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How to choose a crystal oscillator and Buying Guide

  • 2019-11-13 15:07:39

Crystal oscillators come in a variety of packages and feature a wide range of electrical performance specifications. There are several different types: voltage controlled crystal oscillator (VCXO), temperature compensated crystal oscillator (TCXO), oven controlled crystal oscillator (OCXO), and digitally compensated crystal oscillator (MCXO or DTCXO), each type Have their own unique features. If you need to make the device ready to use, you must use VCXO or a temperature-compensated crystal. If the required stability is above 0.5ppm, you need to select the digital temperature-compensated crystal oscillator (MCXO). The analog temperature-compensated crystal oscillator is suitable for applications requiring a stability between 5 ppm and 0.5 ppm. VCXO is only suitable for products with a stability requirement below 5ppm. In environments where out-of-the-box is not required, OCXO can be used if signal stability exceeds 0.1 ppm.



1. Consideration of frequency stability:


One of the main characteristics of a crystal oscillator is stability over the operating temperature, which is an important factor in determining the price of the oscillator. If the higher the stability is required or the wider the temperature range of the crystal is required, the price of the crystal becomes higher.

The range of -40 to +75 °C specified by industrial standards is often just the habit of designers. If -30 ~ +70 ° C is enough, then you do not have to pursue a wider temperature range. Design engineers must carefully determine the actual needs of a particular application and then specify the stability of the oscillator. Asking for too high a indicator means spending more. Crystal aging is another important factor in causing frequency changes. Depending on the life expectancy of the product, there are several ways to mitigate this effect: crystal aging causes the output frequency to change according to the logarithmic curve, which means that this phenomenon is most noticeable in the first year of product use. For example, a crystal that has been used for more than 10 years has an aging rate of about three times that of the first year. This can be improved by special crystal processing, or by adjustment. For example, voltage can be applied to the control pin (ie, voltage control function can be added).




Other factors related to stability include supply voltage, load variation, phase noise, and jitter, which should be specified. For industrial products, it is sometimes necessary to provide indicators for vibration and shock. Military products and aerospace equipment often have more requirements, such as tolerances when pressure changes, tolerances when exposed to radiation, and so on.


2. Output:

Other parameters that must be considered are output type, phase noise, jitter, voltage characteristics, load characteristics, power consumption, and package form. For industrial products, shock and vibration, as well as electromagnetic interference (EMI) parameters are sometimes considered. The crystal oscillator is HCMOS/TTL compatible, ACMOS compatible, ECL and sine wave output. Each output type has its unique waveform characteristics and uses. Attention should be paid to the requirements of tri-state or complementary output. Symmetry, rise and fall times, and logic levels are also specified for some applications. Many DSP and communication chipsets often require tight symmetry (45% to 55%) and fast rise and fall times (less than 5 ns). Phase noise and jitter: The phase noise obtained in the frequency domain measurement is a true measure of short-term stability. It measures up to 1 Hz of the center frequency and typically measures 1 MHz. The phase noise of the crystal oscillator is improved at frequencies away from the center frequency. TCXO and OCXO oscillators, as well as other crystal oscillators using fundamental or harmonic methods, have the best phase noise performance. An oscillator that uses a phase-locked loop synthesizer to produce an output frequency generally exhibits poor phase noise performance than an oscillator that uses a non-phase-locked loop technique.


Jitter is related to phase noise, but it is measured in the time domain. The jitter in microseconds can be measured with an effective value or a peak-to-peak value. Many applications, such as communication networks, wireless data transmission, ATM and SONET requirements, must meet stringent saturation specifications. It is important to pay close attention to the jitter and phase noise characteristics of the oscillators used in these systems.



3. Power and load effects:


The frequency stability of the oscillator is also affected by oscillator supply voltage variations and oscillator load variations. Proper selection of the oscillator minimizes these effects. The designer should verify the performance of the oscillator under the recommended supply voltage tolerances and load. It is not expected that an oscillator that can only be rated for 15pF will perform well when driving 50pF. Oscillators operating above the recommended supply voltage will also exhibit poor waveform and stability. For devices that require battery power, be sure to consider power consumption. Introducing a 3.3V product is bound to develop an oscillator that operates at 3.3V. The lower voltage allows the product to operate at low power. Most commercially available surface mount oscillators operate at 3.3V. Many perforated oscillators using conventional 5V devices are being redesigned to operate at 3.3V.


4. Package:

Similar to other electronic components, clock oscillators are also available in smaller and smaller packages. Various types and sizes of crystal oscillators can be made according to customers' requirements (please refer to the product manual for details). In general, smaller devices are more expensive than larger surface mount or perforated package devices. Therefore, small packages often have a trade-off between performance, output selection, and frequency selection.




5. working environment:

The actual application environment of the crystal oscillator needs careful consideration. For example, high-intensity vibrations or shocks can cause problems for the oscillator. In addition to possible physical damage, vibration or shock can cause erroneous actions at certain frequencies. These externally induced disturbances can cause frequency jitter, increased noise margin, and intermittent oscillator failure. For applications that require special EMI compatibility, EMI is another priority. In addition to using the appropriate PC motherboard layout techniques, it is important to choose a clock oscillator that provides the least amount of radiation. In general, oscillators with slower rise/fall times exhibit better EMI characteristics.


6. Detection:

For the detection of crystal oscillators, it is usually only possible to use an oscilloscope (which needs to be powered by the board) or a frequency meter. A multimeter or other tester cannot be measured. If there is no condition or no way to judge whether it is good or bad, then only the substitution method can be used, which is also effective.



Common faults of crystal oscillators are: (a) internal leakage; (b) internal open circuit; (c) metamorphic frequency offset; (d) leakage of peripheral capacitors connected to it. From these faults, the high-resistance of the multimeter and the VI curve function of the tester should be able to check the faults of (C) and (D), but this will depend on its damage.


Summary: Generally, some allowance should be left when selecting the device 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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