Timekeeping products are almost ubiquitous and an important part of modern society. Precise timing requires a device like a quartz crystal that oscillates at a precise frequency, as well as an integrated circuit (IC) that controls the device. Packaged clock modules typically contain both crystal and control IC's. Electronic oscillator circuits range in frequency 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 Clock (RTC).
Factors determining kHz crystal selection
When selecting kHz crystals for an application, size and the required frequency are the most important considerations, but some other parameters are also important for designing proper circuits.
These include:
Frequency Tolerance, Stability and Aging
Load capacitance (CL)
Equivalent series resistance (ESR)
Drive level (DL)
working temperature
KHz crystals typically have ASIC requirements, which list the values of the required parameters. ASIC information provides a solid starting point for circuit design. The trend toward miniaturization of electronic circuits means that designers need to pay particular attention to these factors because the size and dense packaging of components 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 a crystal specifies a specific frequency, frequency deviations may occur due to stresses in the manufacturing process or strains on the surface during routine operation. The frequency deviation can be summarized by evaluating three parameters: frequency tolerance, frequency stability, and aging.
The frequency tolerance is defined as the difference between the actual and nominal frequencies of the crystal at+25 ° C. Frequency stability refers to the maximum frequency shift caused by temperature within the set temperature range. In order to improve the crystal accuracy, it is recommended to use kHz XO, which allows for the change of frequency with temperature (Fig. 1) and is calibrated accordingly. Finally, aging is a drift in frequency over time. Seals reduce aging effects but may increase size.
Oscillator frequency versus temperature chart
Figure 1: Oscillator frequency varies with temperature and needs to be considered in the sizing process, particularly for extreme environments. Image source: Epson provided through IEEE)
Load capacitance (CL)
The capacitance between two terminals of a crystal is its load capacitance. Designers need to consider external stray capacitance as it causes frequency drift.
The mismatch between CL and circuit capacitance is of particular concern when using smaller crystals because smaller crystals are more sensitive to capacitance variations. Crystals with low CL values are also particularly sensitive to frequency. To design small circuits in confined spaces, designers often select crystals with higher CL values.
Drive level (DL)
DL is the amount of current required to maintain steady oscillation while minimizing structural damage. It is recommended to select crystals with a DL value equal to or greater than the given circuit drive level to avoid frequency instability or premature failure.
working temperature
The influence of temperature on frequency exceeds the range specified by frequency stability index. Designers need to consider not only the operating temperature of the entire circuit, but also the location of the crystals in the circuit, because some areas are easier to heat than others. In addition, the smaller the circuit and crystal, the tighter the component package, and the more heat the entire system generates. In this case, it is best to use crystals that are better calibrated over a certain temperature range, such as DTCXO or RTC products. DTCXO and RTC modules perform well in compact designs and can also be advantageous in applications that require high stability or low power consumption.
KHz crystals and associated modules
Epson produces a large number of kHz crystals, as well as DTCXO and RTC modules. Some of these products and their specifications are briefly introduced below.
FC3215AN series is a 32.768 kHz crystal with an ESR as low as 35 k Ω and a compact package, making it ideal for portable electronics and space constrained applications. FC3215AN series is applicable to various applications, such as wireless module, Internet of Things, medical, industrial, security monitoring equipment, smart meter, consumer electronics and low-power MCU applications. The range supports an extended operating temperature range from - 40 ° C to+105 ° C with 3.2 mm x 1.5 mm x 0.9 mm encapsulation and standard pin assignments.
FC2012AN series 32.768 kHz crystals are similar to the FC3215AN series specifications, but use smaller 2.05 mm x 1.2 mm x 0.6 mm packages and standard pin assignments. Similarly, the FC2012SN series of crystals (Figure 2) are well suited for a variety of applications, such as wearable devices MCU， And wireless modules in the fields of Internet of Things, medical treatment, industry, security, smart meters, etc.