KR20010076621A - Surface acoustic wave oscillator module on low temperature co-fired ceramic substrate - Google Patents

Abstract

PURPOSE: A surface acoustic wave(SAW) oscillator module using low temperature co-fired ceramic substrate is provided to reduce stray capacitance component of the SAW oscillator and broaden a frequency trimming region as much as the frequency shift can be amended, thereby correctly synchronizing the output frequency of a temperature compensation oscillator or a reference frequency generator with a frequency requiring each kind of wireless communication machine. CONSTITUTION: A passive element array installed onto a low temperature co-fired ceramic substrate(17) and includes a selective switch, which is controlled by a digital signal, adjusting the entire inductance or capacitance. A SAW oscillator(10) installed onto a low temperature co-fired ceramic substrate(17) and provides stable oscillated frequency. An IC chip(16) includes a control circuit which controls the passive element array and circuit related to oscillation and temperature compensation, if it is necessary to compensate for them.

Description

Surface acoustic wave oscillator module on low temperature co-fired ceramic substrate

The present invention relates to a surface acoustic wave oscillator using a passive element and a surface acoustic wave (SAW) resonator embedded in a low temperature co-fired ceramic substrate.

In recent years, various portable terminals and wireless communication devices have been developed in the direction of small size, light weight, and low cost, and miniaturization and high integration of components constituting them are required. Of the dozens of components constituting a mobile terminal, a quartz crystal oscillator that is a reference signal source that matches time and frequency synchronization between a base station and a terminal and generates only accurate and constant frequency signals in order to transmit and receive only a fixed frequency signal. Or a temperature compensated oscillator (hereinafter referred to as TCXO) is essential. In particular, in the CDMA (Code Division Multiple Access) system, which uses a phase modulation method to maximize frequency utilization efficiency, the frequency change is very stable within 1 ppm (part per million) against environmental changes such as temperature, load, and vibration. And it needs TCXO which is accurate source of reference signal.

Individual resonator and capacitor components can be combined to form a resonator. However, these resonators have a high quality factor (Q), which results in high phase noise, and the characteristics change with temperature of each component reaches 1000 ppm / ℃. It cannot be used as a reference frequency source that requires stability. Therefore, a crystal crystal resonator having a very high Q of 10,000 or more and a change in resonance frequency with temperature up to 10 ppm has been mainly used.

2 shows the structure of a conventional TCXO, including resonator 30, variable capacitor 31, oscillator and output buffer 32, multiple thermistor resister network 33, and precision. It consists of a precision voltage regulator (34).

The oscillator 32 is a Colpittz or Pierce type oscillator circuit composed of a few transistors, and a crystal crystal acts as an LC resonator necessary for the operation of the oscillator, so that the frequency is very stable and a sinusoidal signal. Generates. If the resonant frequency of the quartz crystal changes slightly (within about 20 ppm) under the influence of temperature change, the capacitance of the variable capacitor 31 or varactor is controlled to change the capacitance minutely in order to bring it back to the original frequency. The thermistor-resistance network circuit section 33 detects the temperature and generates the varactor control voltage. In addition, since the frequency of the output signal may be minutely changed by the variation of the bias voltage of the oscillator, the power supply voltage applied to the entire circuit must be very stable to prevent this, and a precise regulator 34 is required for this purpose.

The resonator 30, which is the core of the conventional TCXO, has a problem that mass production is not easy because it is made by cutting the crystal to an accurate thickness and finely adjusting the thickness of the metal layer.

On the other hand, SAW resonators are manufactured in a semiconductor process and are excellent in mass production, but are not used in communication devices requiring high frequency accuracy due to the large variation in resonance frequency due to changes in the manufacturing process, such as CATV or remote door switch. It has been used mainly for systems that do not require synchronization. Current commercially available SAW resonators can have resonance frequencies up to 1000 ppm from the nominal value indicated in the data sheet. However, as semiconductor process technology continues to evolve in the future, automated SAW resonators may have lower costs than conventional quartz crystal resonators.

Referring to Figure 1 showing the surface acoustic wave oscillator module of the present invention to be described later the structure and operation principle of a conventional SAW resonator as follows. In Fig. 1, reference numeral 10 denotes a SAW resonator.

