Development of High-Stability Miniaturized Oven Controlled Crystal Oscillator

Wan-Lin Hsieh*, Chia-Wei Chen, Chen-Ya Weng, Che-Lung Hsu, Sheng-Hsiang Kao, and Chien-Wei Chiang TXC Corporation

No. 4, Kung Yeh 6th Rd., Ping Cheng Industrial District, Pin Cheng District 32459, Tao Yuan City, Taiwan *wilhsieh@txc.com.tw

Abstract—This paper reports on the development of a high-stability miniaturized Oven Controlled Crystal Oscillator (OCXO) in the size of 9.7 mm x 7.5 mm based on the analog oscillation circuit combined with the conventional temperature sensing circuit using a thermistor and related discrete electrical components. The finite element method (FEM) is implemented to optimize the oven stability of the oven structure. As a result, a highly stable oven performance less than ±1°C variation within the ambient temperature ranging from -40 to 85°C is both numerically analyzed and experimentally demonstrated. Consequently, this result implies that the frequency stability of the miniaturized OCXO can achieve less than ±20 ppb using an AT-cut crystal.

Keywords—Miniaturized OCXO, finite element method (FEM)

I. INTRODUCTION

Over the past few years, small cell solutions have shown their ability to achieve higher radio density and capacity by deploying as standalone networks or integrating with conventional macro cell. In addition, the coverage of next generation 4G-LTE telecommunication will be highly extended by using small cell technology including indoor and outdoor applications. However, for small cell application, the physical size is the key factor affecting the small cell designers to select the frequency control component due to the preferential requirement of the use of a single small package with the cost effective solutions. As a result, much attention has been paid to developing the miniaturized Oven Controlled Crystal Oscillator (OCXO) to meet the stringent system requirement of small cell applications.

In 2012, Ishii et al reported on the development of the digital signal processing-oven controlled crystal oscillator (DSP-OCXO) using an AT cut crystal as temperature sensor, showing the frequency stability is capable to achieve less ±20

4

Fig. 1. A photograph of the 9.7 mm x 7.5 mm miniaturized OCXO.

Fig. 2. Functional block diagram of the miniaturized OCXO. Thermistor is used as the temperature sensor.

ppb within the ambient temperature ranging from -40 to 85°C [1]. However, this digital oven control circuit still suffers from the small fluctuation in temperature stability [2]. To deal with this issue, we propose a high-stability miniaturized OCXO in the size of 9.7 mm x 7.5 mm based on the analog oscillation circuit combined with the conventional temperature sensing circuit using a thermistor, as seen the real appearance of the OCXO in Fig. 1. The functional block diagram of the miniaturized OCXO is shown in Fig. 2. The conventional temperature sensing circuit using thermistor as a temperature sensor is implemented for the temperature control circuit. To optimize the ovenized structure, thermal analysis simulation according to the finite element method (FEM) is utilized [3]. Consequently, a highly stable oven performance less than ±1°C variation from -40 to 85°C is both numerically and experimentally demonstrated.

I. MODELING OF HEAT TRANSFER

OCXO is able to perform a very high degree of frequency stability within a wide operating temperature range [4]. This can be achieved by placing the crystal in a thermally insulated oven structure with a thermostatically controlled heater element. In order to model the ovenized structure of OCXO during the operation, the tree-dimensional simulation of the time dependent process is carried out.

In the simulation, the governing mechanism for the heat transfer modeling in OCXO only considers the conduction effect. Therefore, the general heat conduction equation can be derived from the first law of thermodynamics as shown in below:

∙ 0, (1)

Fig. 3. The heat source for the thermostatically controlled heater is defined as a function of time, including the warm-up and steady step.

where [kg/m ] denotes the density, and [J/(kgK)] is the heat capacity at constant pressure. Fourier’s Law of conduction gives: , (2)

where k [W/(mK)] is the thermal conductivity. It should be noted that the convective effect is ignored by setting the velocity equals to zero in the numerical domain of air.

