Crystal filter

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a crystal filter, and more particularly to a triple mode crystal filter having improved spurious generated on the high frequency side of the pass band.

【0002】

[Conventional technology]

A multi-mode piezoelectric filter that uses an AT-cut quartz substrate as the piezoelectric substrate has good frequency-temperature characteristics, is compact, can obtain a high amount of attenuation, and has robustness. Widely used as an IF filter for machines. FIG. 4 is a diagram showing a configuration of a third-order Overton triple-mode crystal filter using an AT-cut crystal substrate as a piezoelectric substrate. FIG. 4A is a plan view, FIG. 4B is a bottom view, and FIG. 4B is a bottom view. (c) shows a cross-sectional view in QQ. As shown in FIG. 4 (a), electrodes 12, 13 and 14 are arranged close to each other on one main surface along the Z'axis direction of the AT-cut quartz substrate 11 with a predetermined gap g, and the electrodes are arranged. An electrode 15 is provided on the other main surface facing 12, 13, and 14. Then, the lead electrodes 12', 13', 14', and 15'are extended from the electrodes 12, 13, 14, and 15 toward the end of the AT-cut crystal substrate 11, respectively, to form a tertiary overton triple mode crystal filter. To configure.

【0003】

When a high-frequency voltage is applied between the lead electrodes 12'and 15', the symmetric modes S1 and S2 and the antisymmetric mode A1 as shown in Fig. 4 (d) are strongly excited, short-circuiting the lead electrodes 13' and 15. By providing appropriate terminations between the lead electrodes 12'and 15'and between 14' and 15', a triple-mode crystal filter utilizing the three modes S1, A1 and S2 can be constructed. As is well known, narrowing the interelectrode gap g strengthens the coupling between modes S1, A1 and S2 and widens the passband width. Conversely, widening the interelectrode gap g weakens the coupling between modes and narrows the bandwidth. In the case of a filter with a narrow passband, it becomes necessary to add a capacitance in parallel with the terminating resistor between the lead electrodes 12'and 15'and between 14' and 15'.

【0004】

Figures 5 and 6 use an AT-cut crystal substrate, the center frequency of the filter is 109.65MHz (3rd order overton), the lengths in the Z'and X-axis directions are 2.3mm and 2.0mm, respectively, and the electrodes 12, 13 and 14 These are the filter characteristics when the dimensions of are 0.46 mm × 0.8 mm, the gap g between the electrodes is 0.07 mm, and the frequency reduction amount of the electrodes is 400 kHz.

【0005】

[Problems to be Solved by the Invention]

However, in the conventional 3rd order Overton triple mode filter, the center frequency f is as shown in FIG.₀At frequencies higher than + 300KHz, spurious Su due to inharmonic overton is generated, and there is a problem that it is extremely difficult to suppress it to the required 35dB or more. Further, as shown in FIG. 6, spurious SL, which is considered to be caused by higher-order contour vibration, may occur on the lower frequency side than the center frequency, which has a problem of impairing the communication quality of the radio. The present invention has been made to solve the above problems, and has a center frequency f.₀It is an object of the present invention to provide a third-order overton triple-mode crystal filter that suppresses spurious on the high frequency side from + 300 kHz to 35 dB or more and also suppresses spurious generated on the low frequency side from the center frequency.

【0006】

[Means for solving problems]

In order to achieve the above object, the invention according to claim 1 of the quartz filter according to the present invention has three electrodes arranged in close proximity on one main surface of an AT-cut quartz substrate, and the three electrodes are arranged on the other main surface. In a triple mode crystal filter configured by providing electrodes facing the electrodes, the triple mode is characterized in that the arrangement directions of the three electrodes are inclined by 10 to 80 degrees with respect to the Z'axis of the crystal substrate. It is a crystal filter. The invention according to claim 2 is the triple mode crystal filter according to claim 1, wherein one electrode facing the three electrodes is arranged on the other main surface. The invention according to claim 3 is a triple-mode crystal filter configured by arranging three electrodes in close proximity on one main surface of an AT-cut crystal substrate and providing electrodes facing the three electrodes on the other main surface. The triple-mode crystal filter is characterized in that the arrangement directions of the three electrodes are inclined by 10 to 35 degrees with respect to the X-axis or Z'-axis of the crystal substrate. The invention according to claim 4 is the triple-mode crystal filter according to claims 1 to 3, wherein the triple-mode crystal filter is driven by a third-order overton.