Piezoelectric materials, lithium niobate (Lithium niobate, LiNbO ₃ ), lithium tantalate (lithium tantalate, LiTaO ₃ ), quartz, ZnO, AlN, etc. Can be changed to In particular, an inter-digital transducer (IDT) (11, 12) having a constant spacing between electrodes on a surface of a piezoelectric material such as a crystal single crystal is formed by a photolithography process during a semiconductor process. When a signal is applied, only acoustic waves with wavelengths matching the spacing between two adjacent electrode fingers (?? _IDTs ) are excited at the crystal substrate surface, propagated out of the substrate surface, and then converted back into electrical signals in the IDT of the same structure. Can be. In addition, by placing a reflection grating (13) at both ends to reflect the acoustic wave formed on the surface to trap the wave toward the IDT, it is possible to prevent the loss of sound waves, thereby realizing a resonator having a very low loss near the resonance frequency.

The SAW resonator operates on a principle similar to a microstripline filter in which frequency characteristics are determined by the shape of an electrode pattern. The SAW resonator has a feature that the wavelength is very short because the wave is not an electromagnetic wave but a sound wave.

A simplified equivalent circuit for a typical two-terminal SAW resonator with separate input IDT 11 and output IDT 12 is shown in FIG.

Where the values of the motional inductance L _m (36), capacitance C _m (37), acoustic wave loss component R _m (38) and stray capacitance C _T (35) are determined by the metal pattern of the piezoelectric material surface. It is mainly decided.

The metal pattern on the surface that determines the frequency characteristics of the SAW resonator is manufactured by standard lithography processes such as mask fabrication, exposure, and metal layer etching. The resonant frequency may vary slightly from the design value. In addition, the propagation velocity of surface acoustic waves varies slightly depending on the thickness of the metal layer constituting the reflective grating or the depth of the etched crystal surface, and considering the above process variables, the frequency and design value of the actual SAW resonator are roughly different. It is known to vary by 500 to 1000 ppm (Ref. E. Gerber and A. Ballato, Precision Frequency Control , Academic Press, 1985).

In order to compensate for these differences, plasma ion etching can be used to monitor the resonant frequency while varying the thickness of the crystal surface [Ref: R. Subramanian, J. Welter and PVWright, "Production Trimming of SAW Devices using CF4 Chemistry and its effects on SAW Characteristics, "IEEE Ultrasonics Symp., Pp. 255-260, 1996], a method of depositing dielectric material to change substrate surface conditions [M. A. Taslakov and I. D. Avramov, "Frequency Trimming of Surface Transverse Wave Resonators using Resistive Evaporation of Thin SiO Dielectric Film," IEEE Freq. Contr. Symp., Pp. 497-501, 1998, etc., all of which require an additional vacuum process.

In addition, conventionally, the trimmer capacitor attached to the PCB substrate is manually adjusted to trim or adjust the frequency change of several tens of ppm caused by the process of assembling TCXO from the crystal resonator or the SAW resonator and changes in temperature and load. The capacitance 31 was digitally changed by adjusting one by one or by using a capacitor array and a switch. [Reference: Design and Fabrication of a Capacitance Switch Array Usable on a DTCXO, IEEE Trans. Ultrason. Ferroelec. Freq. Contr., Vol. 43, no. 6, pp. 1195-1199, 1996].

However, due to the large stray capacitance C _T (35) component formed by the positive electrode plate constituting the resonator, the resonance frequency can be adjusted only within the range of several tens of ppm, and is generated due to inconsistent process variables such as the resonator electrode thickness. It was impossible to correct the process variation of about 1000 ppm.

An object of the present invention for solving the above problems is to reduce the stray capacitance component of the SAW resonator using an inductor and capacitor switch array embedded in a low-temperature fired ceramic substrate, and to adjust the frequency adjustment region to a process variable of 1000 ppm. It is to provide a module that is wide enough to correct the frequency shift by, so that the output frequency of the reference frequency generator or the temperature compensated oscillator composed of the SAW resonator exactly matches the frequency required by various wireless devices.

1 is a block diagram of a surface acoustic wave oscillator module according to the present invention

2 is a structural diagram of a conventional temperature compensation oscillator

3 is an equivalent circuit diagram of the surface acoustic wave oscillator of FIG.