The heat source for the thermostatically controlled heater is defined as a function of time as shown in Fig. 3. The initial temperature of the heat source is 25°C, and the heating process includes the warm-up step with a heating rate 2.5 °C/s and the steady step that keeps the temperature at 97.5°C.

Convective heat flux boundary condition is adopted to represent the outside wall of the stainless steel cover, exposing to the ambient air: ∙ , (3)

where n denotes the normal to the wall, is the heat transfer coefficient, and represents the temperature of the ambient air. In this paper, the operating temperature range of is from -40 to 85°C.

The thickness of the copper trace (around several micro meters) in the PCB is much thinner than other components

TABLE I. MATERIALS AND PHYSICAL PROPERTIES USED IN THE NUMERICAL MODELING.

Material Density [kg/ ]

Thermal conductivity [W/(mK)]

Heat capacity [J/(kgK)]

Air 1200 0.0257 1003.5

PCB 1900 0.3 1369

Ceramic 3850 31 840

Copper 8700 400 385

Stainless steel 7837 16.9 486

Fig. 4. Illustration of the proposed OCXO using double-heater structure.

(around several millimeters) of the device. As a result, this multi-scale issue may lead to a longer computational time due to the finer mesh is required. To this end, the highly conductively layer is implemented in the numerical model by setting the copper trace as a boundary. It should be mentioned that this assumption still satisfies the heat transfer formulation in (1).

The materials and the physical properties used in the numerical simulation are shown in Table 1, including the ambient air, printed circuit board (PCB), ceramic for the crystal and heater package, copper for the trace embedded in the PCB, and stainless steel for the cover.

II. DESIGN OF OVENIZED STRUCTURE

To optimize the ovenized structure, the thermal analysis based on FEM simulation is utilized to investigate the oven stability. First, we optimize the physical location of the thermistor and show the effect of the relative position between thermistor and heater on the oven controlled circuit. Second, a double-heater structure is proposed to enhance the oven stability.

The illustration of the proposed OCXO is shown in Fig. 4, including two heaters (Heater 1 and Heater 2), three PCB layers, a thermistor, and a TCXO. The passive components, such as chip resistors and capacitors, are neglected in the

Fig. 5. The physical location of the thermistor is analyzed to optimize the oven structure. The simulation results show that the temperature variation of the thermistor is less than 0.3°C in the ambient temperature from -40 to 85°C. The insets represent the temperature distribution when ambient temperature is -40°C before and after optimization, respectively.

Fig. 6. (a) The thermal distribution of the proposed OCXO obtained by simulation is presented when the ambient temperature is 85°C. The double-heater structure is utilized to improve the oven stability. (b) Comparison of crystal temperature between the one-heater and double-heater structure. (c) Temperature increment (ΔT) using double-heater structure compared to the one-heater structure.

Fig. 7. Temperature stability of the crystal referred to 25°C in the ambient temperature ranging from -40 to 85°C obtained by simulation and experiment.

simulation herein. When thermistor is used as a temperature sensor, an accurately temperature detection is critical for the feedback loop of the oven control circuit. For example, when the thermistor does not completely contact with the heater, as shown in the inset in Fig. 5, the temperature of the thermistor is only 85°C when the heat source of heater is set at 97.5°C while the ambient temperature is -40°C. As a result, over 8°C variation of thermistor is obtained (solid line shown in Fig. 5) in the ambient temperature from -40 to 85°C. This large variation indicates that an overheating to the crystal may occur at the low ambient temperature condition since the temperature of thermistor is much lower than the reference temperature set from the feedback loop of the oven control circuit. In other words, this result implies that the physical distance between a crystal and a temperature sensor should be carefully studied to decide the most proper location. To this end, we parametrically analyze the physical location of the thermistor. We found that when the thermistor is completely contact with Heater 1, as seen in Fig. 4 (dashed line), the temperature variation of the thermistor less than 0.3°C variation could be achieved.

