased temperature extremes. 
     
    Another and more serious problem that can occur is activity dips. Activity dips can cause the oscillator to stop oscillating at a specific temperature.
  • Q:What is the difference between a fundamental and an overtone crystal?
    A fundamental mode crystal oscillates at a frequency determined by the physical dimensions of the quartz plate. 
     
    The fundamental frequency is the lowest frequency at which a given resonator plate will oscillate. 
     
    Overtones are frequencies that are approximately odd integer multiples of the fundamental.
  • Q:How do I know if I need a fundamental or an overtone crystal?
    Consult with ARGO Engineering as early in your design process as possible. 
     
    Remember that at some frequency which varies from manufacturer to manufacturer, the use of an overtone is mandated. 
     
    Remember also that within a certain frequency range, the fundamental frequency and the overtone frequencies may overlap. In such a case, your selection may be determined by your application. 
     
    If you are developing an oscillator that is to be extremely stable and accurate, with a high "Q" value, you probably need an overtone. If you require a lot of pullability, you probably need a fundamental. 
     
    Again, consult with your crystal vendor as early in the design process as possible.
  • Q:What are spurs?
    Spurs are frequency responses higher in frequency than the main response but not as high in frequency as the next regular overtone. 
     
    The word "spur" is used as an abbreviation of the word "spurious" but the frequencies described by either of the two are not "spurious" at all. They are regularly occurring natural frequency responses, the amplitude of which is subject to some degree of control through crystal unit design.
  • Q:Do I need to be concerned about spurs?
    Another good question and another unequivocal "maybe.?? Usually, spurs are not a problem with crystal units intended for use in oscillator applications. 
     
    Crystal units intended for use as filters are another matter. The control and suppression of spurious responses in filter crystals is critical. If you must specify some value of spurious response suppression, PLEASE specify the test fixture to be used (IEC 60444 Pi Network is a good choice for oscillator crystals) and a reasonable frequency range over which the test is to be conducted.
  • Q:What are the motional and shunt capacitances of a crystal unit?
    The shunt capacitance (Co) is the capacitance resulting from the presence of the electrodes on the quartz plate plus the capacitance inherent in the crystal holder. 
     
    The motional capacitance is a parameter of the equivalent circuit. It is used as a means of describing the elasticity or "stiffness" of the quartz resonator.
  • Q:What Is trim sensitivity?
    Trim sensitivity is the amount by which the frequency of a crystal oscillating with a specific value of load capacitance will vary if that load capacitance is varied slightly about its nominal value. 
     
    Please see the Technical Note on Frequency Pullability for a more complete explanation.
  • Q:Do I need to be concerned about trim sensitivity?
    A good question and the answer is an unequivocal "maybe.?
     
    If the crystal unit in question has a significant value of trim sensitivity and if a fairly stringent value of frequency tolerance is imposed, it is likely that the manufacturers tolerance on the load capacitors in use will result in actual values of load capacitance that will "pull" the crystal frequency beyond the desired tolerance. 
     
    As an example, an 18.432000 MHz crystal might easily have a trim sensitivity value at 20.0pF of +/20.0ppm/pF. 
     
    Let us suppose that a frequency tolerance of +/-10ppm is specified. If the load capacitor has a tolerance of +/-10%, or 2.0pF, it is entirely possible that an acceptable load capacitor will pull the frequency by +/-40ppm.
  • Q:What is the difference between series resonant and parallel resonant crystals?
    The minimum information needed is the Holder or Package, Frequency, and Correlation (series resonant, or load capacitance if parallel resonant). 
     
    All other specifications will be per our standard specifications for the indicated holder. 
     
    All specifications that differ from our standard specifications must also be supplied when ordered.
  • Q:Will a series crystal work in a parallel circuit and vice versa?
    Yes to both, but a crystal unit manufactured as a series resonant crystal will operate at a frequency higher than expected if used in a parallel resonant circuit. 
     
    A crystal unit manufactured as a parallel resonant crystal will operate at a frequency lower than expected if used in a series resonant circuit.
  • Q:How to determine the approximate load capacitance?
    Use this formula to approximate the value of capacitors needed:
     
                 CL=((C1 x C2) / (C1 + C2)) + C stray
     
    C stray is the stray capacitance in the circuit, typically 2-5pF. 
     
    If the oscillation frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the oscillation frequency. 
     
    When CL =20pF, C1 and C2 will be approximately 27-33pF each, depending on the amount of stray capacitance.
  • Q:What is load capacitance?

    Load capacitance is defined as being the total capacitance present in an oscillator circuit as measured or calculated across the pins of the crystal socket. Load capacitance has the effect of increasing the frequency of a crystal unit.