4a and 4b are expected values for the change of the resonant frequency according to the change of the value of the inductor and the capacitor for the frequency correction

Explanation of symbols on the main parts of the drawings

10: SAW resonator 11: input IDT

12: output IDT 13: grid reflector

15: bonding wire 16: IC chip

17 low temperature fired ceramic substrate 18 inductor

19: capacitor 20a, 20b: metal wiring

21a, 21b: via hole

Surface acoustic wave oscillator module of the present invention for achieving the above object is a low-temperature fired ceramic substrate; A passive element array embedded in the low-temperature fired ceramic substrate and provided with a selection switch controlled by a digital signal, such as an inductor or a capacitor capable of adjusting overall inductance or capacitance; A SAW resonator positioned on the low-temperature fired ceramic substrate, the SAW resonator providing a stable oscillation frequency against changes in the surrounding environment such as temperature, power supply voltage, and load variation; And an IC chip including a circuit for controlling the passive element array, an oscillation circuit, and a control circuit associated with temperature compensation if necessary, wherein each of the components includes metal wiring and via holes on the low-temperature fired ceramic substrate. And it is electrically interconnected by a bonding wire (bonding wire) characterized in that it is possible to automatically correct the oscillation frequency change according to the change of the process variable.

In addition, the low-temperature fired ceramic substrate used in the present invention has a thickness of about 160 μm, and the wire width and the size of the via hole are also reduced in proportion to the thickness of the film, compared to the TCXO implemented on a conventional PCB. The size can be reduced considerably. In addition, by incorporating passive elements such as inductors, capacitors, and resistors, more components can be integrated and a plurality of capacitors and switches can be connected in parallel as shown in FIG. 3. In the present invention, a capacitor switch array (CSA) is used instead of a variable capacitor or a varactor that has been conventionally used for fine adjustment of the resonant frequency. In the case of the varactor, the resonance Q is generally not high. This is because the change in capacitance value with temperature is very large, which lowers the stability of the resonance frequency.

In particular, in the present invention, the capacitor switch array 70 is placed in the portion 39 connected to the ground line of the SAW resonator as shown in FIG. 3, thereby reducing the effect of stray capacitance C _T 35 of the two-terminal SAW resonator and reducing the general capacitance. The frequency fine tuning region determined by the ratio of Cm 37 and C _T 35 was widened.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram of a surface acoustic wave oscillator using a low-temperature fired ceramic substrate of the present invention for generating an accurate reference frequency signal, the low-temperature fired ceramic substrate 17; A passive element array embedded in the low-temperature fired ceramic substrate 17 and provided with a selection switch controlled by a digital signal, and capable of adjusting overall inductance or capacitance; Located on the low-temperature fired ceramic substrate 17, a circuit and oscillation circuit for controlling the SAW resonator 10 and the passive element array that provides a stable oscillation frequency against changes in the surrounding environment, such as temperature, power supply voltage, load variations, etc. And an IC chip 16 including a control circuit related thereto when temperature compensation is required. Each of the components is electrically interconnected with metal wires 20a and 20b, via holes 21a and 21b and bonding wires 15 on the low temperature fired ceramic substrate 17.

The shape of various passive elements 18 and 19 embedded in the low-temperature fired ceramic substrate 17 is referred to US Patent No. 5,661,647 "Low Temperature Co-fired Ceramic UHF / VHF Power Converters".

The IC chip 16 in a die state includes a Colpittz or Pierce type oscillation circuit, logic circuits for controlling a passive element array (capacitor switch array, inductor switch array, etc.), and temperature. If compensation is required, related control circuits are included.

In such a configuration, the SAW resonator 10 resonates at a predetermined frequency by the IC chip 16 including the oscillation circuit and the control circuit, and the frequency correction is performed by the IC chip 16 on the low-temperature fired ceramic substrate 17. This is accomplished by controlling the embedded inductor and capacitor switch arrays 18 and 19. Therefore, the SAW resonator 10 provides a stable oscillation frequency against changes in the surrounding environment such as temperature, power supply voltage, and load variation.

The circuit diagram of FIG. 1 and the equivalent circuit for the SAW resonator are shown in FIG. 3.

Reference numeral 10a denotes an equivalent circuit of the SAW resonator 10 and reference numeral 70 denotes a capacitor for adjusting frequency, and includes a switch 72 in each of the capacitors 19 and forms a capacitor switch array in parallel. Reference numeral 80 designates an inductor switch array as a frequency adjusting inductor, which includes a switch 82 in each of the inductors 18 and is connected in parallel. The oscillation circuit 90, the switch control logic 91, and the PROM 92 are components of the IC chip 16.

In the equivalent circuit 10a of the two-terminal SAW resonator, the series resonant frequency is determined by C _m (37) and L _m (36) as shown in equation (1).

In addition, when the frequency adjusting capacitor 70 or the inductor 80 is connected to the output terminal of the SAW resonator, the maximum frequency adjusting region and resonance Q obtained by changing the value of the capacitor or inductor are given by Equations 2 and 3 .

