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Crystal definition

Quartz crystal oscillator,A quartz resonator is simply referred to as a crystal oscillator, which is made of a quartz crystal piece having a piezoelectric effect.

      This quartz crystal flake generates mechanical vibration when subjected to an applied alternating electric field. When the frequency of the alternating electric field is the same as the natural frequency of the quartz crystal, the vibration becomes very strong, which is the reaction of the crystal resonance characteristic.

With this feature, it is possible to replace LC (coil and capacitor) resonant circuits, filters, etc. with quartz resonators. Quartz resonators are used in household appliances and communication equipment because of their small size, light weight, high reliability, and high frequency stability.

Crystal vibration classification

Divided into the following four categories by major categories

1, simple crystal oscillator (XO)

This is the most basic type, and its stability is completely determined by the inherent characteristics of the crystal resonator itself. Higher frequency crystals in the MHz range are made of quartz rods and are manufactured in a manner that provides relative stability even when the ambient temperature varies from -55 ° C to +125 ° C (-67 ° F to +257 ° F) Frequency of. Even within such a wide temperature range, a properly cut quartz crystal can achieve a stability of ±25 ppm. The performance of crystal oscillators has been greatly improved compared to other passive resonators such as LC oscillating circuits with temperature variations of up to 1% (10,000 ppm) or higher. But for some applications, even 25ppm is not good enough, so additional measures must be taken.

2. Temperature compensated crystal oscillator (TCXO)

If the natural frequency and the temperature stability of the quartz crystal do not meet the application requirements, a temperature compensation unit can be used. The TCXO uses a temperature sensing element and a circuit that produces a voltage curve that is completely opposite to the frequency of the crystal over the entire temperature range, so it is ideal to counteract crystal drift. TCXO's typical stability specifications range from less than ±0.5ppm to ±5ppm, depending on the type and temperature range of the TCXO.

3. Thermostatically controlled crystal oscillator (OCXO)

For some applications, the TCXO's frequency-temperature stability specifications are still not sufficient. In these cases, OCXO may be required. As the name implies, an oscillator with a roasting cavity heats the crystal to a higher temperature, but is still controlled so that the temperature of the crystal remains stable even if the ambient temperature changes greatly. The frequency-ambient temperature stability is significantly improved due to the small changes in the temperature of the crystal and the sensitive portion of the oscillator. The OCXO has a stability of 0.001 ppm over the ambient temperature range. However, this increase in stability is at the expense of increased power consumption, which of course requires energy to provide heat to the oven. A typical OCXO may require 1 to 5 W of power to maintain the internal temperature. After starting up, you also need to wait for the warm-up time of stable temperature and frequency. Depending on the type of crystal oscillator, the warm-up time is usually from 1 minute to more than 10 minutes.

4, voltage controlled crystal oscillator (VCXO)

In some applications, it is desirable to be able to tune or adjust the frequency of the oscillator to phase lock it to a reference in a phase locked loop (PLL) or possibly to adjust the waveform. The VCXO provides this functionality through electronic frequency control (EFC) voltage input. For some specialized components, the VCXO's tuning range specification may range from ±10ppm to ±100ppm (or even higher).

Crystal selection

5 elements

1, the output frequency

The most basic property of any oscillator is the frequency it generates. By definition, an oscillator is a device that accepts an input voltage (usually a DC voltage) and produces a repetitive AC output at a certain frequency. The frequency required is determined by the type of system and how the oscillator is used.

Some applications require low frequency crystals in the kHz range. A common example is the 32.768 kHz hand (clock) crystal. But most current applications require 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 by system requirements. The stability of the oscillator can be simply expressed as: the frequency change due to some reasons divided by the center frequency. (ie: stability = frequency change ÷ center frequency)

3. Input voltage and power

Any type of crystal can typically be designed to operate with the DC input supply voltage already present in the system. In digital systems, it is often desirable to drive the crystal with a voltage that matches the voltage used by the logic devices in the system that the oscillator will drive, so that the logic levels are directly compatible. +3.3V or +5V are typical inputs for these digital units. Other devices with higher power outputs can use higher voltages, such as +12V or +15V. Another consideration is the amount of current required to power the device. XO or TCXO may only require a few mA, so in low voltage systems, the power consumption can be less than 0.01W. On the other hand, some OCXOs may require 5W or 6W at power up.

4, the output waveform

Then select the output waveform to match the load that the oscillator will drive in the system. One of the most common outputs is CMOS - driving the logic level input. The CMOS output will be a square wave that swings between ground potential and the system's Vdd rail. For higher frequencies above about 100 MHz, a differential square wave is typically used. These oscillators feature two 180° out-of-phase outputs with fast rise and fall times and very small jitter. The most common types are LVPECL and LVDS. If the oscillator is used to drive an RF component, such as a mixer or other device with a 50Ω input impedance, a sine wave output of a certain power level is typically specified. The resulting output power is typically between 0dBm and +13dBm (1mW to 20mW), although higher power can be output if needed.

5, package size and shape

The requirements for crystal package will vary widely based on the type and specification of the oscillator. Simple clock oscillators and some TCXOs can be packaged in packages as small as 1.2 x 2.5 mm2; some OCXOs can be as large as 50 x 50 mm2, even larger for certain designs. While some through-hole packages such as dual in-line 4 or 14-pin types are still used for larger components (such as OCXO or dedicated TCXO), most current designs use surface-mount packages. These surface mount configurations can be sealed ceramic packages, or FR-4 based components with built-in I/O.

6, summary

When selecting the device, it is generally necessary to leave some margin to ensure the reliability of the product. The use of higher-end devices can further reduce the probability of failure and bring potential benefits, which should be considered when comparing product prices. In order to balance the "whole performance" of the oscillator, it is necessary to weigh various factors such as stability, operating temperature range, crystal aging effect, phase noise, cost, etc. The cost here does not only include the price of the device. It also includes the cost of using the product for its entire life.

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