Five things to consider when choosing a crystal oscillator
The key component in any oscillator is the resonator which will control the frequency and determine what stability specifications may be achieved. While it is possible to implement a simple oscillator with an LC or RC resonator that may suffice for some applications, the addition of a quartz crystal will greatly improve the frequency stability of the device by several orders of magnitude, often with a minimal cost impact.
1. Output Frequency
The most fundamental attribute of any oscillator is the frequency that it will produce. By definition, an oscillator is a device that accepts an input voltage (usually a DC voltage) and produces a repetitive AC output at some frequency. The frequency that is needed is dictated by the type of system and how it will be used.
Some applications call for low frequency crystals in the kHz range. A common example would be a watch crystal at 32.768 kHz. But most current applications need higher frequency crystals ranging from less than 10 MHz to greater than 100 MHz.
2. Frequency Stability and Temperature Range
The required frequency stability is determined from the system requirements. The stability of an oscillator is simply given as the change in frequency due to some phenomenon divided by the center frequency.
That is: Stability = Change in Frequency ÷ Center Frequency
For example, if the oscillator output frequency is 10 MHz and it changed 10 Hz over temperature, it’s temperature stability would be: 10/10,000,000 = 1×10-6 = 1ppm. Typical stabilities for a crystal oscillator could range from 100ppm to 0.001ppm.
The frequency stability is usually determined by the requirements of the application and will subsequently determine the type of crystal oscillator that will be needed. The temperature range over which the oscillator must operate is a major factor in determining the stability that can be achieved.
Crystal Oscillator Types
Simple Crystal Oscillator (XO): This is the most basic type where the stability is totally determined by the inherent characteristics of the crystal resonator itself. The higher frequency crystals in the MHz range are fabricated from a quartz bar in such a way as to provide a relatively stable frequency even though the ambient temperature may vary as much as -55°C to +125°C (-67°F to +257°F). A stability of ±25ppm is achievable with a properly cut quartz crystal even over this wide of a temperature range. This is a substantial improvement over other passive resonators such as an LC tank circuit which may change 1% or more (10,000ppm). But even 25ppm is not good enough for some applications so additional measures must be employed.
Temperature Compensated Crystal Oscillator (TCXO): If the inherent frequency versus. temperature stability of the quartz crystal is not adequate for an application, a temperature compensated unit may be employed. A TCXO uses a temperature sensing device along with circuitry which generates a voltage curve which is the exact inverse of the crystal over the temperature range and ideally cancels out the drift of the crystal. Typical stability specifications for a TCXO range from less than ±0.5ppm to ±5ppm depending on the type of TCXO and the temperature range.
Oven Controlled Crystal Oscillator (OCXO): For some applications the frequency versus temperature stability of a TCXO will still be inadequate. In these cases, an OCXO may be called for. As the name implies, an oscillator with an oven heats the crystal to an elevated temperature which is controlled so that the temperature of the crystal remains stable even though the ambient temperature may vary widely. Since the temperature of the crystal and the sensitive portions of the oscillator see very little variation, the frequency versus. ambient temperature stability is substantially improved. The stability of an OCXO may be as tight as 0.001ppm over the ambient temperature range. This improved stability, however, comes with the cost of increased power consumption in order to supply the heat to the oven. A typical OCXO may require from 1- to 5-W of power to maintain the internal temperature. A warmup period after turn on is also needed to wait until the temperature and frequency have stabilized, typically from 1 minute to greater than 10 minutes depending on the type of unit.
Voltage Controlled Crystal Oscillator (VCXO): In some applications it is desirable to be able to tune or adjust the frequency of the oscillator in order to phase lock it to a reference in a phase locked loop or possibly to modulate the waveform. A VCXO provides this capability via an Electronic Frequency Control (EFC) voltage input. The tuning range specification for a VCXO may vary from ±10ppm to ±100ppm or even higher for some specialized devices.
TCVCXO and VCOCXO: A TCXO or OCXO will often include an EFC input voltage. This allows adjustment in order to calibrate the output frequency precisely to a nominal value.
3. Input Voltage and Power
Crystal oscillators of any type can usually be designed to operate with a DC input supply voltage that is already available in the system. In a digital system it is normally desirable to use a voltage which matches the voltage used by the logic devices in the system that the oscillator will be driving so that the logic levels will be directly compatible. +3.3 V or +5 V are common inputs for these digital units. Other devices with higher power outputs may use higher voltages such as +12 V or +15 V. Another consideration is the amount of current that is needed to power the device. An XO or TCXO may only need a couple of milli-amps so in a low voltage system they could operate on less than 0.01 W. On the other hand, some OCXO’s could draw 5 W or 6 W at turn on.
4. Output Waveform
The output waveform would then be selected to match the load that the oscillator will be driving in the system. One of the most common outputs would be CMOS to drive logic level inputs. A CMOS output would be a square wave swinging between ground and the Vdd rail for the system. For higher frequencies greater than about 100 MHz a differential square wave is often used. These oscillators have two outputs 180° out of phase with fast rise and fall times and very little jitter. The most popular types are LVPECL and LVDS. If the oscillator is used to drive RF components such as a mixer or other devices with a 50 Ω input, a sinewave output at some power level is usually specified. The output power produced would typically fall between 0 dBm and +13 dBm (1 mW to 20 mW) although higher power may be possible if needed.
5. Package Size and Outline
The package required for a crystal oscillator will vary widely depending on the type of oscillator and the specifications. Simple clock oscillators and some TCXO’s can be housed in packages as small as 1.2- x 2.5-mm while some OCXO’s may be as large as 50- x 50-mm or even larger for some particular designs. Although some through-hole packages such as dual-in-line 4 or 14 pin types are still used for larger parts such as the OCXO’s or specialized TCXO’s, the majority of current designs use surface mount packages. These surface mount configurations may be either a hermetically sealed ceramic package or an FR-4 based assembly with castellations for the I/O’s.
As has been shown, there are many different options to be considered when specifying a crystal oscillator. However, by examining the system in which the unit will be used, the most convenient selections will become obvious such as the input voltages that are available to power the unit and the type of device that the output of the oscillator will be driving. Other constraints of the application such as physical size and the operating environment must also be taken into account. In addition to these basic parameters there is a multitude of other specifications that may be invoked for specific applications. But when all things are considered, it is likely that a crystal oscillator can be found to satisfy the requirements of your system.
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