As mentioned above, the SAW resonator may have a frequency shift from 500 to 1000 ppm in the manufacturing process. If the maximum frequency adjusting region estimated by Equation 2 is smaller than this value from the manufactured resonator model value, the frequency adjusting capacitor No matter how much you adjust it, you cannot adjust it to the desired frequency.

The method of widening the frequency adjustment range is to design a metal pattern to increase C _m or reduce C _T as shown in Equation 2. The former method has a problem that the resonance Q is lowered by Equation 3, the latter. In this case, the number of finger pairs of the IDT electrode needs to be reduced. In this case, the conversion efficiency between the electrical signal and the acoustic wave is reduced, resulting in an increase in loss, and consequently, a resonance Q is rapidly lowered. That is, it is difficult to widen the frequency adjustment region by modifying the metal layer layout of the SAW resonator.

In the present invention, the frequency adjusting capacitor 70 is added in series between the ground terminal of the SAW resonator and the ground point of the entire module to reduce the shunt capacity between the signal terminal of the SAW resonator and the ground. For example, if the capacitance of the frequency adjusting capacitor 70 is adjusted to be equal to C _T , the decentralization capacity of the SAW resonator seen from the input signal source or output load side is reduced by half of C _T.

When the C _T decreases, the circuit simulator can simply confirm that the resonance frequency changes up to 1000 ppm according to the frequency adjusting inductor or capacitor value connected in series with the output terminal of the SAW resonator. The results are shown in FIGS. 4A and 4B. . This is to calculate the S-parameter frequency characteristics by extracting the equivalent circuit model parameters as shown in Fig. 3 from the fabricated 288 MHz SAW resonator and adding passive components for frequency adjustment. The frequency characteristic of FIG. 4B when the SAW resonator is driven by the conventional method without the ground terminal capacitor 70 is compared with FIG. 4A which is the result of calculating the ground terminal capacitor 70 with the adjustment inductor 80. It can be seen that the resonance frequency variation range is greatly increased.

Selection of the values of the capacitor 70 and the adjusting inductor 80 constitutes a capacitor switch array, an inductor switch array by connecting small unit capacitors in parallel and having a selector switch for each of them. This can be done by way of choice.

The technical spirit of the present invention has been described above with reference to the accompanying drawings, but this is by way of example only and not intended to limit the present invention. In addition, it is obvious that any person skilled in the art can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

As described above, in the present invention, a SAW oscillator is mass-produced by adding a capacitor for reducing stray capacitance (C _T ) to the ground terminal side of the SAW resonator and connecting an inductor or a capacitor array to the signal terminal side. Resonance frequency shifting, which was a major obstacle to the problem, is effective in solving the problem. That is, the frequency characteristic change generated in the SAW device fabrication process can be corrected during the module assembly process, and the process can be performed digitally and automatically, thereby lowering the production cost of the oscillator module than the existing crystal oscillator. In addition, since the size of the oscillator module can be reduced by using passive elements embedded in a low-temperature fired ceramic substrate, it is also expected to be used in short-range wireless communication systems such as Bluetooth, which require ultra-small and low cost components.

Claims (2)

In an oscillator module that generates a precise reference frequency, Low-temperature fired ceramic substrates; A passive element array embedded in the low-temperature fired ceramic substrate and provided with a selection switch controlled by a digital signal and capable of adjusting overall inductance or capacitance; A SAW resonator positioned on the low-temperature fired ceramic substrate, the SAW resonator providing a stable oscillation frequency against changes in the surrounding environment such as temperature, power supply voltage, and load variation; And And an IC chip including a circuit for controlling the passive element array, an oscillation circuit, and a control circuit related thereto when temperature compensation is required. The components may be electrically interconnected with metal wires, via holes, and bonding wires on the low-temperature fired ceramic substrate to automatically correct oscillation frequency changes according to changes in process variables. Surface acoustic wave oscillator module. The method of claim 1, And one or more capacitor switch arrays and inductor switch arrays as the passive element array for frequency adjustment or trimming that can correct the deviation of the reference frequency by the process variables of the SAW resonator, The inductor switch array is connected to the signal output terminal of the SAW resonator, and the capacitor switch array is connected to the ground terminal of the SAW resonator to reduce stray capacitance components viewed from the outside of the SAW resonator and to adjust the frequency of the acoustic wave resonator. Surface acoustic wave oscillator module using a low-temperature fired ceramic substrate, characterized in that to extend the area.