Just as most electronic products today contain at least one embedded controller, most also have at least one crystal oscillator. In fact, some multiprotocol networking and telecom equipment can contain 10 or more different crystals.
A crystal oscillator usually sets the processor clock frequency and operational frequencies of networking speed or wireless channels. Crystals provide the accurate timing required by most modern products, in addition to the precision demanded by the FCC in setting operational wireless and networking frequencies.
When designing your products, you can opt to make your own crystal oscillator or design in one of the many available pre-packaged crystal oscillators. In some cases, all you do is connect the appropriate crystal (plus two capacitors) to the processor or other chip, which has the oscillator circuitry built in. Other cases require a separate oscillator.
In these instances, investing the development time and money in designing and building your own crystal oscillator no longer makes economic or time-to-market sense. Electronic design today is more about putting together components and chips to form a system rather than creating detailed circuits. Now, crystal oscillators have evolved into an off-the-shelf subsystem component.
THE MAGIC CRYSTAL
Crystal oscillators are virtually mandatory in more complex modern electronic products. Made of pure quartz, these thin slivers vibrate at a precise and very stable frequency. Their ability to be set to almost any desired frequency and maintain that frequency over a wide range of temperature and voltage variations makes them inordinately better than any RC or LC oscillator.
Quartz is a crystalline structure found in nature and the second most common material found in the earth’s surface next to feldspar. Its chemical composition is silicon dioxide (SiO2), but its piezoelectric characteristics make it special. Piezoelectricity is a material’s ability to generate a voltage when stressed mechanically or to vibrate at a precise frequency if excited by a voltage. This latter characteristic makes quartz the frequency-determining component of choice for most applications.
While quartz crystals are readily found in nature, they can be synthesized. Pure quartz crystal is formed by melting a mined material called lasca in an autoclave and using a seed crystal. Such crystals are then cut into slivers and ground to the desired thickness that sets the frequency of operation.
The geometry and angle of the slice cut from the crystal determines its stability and other characteristics. Different cuts are referred to by designations such as AT, SC, and X cuts. Two plates of silver are deposited on opposite faces of the crystal, and mounting leads are attached to them. The completed assembly is mounted in an enclosure, usually metal.
The crystal itself looks like a series resonant circuit with equivalent inductive, capacitive, and resistive components (Fig. 1a). Placing the crystal in a holder produces a parallel capacitance, with the crystal serving as the dielectric between the two holding plates. This combination produces a unique circuit with both series and parallel resonances (Fig. 1b).
A crystal may be operated in either its series or parallel or anti-resonant modes, depending on the oscillator circuit used. The parallel mode is usually avoided because it’s less stable. However, the frequency range between the series and parallel resonant points is commonly used. This area is known as the parallel mode range.
When operating in the parallel mode, the external capacitance across the crystal will determine the operating frequency. Called the load capacitance, this reactance is any stray or distributed capacitance on the printed-circuit board (PCB) and in the oscillator circuit. Usually in the 3- to 20-pF range, it must be specified when ordering a crystal to be used in a parallelmode circuit.
You can also add a series or parallel capacitor to a crystal to “pull” its resonant frequency over a narrow range. This feature permits minor adjustments to the frequency, as well as the ability to produce a variable-frequency crystal oscillator for use in phase-locked loops (PLLs).
Most crystals also oscillate at higher overtone frequencies. The third and fifth overtones are the most common. An overtone is an approximate third, fifth, or other odd multiple of the primary resonant frequency. A harmonic of a fundamental frequency is an exact multiple, while the overtone is a close but not exact multiple.
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Since the fundamental oscillating frequency of a typical crystal is limited to about 30 to 50 MHz maximum, the overtone mode of oscillation is one way to achieve crystal precision and stability at higher frequencies. When specifying an overtone crystal, it’s important to stress the exact frequency needed so that the manufacturer can produce the appropriate fundamental frequency in the crystal.
Designers should consider 10 key specifications when comparing and selecting crystal oscillators.
Frequency of operation: Crystal oscillators come in a frequency range of approximately 1 to 70 MHz. Special lower-frequency crystals like the popular 32.768-kHz watch crystal are also available. The physical thinness of the crystal limits the upper frequency range. That limit has gone from about 30 MHz in past years to about 200 MHz, thanks to the development of new manufacturing techniques like the inverted Mesa. The operating frequency is usually stated at a temperature of 25°C.
