Crystal oscillators are widely used in military and civilian communication radios, microwave communication equipment, program-controlled telephone exchanges, radio communication test instruments, pagers, mobile phone transmitters, high-end frequency counters, GPS, satellite communications, and remote-controlled mobile devices. They are available in various packages and are characterized by diverse electrical performance specifications. There are several different types: Voltage Controlled Crystal Oscillators (VCXOs), Temperature Compensated Crystal Oscillators (TCXOs), Oven-Operated Crystal Oscillators (OCXOs), and Digitally Compensated Crystal Oscillators (MCXOs or DTCXOs), each with its own unique performance characteristics. If you need your equipment to be ready to use out of the box, you must choose a VCXO or a temperature-compensated crystal oscillator. If a stability requirement of 0.5ppm or higher is needed, a digital temperature-compensated crystal oscillator (MCXO) should be selected. Analog temperature-compensated crystal oscillators are suitable for stability requirements between 0.5ppm and 5ppm. VCXOs are only suitable for products with stability requirements below 5ppm. In environments where out-of-the-box operation is not required, if a signal stability exceeding 0.1ppm is needed, an OCXO can be selected.

Frequency Stability Considerations
One of the key characteristics of a crystal oscillator is its stability within its operating temperature range, a crucial factor determining its price. Higher stability or a wider temperature range generally results in a higher price. The industrial standard range of -40 to +75°C is often merely a matter of design preference; if -30 to +70°C is sufficient, there's no need to pursue a wider temperature range. Design engineers must carefully determine the specific application's needs and then specify the oscillator's stability. Excessively high specifications mean higher costs.

Crystal aging is another significant factor causing frequency variations. Depending on the expected lifespan of the target product, there are various methods to mitigate this effect. Crystal aging causes the output frequency to change logarithmically, meaning this phenomenon is most pronounced in the first year of product use. For example, a crystal used for more than 10 years will age approximately three times faster than in its first year. This can be improved by using special crystal manufacturing processes or by adjusting the voltage applied to the control pins (i.e., adding voltage control functionality).

Other factors related to stability include power supply voltage, load variations, phase noise, and jitter, all of which should be specified. For industrial products, vibration and shock parameters are sometimes also required; military and aerospace equipment often have more stringent requirements, such as tolerances to pressure variations and radiation exposure.

Output Other parameters to consider include output type, phase noise, jitter, voltage characteristics, load characteristics, power consumption, and package type. For industrial products, shock and vibration, as well as electromagnetic interference (EMI), may also be considered. Crystal oscillators can be HCMOS/TTL compatible, ACMOS compatible, ECL, and sinusoidal outputs. Each output type has its unique waveform characteristics and applications. The requirements for tri-state or complementary outputs should be considered. Symmetry, rise and fall times, and logic levels also need to be specified for some applications. Many DSPs and communication chipsets often require strict symmetry (45% to 55%) and fast rise and fall times (less than 5 ns).

Phase Noise and Jitter Phase noise, measured in the frequency domain, is a true measure of short-term stability. It can measure up to 1 Hz from the center frequency and typically up to 1 MHz.

The phase noise of a crystal oscillator improves at frequencies far from the center frequency. TCXO and OCXO oscillators, as well as other crystal oscillators utilizing fundamental or harmonic frequencies, exhibit the best phase noise performance. Oscillators using phase-locked loop synthesizers to generate their output frequencies generally exhibit worse phase noise performance than oscillators using non-phase-locked loop technology.

Jitter is related to phase noise, but it is measured in the time domain. Jitter, expressed in picoseconds, can be measured using RMS or peak-to-peak values. Many applications, such as communication networks, wireless data transmission, ATM, and SONET, require stringent jitter specifications. Close attention must be paid to the jitter and phase noise characteristics of oscillators used in these systems.

Power Supply and Load Effects: The frequency stability of an oscillator is also affected by variations in the oscillator power supply voltage and the oscillator load. Proper oscillator selection can minimize these effects. Designers should test the oscillator's performance under the recommended power supply voltage tolerances and load conditions. An oscillator rated to drive only 15pF cannot be expected to perform well when driving 50pF. Oscillators operating beyond the recommended supply voltage will also exhibit poor waveform and stability.

For battery-powered devices, power consumption must be considered. Introducing a 3.3V product necessitates the development of an oscillator operating at 3.3V.

Lower voltages allow products to operate at lower power. Most commercially available surface-mount oscillators currently operate at 3.3V. Many through-hole oscillators using traditional 5V components are being redesigned to operate at 3.3V.

Packaging
Similar to other electronic components, clock oscillators are increasingly using smaller packages. Dapu Communication Technology Co., Ltd. can manufacture various types and sizes of crystal oscillators according to customer needs (see product manuals for details). Generally, smaller devices are more expensive than larger surface-mount or through-hole packaged devices. Therefore, smaller packages often involve trade-offs between performance, output selection, and frequency selection.

Operating Environment
The actual operating environment of a crystal oscillator needs careful consideration. For example, high-intensity vibration or shock can cause problems for oscillators.

Besides potential physical damage, vibration or shock can cause erroneous operation at certain frequencies. These externally induced disturbances can cause frequency fluctuations, increased noise levels, and intermittent oscillator failure.

For applications requiring specific EMI compatibility, EMI is another priority. In addition to employing appropriate PCB layout techniques, it is crucial to select a clock oscillator that provides minimal radiation.

Generally, oscillators with slower rise/fall times exhibit better EMI characteristics.

Testing Crystal oscillators can typically only be tested using an oscilloscope (which requires power to the circuit board) or a frequency meter. Multimeters or other testing instruments cannot measure their functionality. If there are no available methods or means to determine their condition, substitution is the only effective method.

Common crystal oscillator faults include: (a) internal leakage; (b) internal open circuit; (c) deteriorated frequency deviation; (d) leakage in connected external capacitors. Based on these faults, using the high-resistance setting of a multimeter and the VI curve function of a tester should be able to detect faults in items (C) and (D), but this will depend on the extent of the damage.

In summary, when selecting components, it is generally necessary to leave some margin to ensure product reliability. Selecting higher-end components can further reduce the probability of failure and bring potential benefits; this should also be considered when comparing product prices. To achieve a balanced and reasonable "overall performance" of the oscillator, it is necessary to weigh factors such as stability, operating temperature range, crystal aging effect, phase noise, and cost. Here, cost includes not only the price of the components but also the total cost of using the product over its entire lifespan.