The method of obtaining high-frequency output using AT technology

[Preface]

Last time, we introduced methods of obtaining high-frequency output using frequency doubling circuits and phase-locked loop circuits, or using surface acoustic wave (SAW) resonators. Although these methods can achieve stable high frequencies, they each have their own weaknesses. For example, while phase-locked loop circuits can provide flexibility, the design of oscillation circuits is relatively complex, leading to a deterioration of phase noise characteristics; The circuit design using SAW method is relatively simple and has good phase noise characteristics, but the component itself has the characteristic of large frequency variation with temperature. Usually, quartz crystal manufacturers use AT type quartz crystals more for the following reasons: ① There is a turning point in the room temperature range, and the frequency remains stable over a wide temperature range; ② The high-frequency range covered is relatively wide; ③ Regardless of the length and width dimensions of the quartz plate, the desired high frequency can be obtained by controlling the thickness of the quartz plate, making it suitable for miniaturization Simply rotate the Z-axis to perform quartz wafer cutting, which is convenient for production. However, to directly excite high frequencies from AT type quartz crystals, the quartz plate needs to be processed very thin. There are constraints in terms of processing methods and mechanical strength.

This time, we will introduce the application of Epson's "QMEMS" technology to produce reverse table shaped AT type quartz crystals with only the oscillating part processed to be thinner The method of generating high-frequency and stable reference signals from the body.

【1】 Regarding the reverse table shaped AT type quartz crystal

The word 'Mesa' in the English name originates from Spanish and means' a platform shaped landform surrounded by cliffs and steep edges', usually processed into sections

The semiconductor transistor with a platform shape is called a "platform structure". A reverse truncated AT quartz crystal refers to a structure in which a portion (oscillating part) of an AT cut quartz plate is cut into a concave truncated shape (opposite to the truncated structure). The structure of the inverted truncated AT type quartz crystal is shown in Figure 1. The thinner the quartz plate of AT type quartz crystal, the higher the frequency of vibration it produces. However, the frequency limit for achieving batch and stable production through mechanical grinding is usually around 50MHz starting from the fundamental wave (with a quartz plate thickness of about 30 μ m). If it is necessary to use AT type quartz crystals to obtain frequencies greater than the above limits, high-order vibration modes (third harmonic) are often used to reach 50MHz to 150MHz. Therefore, in order to obtain high frequencies, it is necessary to use complex circuits to control vibration modes such as third harmonic. Epson uses QMEMS technology for photolithography processing to produce a reverse stage structure that only processes the excitation part into a few microns, ensuring the strength of the chip and enabling high-frequency oscillation with the fundamental wave, thus solving the above problems.

【2】 Regarding QMEMS Technology

QMEMS "is a quartz material" QUARTZ "and" MEMS (Micro Electro Mechanical) "with superior performance such as high stability and high precision System， The word "precision machining technology" is composed of "precision machining technology". The miniaturized and high-performance crystal components provided by precision machining (photolithography) using quartz as raw material, corresponding to "MEMS" made of silicon, are called "QMEMS". In addition to the reverse table shaped AT type quartz crystal introduced in this article, products using QMEMS technology are also used for precision machining of tuning fork type crystal unit vibration grooves and table shaped structure machining of AT type quartz crystals. This time, we take the table shaped structure processing of AT type quartz crystal as an example to illustrate QMEMS technology.

The oscillation of AT type quartz crystal is a typical thickness deformation oscillation, and its ideal oscillation state is to oscillate only in the central part, while no oscillation occurs in the surrounding parts. For AT type quartz crystals in the lower frequency band of MHz, chamfering is mostly used to make the thickness of the central part of the quartz plate different from that of the surrounding area, thus achieving this effect. Figure 2 shows the overview of the original mechanical processing method and the QMEMS method using photolithography processing. Due to the use of the weight of quartz chips in mechanical processing, the smaller the quartz chip, the greater the processing difficulty, and the higher the deviation, which affects the characteristics. In contrast, the photolithography processing of QMEMS technology is not affected by the size of the quartz chip, which can keep the shape of the quartz chip uniform. Even ultra small quartz chips can achieve smaller deviations than mechanical processing, thus achieving superior temperature characteristics as shown in Figure 3.

In summary, in the high-frequency field, QMEMS technology can also be used to produce the reverse stage structure shown in Figure 1, which not only maintains the strength of the quartz chip, but also can vibrate at high frequencies with the fundamental wave, thereby providing products with stable performance.

【3】 Products and characteristics using reverse truncated AT type quartz crystals

In Epson's products, the products that use QMEMS technology to achieve high-frequency fundamental oscillation using reverse table AT type quartz crystals are as follows:

① SG-210S * H in SPXO (Simple Packaged Crystal Oscillator) SG-770***/SG-771***；

② The VG-45 * * series in VCXO (Voltage Controlled Crystal Oscillator).

The frequency of VCXO varies with the externally applied control voltage and is mainly used in base stations and optical transmission equipment. In recent years, due to the continuous trend towards high-speed and high-capacity data communication, the market's demand for high-frequency and stable signal sources has been increasing. Therefore, AT type quartz crystals with high frequency, good temperature characteristics, and superior noise characteristics have become a highly anticipated presence.

Figure 4 shows a comparison of distortion components between the VCXO product VG-4513CB using a reverse truncated AT quartz crystal and our company's original product (frequency doubling type).

Figure 4: Comparison of Distortion Characteristics

When using phase-locked loops or frequency doubling circuits to obtain high frequencies (left in Figure 4), the disadvantage is that it will generate noise (distortion components) other than the signal, resulting in a decrease in jitter characteristics. When using a reverse table shaped AT type quartz crystal (as shown on the right in Figure 4), it can vibrate at high frequencies with the fundamental wave, thus avoiding distortion components similar to previous products and achieving low jitter characteristics. The advantage of directly starting high-frequency oscillation with the fundamental wave is significant, and we believe it will become an indispensable key component in the increasing number of basic equipment.

【4】 Afterword

High frequency reference signal sources are indispensable in communication and network equipment in recent years, and there are numerous types of electronic components that meet the high-frequency output requirements and specifications used by customers.

Epson introduced electronic components for obtaining high-frequency output in three parts, including programmable quartz crystal oscillators that enable easy access to any frequency, SAW oscillators that achieve low phase jitter, and AT type quartz crystals that have good temperature characteristics and directly oscillate with the fundamental wave. The above products each have their own unique features, but each product utilizes the high stability and precision characteristics of quartz itself, which we believe can provide customers with greater choices according to their needs. We hope that through this technical explanation, customers can have a better understanding of the high stability of quartz products, which can help customers choose electronic components for various applications.