Higher-frequency oscillators can be obtained by using overtone crystals that take the output to over 200 MHz. In addition, oscillators with built-in PLL frequency multipliers can reach frequencies beyond 1 GHz. When UHF and microwave frequencies are required, the surface-acoustic wave (SAW) oscillator is an option.
Frequency accuracy: Also known as frequency tolerance, frequency accuracy measures how close the crystal frequency is to the desired value as determined by the application. It’s often expressed as a percentage deviation from the specified frequency or in parts per million (ppm). For example, an accuracy of ±100 ppm of a 10-MHz crystal oscillator means that the actual frequency could deviate from 10 MHz by ±1000 Hz:
(100/1,000,000) × 10,000,000 = 1000 Hz
This is the same as 1000/10,000,000 = 0.0001 = 10-4 or 0.01%. Typical oscillator accuracies range from 1 to 1000 ppm, stated at an initial temperature of 25°C. Very high-accuracy crystals are specified in part per billion (ppb).
Frequency stability: This measures how much the frequency deviates from the desired value over a specific temperature range, like 0°C to 70°C and –40°C to 85°C. The stability is also stated in ppm and varies widely depending on the oscillator type from 10 to 1000 ppm (Fig. 2).
Aging: Aging is the change in frequency over a long period of time, usually measured in weeks, months, or years. It’s independent of temperature, oscillator voltage, and other conditions. Most aging frequency change occurs in the first several weeks after the oscillator is turned on. It can be as much as 5 to 10 ppm. After that initial period, the aging frequency change flattens out to a few ppm.
Output: Crystal oscillators are available with different types of output signals. Most are pulse or logic levels, but additionally, there are sine-wave and clipped-sine outputs. Some common digital outputs include TTL, HCMOS, ECL, PECL, CML, and LVDS.
Most digital outputs have a 40%/60% duty cycle, but a 45%/55% output is attainable in some models. A tri-state output may also be available in some models. The maximum load is also specified and is usually given as a fan-out number or as a capacitance in picofarads.
Operating voltage: Most crystal oscillators operate from 5 V dc. But newer designs offer 1.8-, 2.5-, and 3.3-V operation.
Start time: This is the amount of time that the output takes to be stabilized after power turn-on. In some devices, an enable pin is available to switch the oscillator output off and on.
Phase noise: Phase noise is a critical specification at very high frequencies and in applications requiring exceptional stability. This is the rapid short-term random variation in output frequency. Also called jitter, it produces a type of phase or frequency modulation. Measured in the frequency domain with a spectrum analyzer, phase noise is usually stated in terms of dBc/Hz.
A sine-wave output from an oscillator with no phase noise, called the carrier, would be shown as a single vertical straight line at the frequency of operation. The phase noise produces sidebands above and below the carrier. The amount of phase noise is expressed as the ratio of the sideband power amplitude (Ps) to the carrier power amplitude (Pc) in decibel form:
phase noise in dBc = 10 log (Ps/Pc)
Phase noise is measured at increments from the carrier of 10 kHz or 100 kHz, though other frequency increments down to 10 or 100 Hz are also used. The phase noise measurements are commonly normalized to a 1-Hz equivalent bandwidth. Typical phase noise values range from –80 to –160 dBc, depending on the frequency increment from the carrier.
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Pullability: This is a measure of the amount of frequency variation that can be achieved by applying an external control voltage to a voltage-controlled crystal oscillator (VCXO). It represents the maximum deviation possible and is usually expressed in ppm. The control-voltage level is also given, and a linearity value in percent is sometimes provided. Typical dc controlvoltage values fall in the 0- to 5-V range. The linearity of the frequency variation with the control voltage may be an issue.
Packaging: There are many different types of crystal oscillator packages. In the past, metal can packages were the most common, but they’ve been overtaken by newer surface-mount (SMD) packages. Designated as HC-45, HC-49, HC-50, or HC-51, the metal packages commonly have standard DIP through-hole pins. A common SMD package size is 5 by 7 mm. The trend has been to make the packages thinner as demanded by cell-phone manufacturers.
COMMON OSCILLATOR CIRCUITS
Dozens of circuits have been developed for crystals. Most are variations of the common Colpitts, Pierce, and Clapp varieties. The circuit determines whether the crystal operates in its series or parallel mode.