Next, the effect of the additional heater (Heater 2) on the thermal efficiency in the ovenized structure is investigated. It is observed that a huge heat loss from Heater 1 to PCB 1 could be found when Heater 2 is removed as seen in the thermal distribution obtained by the simulation in Fig. 6(a). As seen in Fig. 6 (b), a comparison of crystal temperature between the one-heater and double-heater structure is presented. When Heater 2 provides an additional heat source, the thermal distribution in the oven is more uniform, where the temperature in the steady step of Heater 1 and Heater 2 is set at 97.5°C while the ambient temperature is 85°C. To quantify the improvement of using double-heater structure, the parametrically analysis by changing the temperature of Heater 2 while keeping Heater 1 at 97.5°C is studied. The result in Fig. 6(c) shows the temperature increment (ΔT) using double-heater structure compared to the one-heater structure, where over 1.6°C is improved when Heater 2 is set at 97.5 ̊C.

In consequence, considering the double-heater structure the oven stability of the crystal less than ±0.1°C is successfully achieved as seen in the simulation result in Fig. 7 while the external ambient temperature ranging from -40 to 85°C.

III. EXPERIMENTAL RESULTS

The ovenized structure has been parametrically optimized by analyzing the physical location of thermistor related to the heater and considering the double-heater concept used in the device, the next step is to realize the miniaturized OCXO. As shown in Fig. 7, the oven stability less than ±1°C variation through experimental measurement is obtained. It should be noted that the temperature of crystal is measured using a AC cut blank as a temperature sensor, which has a great linearity between frequency and temperature. Consequently, this result implies that the frequency stability is possible to achieve less than ±20 ppb using an AT-cut crystal. Fig. 8 shows the corresponding output current when the ambient temperature is 25°C, from which the current of warm-up step and steady step is 320 mA and 150 mA, respectively. In addition, the corresponding thermal image captured by the infrared thermal

Fig. 8. The output current when the ambient temperature is 25°C, where the current of warm-up step and steady step is 320 mA and 150 mA, respectively.

imaging equipment is shown in Fig. 9. It is observed that the temperature of Heater 2 is around 95°C.

IV. CONCLUSTION

In this paper, we develop a high-stability miniaturized OCXO in the size of 9.7 mm x 7.5 mm. Thermal analysis according to the FEM simulation is utilized to optimize the ovenized structure. It has shown that the proposed double-heater structure could enhance the oven stability. Therefore, the experimental result shows a highly stable oven performance less than ±1°C variation from -40 to 85°C, indicating that a high frequency stability less than ± 20 ppb can be ahieved if an AT-cut crystal is used.

Fig. 9. Thermal distribution captured by the infrared thermal imaging equipment. The temperature of Heater 2 is around 95°C.

REFERENCES [1] Yasuto Ishii, Kaoru Kobayashi, Tsukasa Kobata, Manabu Ito, Shigenon

Watanabe, Shinichi Sato, Kazuo Akaike “A New Generation DSP-OCXO Using Crystal Temperature Sensor,” International Frequency Control Symposium, IEEE, pp.341-344, 2012.

[2] Kaoru Kobayashi, Yoshiaki Mori, Tsukasa Kobata, Manabu Ito, Shigenori Watanabe, Shinichi Sato, Kazuo Akaike, “High-Performance DSP-TCXO Using Twin-Crystal Oscillator,” International Frequency Control Symposium, IEEE, pp.1-4, 2014.

[3] COMSOL Multiphysics Version 4.3a User Guide, October, 2012.

[4] Vig, John R. “Introduction to quartz frequency standards,“ No. SLCET-TR-92-1. ARMY LAB COMMAND FORT MONMOUTH NJ ELECTRONICS TECHNOLOGY AND DEVICES LAB, 1992.