【0007】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1 is a diagram showing a configuration of a tertiary overton triple mode crystal filter according to the present invention, in which FIG. 1A is a plan view and FIG. 1B is a bottom view. As shown in Fig. 1 (a), the Z'axis direction of the AT-cut crystal substrate 1 is the long side, the X-axis direction is the short side, and the Y'axis direction is the thickness. 'Inclined by θ degrees with respect to the axial direction, a predetermined gap G is arranged on one main surface with a predetermined gap G, and an electrode 5 is provided on the other main surface facing the electrodes 2, 3 and 4. Then, the lead electrodes 2', 3', 4', and 5'are extended from the electrodes 2, 3, 4, and 5 toward the end of the crystal substrate 1, and the lead electrodes 3'and 5'are short-circuited. , Lead electrodes 2'and 5'are input terminals and lead electrodes 4'and 5'output terminals, and the input / output terminals are appropriately terminated to form a third-order overton triple mode crystal filter. As shown in FIG. 1A, the distance between the end of the crystal substrate and the ends of the electrodes 2 and 4 must be maintained at a predetermined distance in order to prevent leakage of vibration energy.

【0008】

Since the multi-mode crystal filter uses multiple electrodes, even-numbered modes (anti-symmetrical modes A1, A2, ... Ai) and odd-numbered modes (symmetrical modes S1, S2, ... Si) are excited in addition to the main vibration. It becomes easier and inevitably more modes are excited than a single electrode crystal. Further, in the case of excitation in overton, for example, the third-order overton mode, even if the flatness and parallelism deviate slightly from the ideal state, more modes are likely to be excited than in the case of using in the fundamental wave mode. The inventor of the present application experimented by changing the size of the crystal substrate, the size of the electrode, the thickness of the electrode, etc. in order to suppress the spurious caused by the harmonic overton generated on the high frequency side of the passing region. In the case of the triple mode filter, no effective means for suppressing the spurious could be found.

【0009】

Therefore, we experimented with how the spurious changes when the arrangement direction L of the three electrodes is tilted from the Z'axis direction of the AT-cut quartz substrate. As shown in FIG. 7, when the electrode arrangement direction of the dual mode crystal filter is tilted from the Z'axis direction to the X axis direction, the coupling between the two modes S1 and A1, that is, the passage of the filter Various studies have been conducted on how the bandwidth changes. In other words, since the X-axis is the wide band and the Z'axis is the narrow band, it is common to select either the X-axis or the Z'-axis as needed. It is a target, and it has not been considered to use other arrangement directions. Moreover, the relationship between the spurious generated on the high frequency side of the pass band and the electrode arrangement direction has not been discussed so far.

【0010】

In FIG. 2, the parameters shown in FIGS. 5 and 6 are used for the electrode dimensions, the electrode gap G, the electrode film thickness, etc., and the angle θ between the array directions L and the Z'axis of the electrodes 2, 3 and 4 is changed. If the center frequency f₀It shows how the maximum value of spurious changes in the high frequency range above + 300kHz. In FIG. 2, the inclination angle θ of the electrode arrangement is taken as the horizontal axis, and the vertical axis shows the suppression level of spurious. From this figure, the spurious decreases as the electrode arrangement direction L is tilted from the Z'axis, and when the inclination angle θ becomes 10 degrees, the spurious level improves by about 10 dB compared to the conventional one with the arrangement direction L as the Z'axis. After that, the suppression level reaches the maximum near θ = 20 degrees, and although not shown, the improvement effect of about 10 dB is maintained even if it exceeds 45 degrees. On the other hand, even when the array direction L is tilted with respect to the X axis, there is an improvement of about 10 dB when the tilt reaches 10 degrees, the maximum value is reached near 20 degrees, and the improvement of about 10 dB is maintained thereafter. A similar tendency was seen. In other words, it was confirmed that the improvement was achieved in the range of 10 to 80 degrees from the Z'axis. It is considered that most of the spurious on the high frequency side is generated by the inharmonic overton mode, and when the mode is observed by the X-ray topography method, it may be distributed symmetrically in the Z'axis and X-axis directions. It is often seen. Therefore, by arranging the electrodes at an angle from the Z'axis direction, the ratio of the electrodes picking up the charges excited by the inharmonic overton mode becomes small, and it is thought that the spurious caused by the inharmonic overton mode is suppressed. Is done.