In series-mode oscillators, one of the two logic inverters is biased into the linear region for amplification by R1 (Fig. 3). C1 is a dc blocking capacitor. R2 sets the optimum crystal drive current. The crystal operates in the series mode. This type of oscillator isn’t widely used because it’s less stable than one that operates in the parallel mode.
For example, Pierce oscillators are commonly used inside embedded controllers (Fig. 4). The crystal along with C1 and C2 are external to the processor. Again, R1 biases the inverter into the linear region. Capacitors C1 and C2 provide the necessary 180° phase shift along with the crystal.
The frequency of oscillation is higher than the series resonant frequency, but lower than the parallel resonance figure of the crystal. Capacitors C1 and C2 do affect the frequency of oscillation. The series combination of C1 and C2 plus the external stray capacitance (CS ) of the PCB in parallel with the crystal form what is called the load capacitance:
CL = (C1C2 )/(C1 + C2) + CS
C1 and C2 are normally equal and fall in the 10- to 30-pF range. Stray capacitance is typically 2 to 5 pF. The crystal manufacturer will require you to specify this load capacitance for the desired frequency.
TYPES OF CRYSTAL OSCILLATORS
There are four basic types of packaged crystal oscillators: clock oscillators (XOs), voltage-controlled crystal oscillators (VCXOs), emperature-compensated crystal oscillators (TCXOs), and oven-controlled crystal oscillators (OCXOs).
The base XO is a crystal packaged with its oscillator. The frequency is usually fixed, but in some designs a trimmer capacitor may be provided to make adjustments for aging. XOs are for the least critical designs, usually as clock oscillators for processors or other digital chips. Typical accuracies range from 10 ppm to several hundred, with aging from ±1 to ±5 ppm/year (Fig. 5).
TCXOs incorporate circuitry to compensate for the frequency variations that accompany temperature variations. This results in a far more precise and stable output frequency that’s demanded by many applications. Cell phones and two-way radios are common examples.
The simplest form uses a thermistor temperature sensor in a circuit that operates a varactor (voltage variable capacitor) in a feedback circuit to keep the crystal frequency more constant. More elaborate schemes.
For instance, microprocessor-controlled crystal oscillators (MCXOs) use an embedded controller to process the temperature input according to a desired algorithm to operate a “pull” varactor from a digital-to-analog converter (DAC). Oscillator frequencies are available from about 1 to 60 MHz. Typical stability specifications range from ±0.2 to ±2.5 ppm, with aging rates of ±0.5 to ±2 ppm/year.
VCXOs are XOs optimized for external frequency control by way of a dc input, which varies an internal varactor pulling capacitor to provide a narrow range of output frequency adjustment. They are designed primarily for use as the VCO in a narrowband phase-locked loop (PLL).
Commonly available frequencies range from 1 to 60 MHz. Typical pullability ranges from ±10 to ±2000 ppm. Aging rate is commonly ±1 to ±5 ppm/year. A temperature- compensated version of this called a VCTCXO or some variation thereof is also available.
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Silicon Labs offers XOs and VCXOs that use an internal multiplier PLL with a DSP filter for outputs in the 10-MHz to 1.4-GHz range (Fig. 6). The Si530 and Si550 are programmed by the manufacturer, while the Si570 is user programmable via an I2C bus. Stability options are from ±20 to ±200 ppm. The surfacemount package measures 5 by 7 mm.
OCXOs put the crystal and sometimes the whole oscillator circuit in a small oven. A dc heating element in a feedback loop keeps the temperature virtually constant for very precise and stable output frequency. They are the best choice for critical applications like cellular basestations, telecom, local-area and wide-area networks (e.g., Sonet), and GPS.
Yet they draw much more power, with 1 to 3 W typical. The typical stability figure is ±1 × 10-8. A typical aging rate is from about ±0.2 ppm/year to ±2.0 × 10-8/year. Even improved accuracy and stability can be obtained with an OCXO that encloses one OCXO inside a second oven.
The type of oscillator you need depends specifically upon your application (see the table). David Meaney, engineering sales manager for Fox Electronics, says the highest crystal oscillator volume lies in cell phones and consumer products, followed closely by networking and computer- related applications. It’s essential to work closely with the crystal oscillator manufacturer to ensure you get exactly what you want and need.