Five key points to consider when choosing a quartz crystal

In the design, most electronic systems need some kind of oscillator as their key function block. Some typical uses include: clocks in digital systems for synchronous operation; stable RF signals for receivers or transmitters; precision frequency references for accurate measurements; or real-time clocks for precise timing. The specification of the system and the role of the required oscillator will determine most of the crystal's parameters.

The key component in any oscillator is a resonator that will control the frequency and determine what level of stability can be achieved.

Although simple oscillators implemented with LC or RC resonators can meet the requirements of some applications, the addition of quartz crystals can greatly increase the frequency stability of the devices by several orders of magnitude, and the cost usually required is small.

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 a 32.768 kHz hand (clock) watch crystal. However, 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 the system requirements. The stability of the oscillator can be simply stated as the frequency change due to some reason divided by the center frequency.

Namely: stability = frequency change ÷ center frequency

For example, if the oscillator output frequency is 10 MHz and the temperature changes by 10 Hz, the temperature stability is: 10/10,000,000=1x10-6=1ppm. The typical stability of the crystal can be between 100ppm and 0.001ppm. Frequency stability is usually determined by the application requirements, and then determine the type of crystal 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 type

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 such a way that the ambient temperature can vary from -55°C to +125°C (-67°F to +257°F). Stable frequency. Even at such a wide temperature range, a properly cut quartz crystal can achieve ±25ppm stability. Compared to other passive resonators, such as LC oscillating circuits whose temperature varies up to 1% (10,000 ppm) or more, the performance of the crystal oscillator has been fundamentally improved. But for some applications, even 25 ppm is not good enough, so additional measures must be taken.

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. TCXO uses a temperature sensing device and a circuit that generates a voltage curve that is completely opposite to the frequency change of the crystal over the entire temperature range, so that crystal drift can be ideally cancelled. TCXO's typical stability specifications range from less than ±0.5ppm to ±5ppm depending on the type and temperature range of the TCXO.

Constant temperature controlled crystal oscillator (OCXO): For some applications, the frequency-temperature stability of the TCXO is still not satisfactory. In these cases, OCXO may be required. As the name implies, an oscillator with a baking chamber heats the crystal to a higher temperature, which is controlled so that the temperature of the crystal remains stable even though the ambient temperature may vary greatly. Due to the small changes in the temperature of the crystal and the sensitive part of the oscillator, the frequency-ambient temperature stability is significantly improved. In the ambient temperature range, OCXO stability can reach 0.001ppm. However, this increase in stability is at the expense of increased power consumption. Providing heat to the baking chamber of course requires energy. 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 when the temperature and frequency are stable. It depends on the type of crystal. The warm-up time is usually from 1 minute to more than 10 minutes.

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, or it may also be to modulate the waveform. The VCXO provides this function via electronic frequency control (EFC) voltage input. For some dedicated devices, the VCXO's tuning range specification may be ±10ppm to ±100ppm (even higher).

TCVCXO and VCOCXO: The TCXO or OCXO usually includes the EFC input voltage. This allows adjustments to accurately calibrate the output frequency to a nominal value.

3, input voltage and power

Any type of crystal oscillator can usually be designed to operate with the DC input supply voltage already in the system. In digital systems, it is often desirable to drive the crystal using a voltage that matches the voltage used by the logic device in the system that the oscillator will drive, so that the logic levels are directly compatible. +3.3V or +5V is a typical input for these digital units. Other devices with higher power output can use higher voltages, such as +12V or +15V. Another consideration is the amount of current needed to power the device. XO or TCXO may only need a few mA, so in a low-voltage system, its 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 - to drive logic level inputs. 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, differential square waves are often used. These oscillators have 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 50-Ω input impedance, a power level sine wave output is usually specified. The resulting output power is usually between 0dBm and +13dBm (1mW to 20mW), although higher power can be output if needed.

5, package size and shape

Based on the oscillator type and specifications, the requirements for the crystal package will be very different. Simple clock oscillators and some TCXOs can be housed in packages as small as 1.2 x 2.5mm2; some OCXOs can be as large as 50 x 50mm2, and even larger for some specific designs. Although some via packages such as dual inline 4 or 14 pin types are still used for larger components (such as OCXOs or dedicated TCXOs), most current designs use surface mount packages. These surface mount configurations can be sealed ceramic packages, or FR-4 based components with a construction for I/O.

In summary, there are many different options to consider when choosing a crystal. However, by examining the system that uses the crystal oscillator, the most convenient choice will become apparent; for example, factors such as the input voltage that can be used to power the crystal oscillator and the type of device the oscillator output will drive. It must also consider other constraints applied, such as physical dimensions and operating environment. In addition to these basic parameters, there are many other specifications to consider for specific applications. However, after considering these factors, it is likely to find a crystal that meets the system requirements.