【0011】

However, as the tilt angle θ from the Z'axis increases, the coupling between the S1, A1, and S2 modes changes, so if the electrode configurations are the same, the pass bandwidth of the filter will deviate from the desired value. become. This change can be set to a desired value by changing the gap G between the electrodes, the electrode size, etc., but it is not so preferable because it requires a complicated work of design change. Therefore, considering that the effect of improving spurious suppression due to the inclination in the arrangement direction is maximized near 20 degrees, it is desirable to set the inclination to 10 to 35 degrees, and the conventional electrode pattern design method is applied as it is. You can also enjoy the merit of being able to do it.

【0012】

In Fig. 3, an AT-cut crystal substrate is used, the center frequency of the filter is set to 109.65 MHz (third-order overton), the lengths in the Z'and X-axis directions are 2.3 mm and 2.0 mm, respectively, and the electrodes 12, 13 and These are the filter characteristics when the dimensions of 14 are all 0.46 mm × 0.8 mm, the gap G between the electrodes is 0.07 mm, the amount of frequency decrease of the electrodes is 400 kHz, and the inclination angle θ of the electrode arrangement L is 20 degrees. From this figure, the center frequency f₀Spurious in the high frequency range of + 300kHz or higher is suppressed to about 40dB, and it can be seen that the difference in spurious suppression level is significantly improved to about 10dB compared to the filters shown in FIGS. 5 and 6. It was also clarified that the spurious of the higher-order contour system generated on the low frequency side is also suppressed.

【0013】

In the above description, an example in which three electrodes are arranged on one main surface and one electrode is arranged on the other main surface has been described, but it is needless to say that three counter electrodes may be used on both sides. Furthermore, in the above explanation, the third-order Overton triple-mode crystal filter has been described as an example, but it has also been confirmed that the present invention can be applied to a triple-mode crystal filter driven by a fundamental wave and spurious on the high frequency side of the center frequency can be suppressed. It was. Further, although the description has been made assuming that the sizes of the three electrodes are all the same, the same effect can be obtained even if the sizes of the electrodes are different so as to match the input / output impedance with the external circuit.

【0014】

[Effect of the invention]

Since the present invention is configured as described above, the invention according to claim 1 exhibits an excellent effect that spurious generated on the high frequency side of the center frequency can be significantly suppressed. The invention according to claim 2 exhibits an excellent effect that manufacturing and frequency adjustment are facilitated by using one electrode as one. The invention according to claim 3 is effective in increasing the bandwidth. Since the invention according to claim 4 is driven by a third-order overton, it has an excellent effect of easily realizing a high-frequency filter having less spurious.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows the structure of the 3rd order overton triple mode crystal filter which concerns on this invention, (a) is a plan view, (b) is a bottom view.

[Figure 2]

It is a figure which shows the relationship between the angle θ and spurious suppression when the electrode arrangement direction is inclined by the angle θ from the Z'axis.

[Fig. 3]

It is a figure which shows the filter characteristic of the 3rd order overton triple mode crystal filter according to this invention.

[Fig. 4]

It is a figure which shows the structure of the conventional 3rd order overton triple mode crystal filter, (a) is a plan view, (b) bottom view, (c) is a sectional view, (d) is a view of three modes to be excited. It is a figure which shows the displacement distribution.

[Fig. 5]

It is a figure which shows the filter characteristic of the conventional 3rd order overton triple mode crystal filter.

[Fig. 6]

It is a figure which shows the filter characteristic of the conventional 3rd order overton triple mode crystal filter.

[Fig. 7]

It is a figure which shows the relationship between the crystal axis direction and the electrode arrangement direction of the conventional double mode crystal filter.

[Explanation of symbols]

1 ... Crystal board 2, 3, 4, 5 ... Electrodes 2', 3', 4', 5'... lead electrode θ: The angle between the electrode arrangement direction and the Z'axis direction L ... Electrode arrangement direction G ... Electrode gap d ... Distance between the end of the crystal substrate and the end of the electrode