Timing products are almost ubiquitous and an important component of modern society. Accurate timing requires devices such as quartz crystals that can oscillate at precise frequencies, as well as integrated circuits (ICs) that control the devices. Encapsulated clock modules typically contain both crystals and control ICs. The frequency range of electronic oscillator circuits varies from kilohertz (kHz) to megahertz (MHz).
KHz crystals can be sold separately or integrated into other products such as crystal oscillators (CXO), digital temperature compensated crystal oscillators (DTCXO), and real-time clocks (RTC).
Factors determining the selection of kHz crystals
When selecting kHz crystals for applications, size and desired frequency are the most important considerations, but other parameters are also crucial for designing suitable circuits.
This includes:
Frequency tolerance, stability, and aging
Load capacitance (CL)
Equivalent series resistance (ESR)
Drive level (DL)
Operating Temperature
KHz crystals typically have requirements for application specific integrated circuits (ASICs), which list the values of the required parameters. ASIC information can provide a solid starting point for circuit design. The trend towards miniaturization of electronic circuits means that designers need to pay extra attention to the above factors, as the size and dense packaging of components can affect the characteristics and performance of crystals. However, the photolithography manufacturing process ensures that the miniaturization of crystal oscillator circuits does not affect the parameters required for their efficient operation.
Frequency tolerance, stability, and aging
Although crystals specify specific frequencies, frequency deviations may still occur due to stress during manufacturing or surface strain during daily operation. Frequency deviation can be summarized by evaluating three parameters: frequency tolerance, frequency stability, and aging.
Frequency tolerance is defined as the difference between the actual frequency and nominal frequency of a crystal at+25 ° C. Frequency stability refers to the maximum frequency shift caused by temperature within a set temperature range. To improve the accuracy of the crystal, it is recommended to use kHz XO, which can take into account the frequency variation with temperature (Figure 1) and perform corresponding calibration. Finally, aging refers to the drift of frequency over time. Sealing can reduce aging effects, but may increase size.