Frequently Asked Questions
Crystal 

Please note that while most of these questions and answers are applicable to a variety of crystal unit types, they are intended to be specific to the "thickness-shear" "AT" and "BT" crystal units.

1. What is a Piezoelectric Quartz Crystal Unit? For answer click here.

2. What information is necessary to order a crystal? For answer click here.

3. What is the difference between series resonant and parallel resonant crystals? For answer click here.

4. The load capacitance (CL) of my parallel resonant crystal is rated at 20pF. How do I calculate the value of the load capacitors used in my parallel resonant oscillator circuit? For answer click here.

5. What is load capacitance? For answer click here.

6. Will a series crystal work in a parallel circuit and vice versa? For answer click here.

7. Can I get crystals rated for -40 C to +85 C? For answer click here.

8. What happens if I operate a crystal outside of its specified temperature range? For answer click here.

9. What is the difference between a fundamental and an overtone crystal? For answer click here.

10. How do I know if I need a fundamental or an overtone crystal? For answer click here.

11. What happens if I operate a crystal over its maximum drive level spec? For answer click here.

12. 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? For answer click here.

13. Can I safely change from a conventional crystal to a "surface mount device"? For answer click here.

14. What is an activity dip and do I need to worry about them? For answer click here.

15. What are spurs? For answer click here.

16. Do I need to be concerned about spurs? For answer click here.

17. What are "coupled modes" and do I need to worry about them? For answer click here.

18. Why don't HC49S and surface mount crystals pull as much as HC49U crystals? For answer click here.

19. What is pullability? For answer click here.

20. Why can't I get a crystal with symmetrical pulling characteristics? For answer click here.

21. What is trim sensitivity? For answer click here.

22. Do I need to be concerned about trim sensitivity? For answer click here.

23. What are the motional and shunt capacitances of a crystal unit? For answer click here.

24. What are the differences between AT-cuts and AT-strip cuts? For answer click here.

25. What is the difference between a "crystal" and a "strip resonator"? For answer click here.

26. What about "ceramic resonators?" For answer click here.

27. Why is there more than one holder designator for the same package? For answer click here.

28. What is the difference between AT- and BT-cuts? For answer click here.

29. What is a Thickness-shear AT or BT cut Crystal Unit? For answer click here.

30. Why does it take so long to get a two piece sample? For answer click here.

 


Answers

Please note that while most of these questions and answers are applicable to a variety of crystal unit types, they are intended to be specific to the "thickness-shear" "AT" and "BT" crystal units.

1. What is a Piezoelectric Quartz Crystal Unit?

A Piezoelectric Quartz Crystal Unit is an electronic component used in frequency control, filtering, and clocking applications. It consists of a quartz resonator equipped with electrodes, housed in a hermetically sealed package that provides some means of connection to a circuit. Piezoelectric Quartz Crystal Units are usually called "crystals."

2. What information is necessary to order a crystal?

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.

3. What is the difference between series resonant and parallel resonant crystals?

All quartz crystal resonators have a series resonant frequency (fs, frequency of lowest impedance). At this frequency, the crystal appears resistive in the circuit. Crystals can be "pulled" from this series frequency by adding reactance (capacitance) in series with the crystal. When operated in combination with an external load capacitance (CL) the crystal oscillates in a frequency range slightly above its series resonant frequency. This is the parallel (load resonant) frequency.

When ordering a parallel crystal, always specify the nominal parallel resonant frequency and the amount of load capacitance in picoFarads (pF). Alternatively, a standard value of CL (such as 20pF) can be ordered; the capacitor values are then calculated to match the crystal CL. Note that a series type will oscillate in a parallel oscillator, and vice versa. The observable difference is a frequency offset of less than + 1000PPM (0.1%) from the nominal frequency.

4. The load capacitance (CL) of my parallel resonant crystal is rated at 20pF. How do I calculate the value of the load capacitors used in my parallel resonant oscillator circuit?

Use this formula to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + Cstray

Cstray 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.

5. 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. Please see the tutorial section for a complete explanation.

6. 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.

7. Can I get crystals rated for -40 C to +85 C?

Yes, this industrial temperature range (-40 C to +85 C) is optional on most crystals. Depending on the crystal package, temperature ranges of up to -55 C to +105 C may be available. Contact Fox's engineering department to discuss your specific requirements.

8. 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.

9. 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.

10. How do I know if I need a fundamental or an overtone crystal?

Consult with Fox 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.

11. 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.

12. 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.

13. Can I safely change from a conventional crystal to a "surface mount device"?

Do 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, SMDs have higher resistance, differing values of shunt and motional capacitances, and are more sensitive to drive level. The pullability characteristics of these devices may differ significantly from a conventional crystal. One should undertake a fairly exhaustive qualification sequence before making the switch.

14. 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.

15. 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. Please see the Tech Note on Spurious Responses for a more complete explanation.

16. 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.

17. What are "coupled modes" and do I need to worry about them?

This is an excellent question. Guess what? Please see the Technical Note on coupled modes in quartz crystals. The short answer is that you as a user should not have to be concerned with coupled modes.

18. 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 geometries. 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.

19. 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. Please see the Technical Note on Frequency Pullability for a more complete explanation.

20. 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. Please see the Technical Note on Frequency Pullability for an explanation.

21. 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.

22. 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 manufacturer's 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 may pull the frequency by +/-40ppm.

23. What are the motional and shunt capacitances of a crystal unit?

The shunt capacitance (C0) 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. Please see the Technical Note on the Equivalent Circuit for a more complete explanation.

24. What are the differences between AT-cuts and AT-strip cuts?

The differences are in size, shape and some performance characteristics. AT-cut crystal blanks are circular, and AT-strip cut crystal blanks are rectangular. Both types are AT-cuts, but several differences in performance characteristics exist because of the different geometries of the blanks. In general, given the same frequency and overtone, the AT-strip will have a lower value of Shunt Capacitance (CO) and Motional Capacitance (C1). It will also have a higher value of Effective Series Resistance (ESR) and Motional Inductance (L1). Because pullability is a function of CO and C1, AT-strip cut crystals have less pullability than circular AT-cut crystals.

25. What is the difference between a "crystal" and a "strip 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 "strip resonator" uses a rectangular quartz plate. The operating characteristics of the two may differ significantly.

26. What about "ceramic resonators?"

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 knowlege, 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, we 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.

27. Why is there more than one holder designator for the same package?

There are three ways to seal a crystal package: solder seal, cold weld and resistance weld. Most manufacturers use the resistance weld method because it is superior to solder seal and less expensive than cold weld. Since not all customer prints have been changed to include the resistance weld crystals, a cross reference is provided below.

Resistance weld Cold weld Solder seal
HC49U HC43U HC18U
HC51U HC47U HC33U
HC80U HC45U  
HC50U HC42U HC25U
HC48U   HC6U

28. What is the difference between AT- and BT-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 pullability.

29. What is a Thickness-shear AT or BT cut Crystal Unit?

"Thickness-shear" describes the motion of the quartz plate as it oscillates. "AT" and "BT" describe resonator plates cut from the original stone at precise angular orientations that are well known to quartz crystal unit manufacturers.

30. Why does it take so long to get a two piece sample?

Even a relatively simple, straightforward crystal unit requires ten to fifteen separate and distinct manufacturing stages. If you specified the full range of your unique requirements, chances are that your samples had to be manufactured from scratch. In many cases, a 'standard' stock crystal could be used for prototyping. Then, if required, a full custom crystal could be manufactured.