1. Field of the Invention
The present invention relates generally to temperature compensated crystal oscillators for easily adjusting output frequencies, and more particularly to a temperature compensated crystal oscillator and method for adjusting an output frequency thereof, which can adjust the output frequency of the crystal oscillator by inserting a thin film resistor between layers comprising a layered structure and trimming the thin film resistor through a bottom layer a laser beam.
2. Description of the Prior Art
Crystal oscillators using crystal vibrating chip are essential parts to generate oscillation frequencies for controlling transmission and reception of signals between mobile communication terminals. The crystal oscillators have excellent frequency stability compared with other oscillators. A generally used crystal oscillator is a temperature compensated crystal oscillator (TCXO) for solving the problem of the variation of an oscillation frequency due to ambient temperature,
FIG. 1 is an equivalent circuit diagram of a conventional temperature compensated crystal oscillator. As shown in FIG. 1, the temperature compensated crystal oscillator comprises a frequency adjusting circuit 10, a temperature compensation circuit 20, a crystal oscillator 30 and an oscillation circuit 40 realized as an IC chip. The temperature compensation circuit 20 controls the crystal oscillator 30 to resonate at a predetermined frequency to correspond to capacitance and inductance varied according to ambient temperature using a thermistor. Then, the crystal oscillator 30 oscillates at the compensated resonance frequency through the oscillation circuit 40.
The temperature compensation crystal oscillator additionally has the frequency adjusting circuit 10 so as to provide a correct output frequency at room temperature. In the temperature compensated crystal oscillator, it is impossible to adjust inductance as in a voltage controlled oscillator (VCO). Therefore, the temperature compensated crystal oscillator generally uses a method for adjusting a trimmer capacitor or a trimmable chip resistor. Especially, the trimmable chip resistor 9, which is favorable in terms of an arrangement area and easiness of trimming operation, is generally used as shown in FIG. 1.
FIGS. 2a and 2b are respectively a side sectional view and a schematic perspective view of a conventional temperature compensated crystal oscillator 50. The temperature compensated crystal oscillator 50 is an embodiment of the temperature compensated crystal oscillator of FIG. 1, and shows the structure for adjusting a frequency using a trimmable chip resistor. As show in FIG. 2a, the temperature compensated crystal oscillator 50 has a structure in which a crystal oscillating unit 53, parts 55 for temperature compensation and oscillation circuits, and a trimmable chip resistor 59 are mounted on the top layer 51 of layered structure comprised of two layers. Further, a metal case 57 is covered on the upper surface of top layer 51, such that a mounting area on the upper surface of top layer 51 is shielded from external electrical and mechanical influences. Because the case 57 is made of a metal, a hole 57a is formed at the metal case 57 so as to trim the chip resistor 59 with a laser beam, as shown in FIG. 2b, and then the chip resistor 59 is trimmed through the hole 57a. 
However, the trimmable chip resistor employed in the conventional temperature crystal oscillator of FIG. 2 has a size of several mm2. As a result, it requires a considerable mounting area compared with the crystal oscillator having a size of only approximately 5.0xc3x973.2 mm2 or 4.7xc3x972.9 mm2. Consequently, the size of trimmable chip resistor increases the size of a final product, and makes the miniaturization suitable for mobile communication terminals difficult.
Accordingly, in this technical field, it is strongly required to develop a temperature compensated crystal oscillator having a new structure and a method of adjusting the output frequency using the new structure, wherein a trimmable chip resistor can be mounted without greatly increasing the product size and can be trimmed in the state of final product, that is, at the last step of the process.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a temperature compensated crystal oscillator, in which a planar thin film resistor is formed on the upper surface of a bottom layer of a layered structure, thus minimizing an installation area occupied by the thin film resistor for adjusting an output frequency.
Another object of the present invention is to provide an output frequency adjusting method, by which an output frequency can be adjusted in the state of a final product by trimming a planar thin film resistor with a laser beam through the lower surface of a bottom layer having an upper surface on which the planar thin film resistor is arranged.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a temperature compensated crystal oscillator including a crystal oscillating unit and at least one part for temperature compensation and oscillation circuits, comprising a plurality of layers having an upper layer on which the crystal oscillating unit and the part are mounted, and at least one layer on which conduction patterns are formed; and a planar thin film resistor arranged on an upper surface of a bottom layer of the layered structure for adjusting an output frequency of the temperature compensated crystal oscillator.
The present invention can be provided in two types, according to a mounting structure formed on the upper layer.
In a preferred embodiment of this invention, a layered structure is comprised of a first layer on which conduction patterns for mounting the crystal oscillator and the part are formed, a second layer arranged under the first layer and provided with an upper surface on which other conduction patterns connected to the conduction patterns of the first layer are formed, and a third layer arranged under the second layer, wherein the first layer is the upper layer on which the crystal oscillating unit and the part are mounted, and the third layer is the bottom layer on which the planar thin film resistor is formed.
Further, in another embodiment of this invention, a layered structure is comprised of a first layer having an upper surface on which the crystal oscillating unit is arranged and having a cavity formed therein, a second layer on which conduction patterns for mounting the part for temperature compensation and oscillation circuits are formed at a region exposed to the cavity of the first layer, and a third layer arranged under the second layer, wherein the first and second layers compose an upper layer on which the crystal oscillating unit and the part are respectively formed on upper surfaces of the first and second layers, and the third layer is the bottom layer on which the planar thin film resistor is formed on its upper surface.
Moreover, in the preferred embodiment of this invention, at least one fourth layer is additionally arranged between the second and third layers so as to sufficiently realize signal lines of the temperature compensated crystal oscillator, wherein other conduction patterns connected to conduction patterns of other layers can be formed on the upper surface of the fourth layer. In this case, the second to fourth layers can be preferably manufactured as a single printed circuit layer so as to form signal lines, such as the temperature compensation circuit, the oscillation circuit and the frequency adjusting circuit, in one process.
More preferably, the conduction patterns formed on the upper surface of the fourth layer are formed at a remaining region except a region vertically overlapped with a region, at which the planar thin film resistor is formed, so as to reduce the risk of damage caused by a laser beam trimming process. Most preferably, a laser beam blocking film covering a region on the upper surface of the fourth layer, vertically overlapped with a region at which the planar thin film resistor is formed, can be additionally formed. At this time, the laser beam blocking film is preferably made of a thin metal film.
In accordance with another aspect of the present invention, there is provided a method for adjusting an output frequency of a temperature compensated crystal oscillator comprised of a layered structure, comprising the steps of providing a temperature compensated crystal oscillator in which a planar thin film resistor is arranged on an upper surface of a bottom layer of a layered structure; and trimming the planar thin film resistor arranged on the upper surface of the bottom layer by irradiating a laser beam on the lower surface of the bottom layer so as to obtain a predetermined output frequency.
Furthermore, in the temperature compensated crystal oscillator, a laser beam blocking layer made of a thin metal film is preferably arranged at a region on the upper surface of another layer to be arranged over the bottom layer so as to include, a region vertically overlapped with the region at which the planar thin film resistor is formed. Further, a mark is formed at a region on the lower surface of the bottom layer, corresponding to the position of the planar thin film resistor formed on the upper surface of the bottom layer, and a laser beam is irradiated along the mark, thus allowing the trimming process to be easily performed.