  • Q:What happens if I operate a crystal over its maximum drive level spec?

    Exceeding the maximum drive level (power dissipation) of a crystal can lead to an increased rate of aging, Drive Level Dependency (DLD) problems, increased number and intensity of activity dips that can stop oscillation and, at very high drive levels, breakage of the crystal blank.

  • Q:I have an existing design that uses a HC49U. What should I consider if I change to the smaller HC49S crystal, or an SMD crystal?

    Drive Level - HC49U crystals are typically rated 1.0mW max, while the HC49S and most SMD crystals are rated at only 0.5mW or 0.1mW maximum. See question 11 for more information on exceeding the maximum drive level of a crystal.

    Pullability -some applications require tuning the frequency of the oscillator, either by mechanically or electrically changing the value of load capacitance.

    The HC49S and surface mount crystals have less pullability than the HC49U. It is important to verify whether the HC49S or the surface mount crystal will have sufficient pull range for the application. Generally, an HC49S has one half the pullability as an HC49U.

    ESR - The Equivalent Series Resistance (ESR) is generally higher with the HC49S and surface mount crystals, and can cause a problem if the oscillator circuit does not have sufficient loop gain.

  • Q:Can I safely change from a conventional crystal to a "surface mount device"?

    I would not make the change without a thorough investigation. Some of the SMD (surface mount device) crystal units are comparable to conventional crystal units, others are not.

    In general, SMD's have higher resistance, differing values of shunt and motional capacitance, and are more sensitive to drive level. The pullability characteristics of these devices may differ significantly from a conventional crystal.

    I would recommend a fairly exhaustive qualification sequence before making the switch.

  • Q:What is an activity dip and do I need to worry about them?

    "Activity dips" are sudden increases in the resistance of a crystal that may perturb the frequency. 

  • Q:What is pullability?

    Pullability is the amount by which the frequency of a crystal will change when the circuit condition is switched from series to parallel resonance.

    Pullability is also used to describe the frequency change that occurs when the load capacitance is switched from one value to another.

  • Q:Why can't I get a crystal with symmetrical pulling characteristics?

    Quartz crystals are not linear devices and they do not behave in a linear way. But, it is theoretically possible to achieve symmetrical pulling.

  • Q:Why don't HC49S and surface mount crystals pull as much as HC49U crystals?

    The HC49U crystal utilizes a circular AT cut crystal blank. Due to their small size, most surface mount crystals use a rectangular AT strip cut crystal blank. ("Strip" refers to the rectangular shape of the blank.)

    While both are AT cut blanks, several differences exist due to their different geometry. In general, given the same frequency and overtone, an AT strip cut will have a lower Shunt Capacitance (CO) and Motional Capacitance (C1) than the AT cut. Because pullability is a function of the capacitance ratio CO/C1, AT strip crystals have less pullability.

  • Q:Typical Crystal Design questions to consider…
    Before beginning a design or purchase of a crystal , there are system parameters which must be considered. Below are questions which need to be determined by your system. These parameters will determine the crystal specifications. 
     
    (1) On what crystal frequency do you wish to operate?
     
    (2) How much can the frequency be off at room temperature(25℃)?
     
    (3) What is the temperature range over which the crystal will operate?
     
    (4) How much can the crystal change frequency over the temperature range?
     
    (5) Is the crystal to be operated at Series or Parallel resonant?
     
    (6) If operated at parallel,what is the parallel capacitance in picofarads(pF)?
     
    (7) Is pullability important?
     
    (8) What holder type or can size do you require?
  • Q:What is a Piezoelectric Quartz Crystal Unit?
    A "ceramic resonator" is a resonator fabricated from a piezoelectric ceramic material, of which there are several. These materials are not naturally piezoelectric, the property is induced in them during manufacture. Some of the materials used for this purpose are quite remarkable and represent major engineering accomplishments. 
     
    To our knowledge, however, none of these devices can compete with quartz in terms of frequency stability, particularly over a range of temperatures, nor can their operating frequencies be held to any real accuracy. These types of resonators exhibit "Q" values much lower than those manufactured from quartz. 
     
    These devices, I believe, tend to age far more than do quartz crystals. Still, if a very low cost, loosely specified device is suitable for an application, piezoelectric ceramics have much to commend them.
  • Q:Can I get crystals rated for-40C to + 85C?
    Yes, this industrial temperature range (-40 ºC to + 85 ºC ) is optional on most crystals. Depending on the crystal package, temperature ranges of up to -55 ºC to +105 ºC may be available. 
     
    Contact ARGO's engineering department to discuss your specific requirements.