Silicon Frequency Controller (SFC) Technology (I)

The main component of crystal is silicon dioxide, commonly known as quartz. Quartz has extraordinary mechanical and piezoelectric properties, making it a basic clock device since the mid-1840s. Today, wherever a clock is needed, engineers first think of crystals, but as the application continues to deepen, some inherent defects of crystals are also exposed. Today, new technologies are constantly emerging and bringing great changes.

　　Crystal characteristics and parameters

　　Encapsulation

　　The crystal package is shown in Figure 1, which consists of three parts: a metal cover, a quartz plate with electrodes, and a ceramic base. Generally speaking, nitrogen needs to be filled into the sealed shell.

　　Figure 1 Crystal package diagram

　　Currently, almost all ceramic seals are provided by three Japanese companies, but due to the earthquake and tsunami in Japan, the production has been severely affected. It will be difficult to meet the market demand for a long time in the future.

　　Quartz material

　　Quartz is the main part of crystal due to its inherent piezoelectric properties. However, it must be cut and polished before it can be used. Due to its very thin thickness, although protective measures have been taken, its shock resistance has always been a concern.

　　Accuracy

　　The so-called accuracy refers to the degree to which the actual clock frequency deviates from the standard clock frequency. It can be expressed as:

　　Error (PPM) = (Factual-Ftarget) / Ftarget * 10E6

　　Error: Accuracy

　　Factual: actual frequency

　　Ftarget: standard frequency

　　PPM: parts per million

　　In the application of crystals, there are several aspects to consider:

　　Frequency tolerance: It is the value by which the actual frequency deviates from the standard frequency at normal ambient temperature (25°C+/-5°C).

　　Frequency temperature characteristics: refers to the value of the actual frequency deviating from the standard frequency within the entire temperature range. There are usually three temperature ranges: 0°C--70°C, -20°C--70°C and -40°C--85°C.

　　Aging: The frequency deviation caused by the change of the internal characteristics of the crystal over time is called crystal aging. Generally speaking, the accuracy of the crystal is affected by aging by 5PPM in the first year, and about 1-3PPM each year thereafter. If the design life cycle of a product is 10 years, the frequency accuracy change caused by aging can be as high as 32PPM.

　　Changes in load capacitance accuracy cause changes in frequency: This factor is often overlooked. There are two operating modes in crystal applications, serial oscillation mode and parallel oscillation mode. The parallel mode has become the mainstream in the market due to its flexible design and high output accuracy. Figure 2 is the equivalent circuit diagram of the parallel oscillation mode: