Definitions of frequently used key words when working with crystals.
Quartz crystal aging applies to the cumulative change in frequency over time, which results in a permanent change in the operating frequency of the crystal unit. The rate of change of frequency is fastest during the first 45 days of operation. Many interrelated factors are involved in aging, some of the most common being: Internal contamination, excessive drive level, surface change of the crystal, various thermal effects, wire fatigue and frictional wear. Proper circuit design incorporating low operating ambients, minimum drive level and static pre – aging will greatly reduce all but the most severe aging problems.
A major factor in the aging rate of a quartz crystal is in the method of encapsulation since the sealing of the crystal case leaves contamination and oxygen within the crystal environment. Where crystals are concerned, the 2 most common methods of encapsulation for through hole crystals are resistance weld and cold weld. Typical aging rates are as follows:
|Method of sealing||Resistance weld (HC49/U)||Aging per annum (1st Year)||5ppm Typ.|
|Method of sealing||Cold weld (HC43/U)||Aging per annum (1st Year)||2ppm Typ.|
Typical aging rates for SMD crystals are as follows:
|Type of crystal Seal / Package||Aging per annum (1st Year)|
|Plastic eg. ACT86SMX||±5ppm Max.|
|Metal Seam Weld eg. ACT753||<3ppm Max.|
|Glass Seal Ceramic eg. ACT632||±5ppm Max.|
The speed of data transmission (how much data can be transmitted in one second)
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them became known as “crystal oscillators”.
Quartz crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. More than two billion crystals are manufactured annually. Most are small devices for consumer devices such as wristwatches, clocks, radios, computers, and cellphones. Quartz crystals are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.
The output frequency that is divided by the internal IC.
Drive level is the level of power dissipated in the crystal as a result of the operating circuit. Rated or test drive level is the power at which the crystal is specified and any deviation from the rated level will affect the crystal performance: therefore, the actual drive level should reasonably duplicate that specified. AT-Cut crystals can withstand a considerable overdrive without physical damage: however, the electrical parameters are degraded at excessive drive. Low frequency crystals (especially flexural mode crystals) may fracture if overdriven. Drive level ratings range from 5µW below 100kHz to about 10mW in the 1 – 30 MHz region for Fundamental Mode crystals. Overtone crystals which are generally used above 30 MHz are often rated at 1 – 2 mW
|GENERAL TABLE OF MAXIMUM DRIVE LEVELS|
|Frequency range||Mode||Drive level|
|Up to 100 kHz||Flexural||5µW|
|1 – 4 MHz||Fundamental||1µW|
|4 – 20 MHz||Fundamental||0.5µW|
|20 – 200 MHz||Overtone||0.5µW|
Equivalent series capacitance (C1)
Energy distortion to the (equivalent) internal charge capacitance component of the crystal resonator, at the series resonant frequency.
Equivalent series resistance (ESR)
As can be seen from the equivalent circuit of a crystal, there is the parameter R, which is defined as the motional resistance.
This value has a direct relationship to the Q-Factor and Activity of the crystal. Activity can also be termed ESR or Equivalent Series Resistance. The lower the ESR values, the higher will be the amplitude of oscillation in the oscillator circuit. The actual value of ESR is somewhat dependent on the load capacity presented to the crystal in use, and can be calculated by the following formula:
ESR = R1 ( 1 + Co / CL )²W
It is extremely important to take into account the ESR value when designing a crystal oscillator, as crystals with a high ESR value will be less inclined to begin oscillation than one with a low ESR.
Number of waves (cycles) per second. The relationship between frequency and cycle is:
fo (Hertz Hz) = 1 / T (Sec.)
1 kHz = 1ms, 1 MHz = 1µS, 1 GHz = 1nS
Frequency tolerance precision (f / f)
Under specified conditions at an ambient temperature of 25 °C, the difference in actual (measured) frequency from the nominal frequency.
Frequency stability (f / fo)
Within standard temperature and operational voltage ranges, the drift in the output frequency. The output frequency drift including frequency temperature characteristics and frequency voltage characteristics response to ambient temperature.
Taking the frequency at 25°C as the reference, the change in frequency in response to ambient temperature.
