This guide is intended for electronics engineers who may not be fully familiar with specifying quartz crystals and offers general guidance. TechPoint Golledge manufacturers two main types of crystal, AT cut and tuning fork; for specific advice please contact our Applications Support Team.
AT-Cut
These cover a wide frequency range. The minimum frequency is dictated by the case size. 1MHz for the largest case in production today. The smaller the case size the higher the minimum frequency. Maximum frequency can go up to 80MHz for regular production techniques or up to 315MHz for plasma etched inverted mesa. Overtone crystals are available up to 125MHz. The cut applies to a narrow range of angles that the slice is cut from the bulk quartz relative to the crystal lattice. Within this range the crystal manufacturer can vary the angle to optimise the temperature characteristic for the temperature range of interest. The cut angle does not form part of the crystal specification, instead it is derived from the temperature stability and temperature range specifications.
Tuning Fork
These are only readily available for one frequency, 32.768kHz. The cut is XT. This gives a parabolic temperature characteristic. There are no customer options for the temperature characteristic. The second order coefficient is fixed with a ±5% tolerance and the turnover, nominally 25°C, has a tolerance ±5°C. Tuning fork crystals are always used in the parallel resonant mode.
Other Cuts
There are other types, for example the SC-Cut used in high performance OCXOs, but these are not very often deployed and are not manufactured by TechPoint Golledge.
Below are the minimum set of requirements we would like from a customer to specify a crystal. Often if designing for a specific chip you will find guide values for these parameters in the data sheet for the chip or associated application notes.
Frequency
Usually self-evident and is set by the application.
Case Size
The sizes available are constrained by the frequency and ESR requirement. Both these can place constraints on how small you can go. Specifying too large a case size can lead to designing in a part that me close to going obsolete.
Load Capacitance
Most crystals are used in a parallel resonant mode where the crystal operates on the inductive region of its characteristic which is resonant with an externally applied capacitance. This externally applied capacitance is referred to as the load Capacitance. In rare cases the crystal is used in its series resonant mode and then this parameter is not applicable. These cases are usually for overtone crystals in discrete oscillator circuits.
Tolerance
This is the allowed manufacturing variation from the nominal frequency measured at 25°C. Normally the parallel resonant frequency with the nominal load capacitance is specified.
Stability
This is the amount the resonant frequency varies from that measured at 25°C.
Operating Temperature Range
The temperature range for which the stability is specified. If no other information is given this will be taken as the storage temperature range.
ESR
This stands for Equivalent Series Resistance. Strictly speaking this is the resistive part of the crystal impedance seen at its resonance with the nominal load capacitance. The industry norm has become to quote the ESR measured at the series resonance, where it is equal to the motional resistance, as this gives a slightly lower figure. The lower the value the better all other parameters being fixed.
Overtone
Usually the first overtone, referred to as the fundamental. Crystals made for the lower to mid VHF region can use the 3rd or 5th overtone. These being approximately 3 or 5 times the fundamental. These are for specialist applications, often with discrete component built oscillators.
Optional Specifications
The above will give a crystal supplier the basics to select a suitable crystal for an application where the specifications are not tight for instance USB 1 and 2 or a microcontroller where the timing is not critical. Where more accurate timing is required or less usual operating conditions apply then the parameters below should be considered.
Drive Level
This is the power dissipated in the crystal while working in its oscillator circuit. When characterising a crystal a supplier will use one drive level to measure most of the parameters. This level will be indicated in the data sheet as the typical drive level. For AT cut crystal frequency and motional resistance can vary with high drive level. The onset of this variation with rising drive level is given by the maximum drive level in the data sheet. Tuning fork crystals can be damaged by excessive drive so the maximum given in the data sheet should not be exceeded.
Storage Temperature Range
This is as the name suggests. On rare occasions, for extreme operating conditions, this will be less than the operating temperature range.
Motional Capacitance
If you are tuning the crystal by varying the load capacitance then it becomes important to specify this parameter. This sets the tuning range that will be achieved with a given range of load capacitance. Please consult us for typical values for a given case size and frequency.
Ageing
All crystals have a long term drift in their resonant frequency. This follows an exponential decay like pattern. Normally the first year’s drift is specified assuming storage at room temperature. The accumulated drift over longer periods can be specified. The ageing process is accelerated by storage at higher temperatures.
DLD
Drive Level Dependence. This is measured for a defined range of drive levels. Shifts in series resonant frequency and motional resistance are recorded. For small surface mount crystals this indicates the limit of the range over which the crystal has a characteristic that changes little with drive level. Going beyond this range gives rise to extra frequency error and AM to PM conversion degrading the phase noise performance of the oscillator. A different shape of DLD with step changes in frequency and spikes in the motional resistance graph are an indication of surface contamination of the crystal.
Perturbations
An AT cut crystal’s temperature characteristic can normally be accurately described by a 5th order polynomial. To calculate the perturbations perform a curve fit on the experimental data to derive the optimum polynomial coefficients. Then calculate the greatest deviation from the polynomial curve of the measured data. This is the perturbation. Specify these if you are performing your own temperature compensation of the crystal.
Calculating Overall Frequency Error.
This industry standard method is to simply add Tolerance, Stability, error due to variation in the load capacitance and aging over the lifetime of the product. A more scientifically correct approach is to use a statistical method. The above error sources are uncorrelated, so the simple addition gives a pessimistic view.