About Crystal Oscillator

When comparing and selecting a crystal, the designer should consider the following 10 key parameters.

1, the frequency range of the operating frequency crystal oscillator is generally between 1 and 70MHz. But there are also special low-frequency crystals such as the universal 32.768kHz clock crystals. The physical thickness of the crystal limits its frequency limit. Thanks to the development of manufacturing technologies such as reverse countertops (inverted mesa), the frequency limit for crystals has increased from 30MHz in previous years to 200MHz. The operating frequency is usually given at 25°c at the operating temperature.

A higher frequency crystal with a frequency of more than 200MHz output can be achieved using a pan-frequency crystal. In addition, the crystal oscillator with the built-in PLL frequency multiplier can provide frequencies above 1GHz. When UHF and microwave frequencies are required, the acoustic wave (SAW) oscillator is a choice.

2, frequency accuracy: Frequency accuracy is also called frequency tolerance, the indicator measures the actual frequency of the crystal oscillator in the application of the frequency value of the approximate degree. The commonly used method of the table is the offset percentage or the million-percent (ppm) ratio of a particular frequency. For example, for a 10MHz crystal with a precision ±100ppm, its actual frequency is between 10mhz±1000hz.

(100/1,000,000) x10,000,000=1000hz

It has the same meaning as the following: 1000/10,000,000=0.0001=10-4 or 0.01%. The typical frequency accuracy range is 1 to 1000ppm, given at the initial 25°c. The crystal oscillator with high precision is given at 1 billion (ppb).

3. Frequency stability The indicator measures the degree of divergence between the actual frequency and the nominal frequency within a specific temperature range (e.g. 0°c to 70°c and-40°c to 85°c). Stability is also given in ppm, which varies greatly from 10 to 1000ppm depending on the type of crystal (Figure 2).

4, aging refers to the frequency of the long-term passage of time and the change, generally in weeks, months or years of calculation. It is independent of temperature, voltage and other conditions. Major frequency changes occur during the first few weeks of the use of the crystal oscillator. This value can be between 5 and 10ppm. After the initial period, the rate of frequency change caused by aging will be slowed to several ppm.

5, the output has a different kind of output signal crystal oscillator. The output is mostly pulse or logic level, but there are also sine and embedded sine wave outputs. Some common digital outputs include: TTL, Hcmos, ECL, PECL, CML, and LVDS.

Many of the digital outputs have a duty ratio of 40%/60%, but some models can achieve a 45%/55% output duty-free ratio. Some models also offer three-state outputs. The maximum load is also usually given in the form of fan or capacitance (PF).

6. Working voltage many crystal oscillator works in 5V DC. But new products can work at 1.8, 2.5 and 3.3V.

7. Start-Up time the specification measures the time required for the system to stabilize the output after power-on. Within some devices, there is a enable pin that controls the output of the crystal oscillator on/off.
8. Phase noise is a key indicator when the frequency is high or the application requires an ultra-stable frequency. It is apparent that the output frequency is short of random drift. It is also called jitter, which produces some kind of phase or frequency modulation. The indicator is measured in frequency range with Spectrum analyzer, and dbc/hz is generally used to express phase noise.

9, Adjustable (pullability) The indicator is the frequency at which the voltage can be generated by applying an external control voltage to a controlled crystal oscillator (VCXO). It represents the maximum possible frequency variation, usually expressed in ppm. The control voltage level is also given, and sometimes a linear value expressed as a percentage is provided. The typical DC control voltage range is 0 to 5V. The linear relationship between the frequency change and the control voltage can be a problem.

10, packaging crystal oscillator has many kinds of packaging patterns. In the past, the most common is the metal shell package, but now it has been replaced by the updated table-mount (SMD) package. Metal packages named HC-45, HC-49, HC-50, or HC-51 typically use standard dip through hole pins. The common SMD package size is 5x7mm. Due to the requirements of cellular handset manufacturers, the trend of SMD packaging is more and more thin.

Type of crystal oscillator

There are 4 basic integrated crystals: Clock Crystal (XO), Voltage controlled crystal (VCXO), temperature compensated crystal (TCXO) and thermal cavity controlled crystal oscillator (OCXO).

The base XO is the encapsulation of crystals with their oscillators. The frequency is generally fixed, but in some designs, an adjustable capacitor is provided to compensate for the frequency drift caused by aging. The XO is used in the least critical design, and is typically used as a clock crystal for a processor or other digital chip. Typical accuracy is from 10ppm to hundreds of ppm, and the aging indicator is ±1 to ±5ppm/year (Figure 5).

The TCXO has built-in circuitry to compensate for the frequency variations that accompany temperature changes. Therefore, the TCXO can provide a much more accurate and stable output frequency required by many applications. Mobile phones and two-way wireless communication are common examples of applications.

The simplest way is to use a thermistor temperature sensor in the circuit to manipulate a piezoelectric capacitor in the feedback circuit to make the crystal oscillator frequency more stable.

A more precise approach

For example, a microcontroller-controlled crystal oscillator (MCXO) uses an embedded controller to process the temperature input based on the desired algorithm and use a digital-to-analog converter to manipulate a "traction-adjusted" voltage-change capacitor. The available crystal frequencies are from about 1 to 60MHz. Typical stability parameters range from ±0.2 to ±2.5ppm, and the aging rate is ±0.5 to ±2ppm/years.

The VCXO and XO are optimized for external frequency control in the form of DC input, and the external voltage controls an internal voltage-variable capacitor to change its capacitance to achieve a small range of output frequency adjustments. They are primarily designed for use as VCOs in narrow-band PLLs.

Its common frequency range is from 1 to 60MHz. The typical variable range is from ±10 to ±2000ppm. The aging rate is usually in ±1 to ±5ppm/years. Its temperature-compensated version is called Vctcxo, and there are some other variant combinations.

Silicon Labs offers an internal multi-PLL with a DSP filter in the 10MHZ to 1.4GHz output frequency range of XO and Vcxo (Figure 6). Si530 and Si550 are programmed by the manufacturer, and the Si570 is user-programmable via the I²c bus. The range of stability is selectable from ±20 to ±200ppm. The size of the surface package they use is 5x7mm.
Ocxo The crystal and sometimes the entire oscillator circuit in a small heating chamber. A DC heating element in the feedback loop makes the temperature of any precise and stable output frequency almost constant. OCXO is the best choice for critical applications such as cellular base stations, telecommunications, local area and wide area networks such as SONET and GPS.

But the OCXO consumes much more power, typically between 1 and 3 watts. The typical stability indicator is ±1x10-8. The typical aging rate is between ±0.2ppm/years and ±2.0x10-8/years. If another ocxo is placed in the second hot chamber, the OCXO architecture can also be implemented with higher accuracy and stability.

About Crystal Oscillator