Frequency voltage characteristics
Taking the output frequency at the central voltage in the operating voltage range as the reference, the change in output frequency to voltage. Causes of this change are changes in crystal deformation, and changes in IC internal constants for IC’s mounted in the oscillator and RTC. The effects of the ICs are larger.
First harmonic crystal vibration state. The AT resonator frequency is determined by the AT resonator thickness of the crystal, but even with the same thickness the third overtone will be approx. 3 times the frequency of the fundamental. With tuning fork type resonators, the second overtone is about six times the fundamental.
The hertz is equivalent to cycles per second, named after Heinrich Hertz.
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of sinusoidal waves, particularly those used in radio and audio applications.
In English, hertz is used as both singular and plural. As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz, 106 Hz), GHz (gigahertz, 109 Hz) and THz (terahertz, 1012 Hz). One hertz simply means “one cycle per second” (typically that which is being counted is a complete cycle); 100 Hz means “one hundred cycles per second”, and so on.
|SI multiples for hertz (Hz)|
|10-1 Hz||dHz||decihertz||101 Hz||daHz||decahertz|
|10-2 Hz||cHz||centihertz||102 Hz||hHz||hectohertz|
|10-3 Hz||mHz||millihertz||103 Hz||kHz||kilohertz|
|10-6 Hz||µHz||microhertz||106 Hz||MHz||megahertz|
|10-9 Hz||nHz||nanohertz||109 Hz||GHz||gigahertz|
|10-12 Hz||pHz||picohertz||1012 Hz||THz||terahertz|
|10-15 Hz||fHz||femtohertz||1015 Hz||PHz||petahertz|
|10-18 Hz||aHz||attohertz||1018 Hz||EHz||exahertz|
|10-21 Hz||zHz||zeptohertz||1021 Hz||ZHz||zettahertz|
|10-24 Hz||yHz||yoctohertz||1024 Hz||YHz||yottahertz|
|Note: Common prefixed units are in bold face|
Insulation resistance (IR)
Resistance between leads, or between lead and case package (conductive package).
Jitter is the deviation in or displacement of some aspect of the pulses in a high-frequency digital signal. The deviation can be in terms of amplitude, phase timing, or the width of the signal pulse. Another definition is that it is “the period frequency displacement of the signal from its ideal location.” Among the causes of jitter are electromagnetic interference (EMI) and crosstalk with other signals. Jitter can cause a display monitor to flicker; affect the ability of the processor in a personal computer to perform as intended; introduce clicks or other undesired effects in audio signals, and loss of transmitted data between network devices. The amount of allowable jitter depends greatly on the application.
Jitter is the time variation of a periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of periodic signals. Jitter is a significant, and usually undesired, factor in the design of almost all communications links (e.g., USB, PCI-e, SATA, OC-48).
Jitter can be quantified in the same terms as all time-varying signals, e.g., RMS, or peak-to-peak displacement. Also like other time-varying signals, jitter can be expressed in terms of spectral density (frequency content).
Jitter period is the interval between two times of maximum effect (or minimum effect) of a signal characteristic that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse.
Load capacitance (CL)
Effective capacitance (series equivalent charge capacitance) of the oscillation circuit as seen from the pins of the crystal oscillator. This capacitance is determined as a condition when the crystal oscillator is connected to the oscillation circuit, and will determine the output frequency. Load capacitance approximation :
|CL = CG X CD / ( CG + CD ) + CS|
|(CS = stray capacitance)|
Maximum drive level (GL)
Rating for the drive level. Current or power input over this level may result in characteristic degradation or destruction.
Maximum supply voltage (VDD – GND)
Maximum rated value for power input to the power supply pin. Input over this value may result in characteristic degradation or destruction.
Nominal frequency (f)
Nominal value of frequency of crystal resonator.
Oven Controlled Crystal Oscillator. Oscillator with additional circuitry and packaging to keep the environment, and thus the temperature range over which the oscillator has to operate, constant.
Operating temperature range (Tsol)
Temperature range where specification characteristics are fulfilled.
Operating voltage (VDD)
Voltage input to VDD pin which will support continuous operation with specification characteristics.
Origin frequency (fo)
Oscillation source frequency of oscillator inside oscillation system.
Circuit needed to oscillate crystal resonator. Circuit will differ with type of resonator and frequency.
Oscillation start time (Tosc)
The time from power on until the waveform stabilises. However, voltage rise times depend on the power supply, therefore the time is measured from a specific set of initial conditions.
Crystal with additional circuitry that forms the crystal output into a square wave.
Output enable (OE)
Output is switched to high impedance, and wired OR connection can be used to select multiple outputs (frequency). OE pin – low. Output is high impedance = disabled. Oscillation is not stopped, so the clock after disabled is cleared is not synchronised with OE (clock is continuous).
Output fall time (tTHL)
The time it takes for the output waveform to change from the high voltage (high level) to the low voltage (low level). Also called waveform fall time.
Output frequency (fo)
The output frequency from the oscillator circuit or the crystal oscillator system.
Output load conditions
The types and quantities (power) of the loads that can be connected to the oscillator.
Calculated for TTL-1 as:
- IOH = -40µA, IOL = 1.6mA
For LS TTL-1 as:
- IOH = -40µA, IOL = 0.4mA
For CMOS-1 = 5pF
- IOH = 0, IOL = 0
but peak current is 0.3mA in transition
Output rise time (tTHL)
The time it takes for the output waveform to change from the low voltage (low level) to the high voltage (high level). Also called waveform rise time. Often specified between 0.4V and 2.4V for TTL or 10% to 90% for CMOS.
Because of the physical properties and geometry of an AT Cut quartz blank, a crystal can vibrate at many frequencies. The lowest frequency is called the fundamental frequency and can be supplied up to about 45 Mhz. Higher frequencies (to over 300 MHz) are achieved by operating the crystal at odd overtones, 3rd, 5th, 7th, 9th and 11th etc. and tuning the circuit so that the crystal oscillates at its designed overtone frequency.
Overtone crystals are specially processed for plane parallelism and surface finish in order to enhance their performance at the required overtone frequency. The overtone frequency is higher than the equivalent harmonic multiple of the fundamental by approximately 25 kHz per overtone.
The pullability of a crystal refers to a crystal operating in the parallel mode and is a measure of the frequency change as a function of load capacitance. Pullability is important to the circuit designer who wishes to achieve several operating frequencies with a single crystal by means of switching various values of load capacitance.
Quartz is the second most abundant mineral in the Earth’s continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2.
Synthetic and artificial treatments:- Not all varieties of quartz are naturally occurring. Due to natural quartz being so often twinned, much of the quartz used in industry is synthesized. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal process. While these are still commonly referred to as quartz, the correct term for this material is silicon dioxide.
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator’s bandwidth relative to its centre frequency. Higher Q indicates a lower rate of energy loss relative to the stored energy of the oscillator; the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Oscillators with high quality factors have low damping so that they ring longer.
The quality factor of oscillators varies substantially from system to system. Systems for which damping is important (such as dampers keeping a door from slamming shut) have Q = 1⁄2. Clocks, lasers, and other resonating systems that need either strong resonance or high frequency stability need high quality factors. Tuning forks have quality factors around Q = 1000. The quality factor of atomic clocks and some high-Q lasers can reach as high as 1011 and higher.
Recommended drive level
Excitation level for optimum oscillation characteristics.
Shunt capacitance (Co)
Charge capacitance between the 2 electrodes in the crystal oscillator.
Soldering conditions that can be assured at mounting. Temperatures or times over these limits may result in characteristic degradation or destruction.
Function that halts crystal resonator oscillation and frequency division. Cuts the current consumed by the oscillators circuit and the frequency division stage.
ST pin – High or open: specified frequency output.
ST pin – Low: output is high level, clock stops.
Because oscillation is halted, there is a delay of maximum 10mS, (0.3mS TEP), before clock output stabilises. If ST is also dropped to low, output is high impedance but output is also unstable after function is restarted for the same reason.
Storage temperature (Tstg)
Maximum absolute rating for the discharged state (no input of voltage, current or power). Exposure to temperature over this level may result in characteristic degradation or destruction. To assure precision, store at room temperature whenever possible.
Temperature Compensated Crystal Oscillator. Oscillator with additional circuitry that incorporates a feedback loop, where changes in temperature are reflected by changes in voltage, which in turn change the frequency to compensate for the temperature change.
Voltage Controlled Crystal Oscillator. Oscillator with additional circuitry that allows the frequency to be changed by varying an external voltage. (See Pullability)