JPH10270983A - Surface acoustic wave bandpass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave band pass filter with less loss in a wide band and a pass band by setting a ratio between the metallic electrode finger width of an acoustic electric transducer and an electrode interval and a ratio between the metallic stripe width of a reflector and a strip interval to be specified values and allowing both values to have a specified relation. SOLUTION: The ratio a/b between the width (a) of the metallic electrode finger 2 and the electrode interval (b) is set to be k1 and the ratio c/d between the width (c) of the metallic stripe 3 of the reflector and the interval (d) of the metallic stripe 3 to be k2. These ratios are set so as to satisfy the conditions k1=1.0, 0.8<=k2<=1.2 and k2<k1<(d/b)k2+(d/b-1). Thus, k1 and k2 are most suitably set so that the loss becomes a minimum. Thus, the filter which has the superior frequency characteristic and has the wide band and the low loss is obtained without enlarging the device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave bandpass filter, and more particularly, to a surface of a substrate made of a piezoelectric material or a non-piezoelectric material and propagating a surface acoustic wave. With a plurality of divided input and output acoustoelectric transducers and reflectors, VHF,
The present invention relates to a surface acoustic wave bandpass filter operating in the UHF frequency range.

[0002]

2. Description of the Related Art As a circuit used in the frequency range of VHF and UHF, a filter using a surface acoustic wave has been developed as an alternative to a circuit combining a conventional lumped circuit element of a capacitance element, a resistance element and an inductance element. And
It is used for various communication devices and television receivers. These surface acoustic wave filters have many advantages, such as being small in size and having stable operation characteristics, but still have problems to be solved.

In a conventional surface acoustic wave filter, a ratio (a / a / d) of an electrode finger width a to an electrode finger interval b of electrodes constituting the filter is used.
b) k1 and the ratio (c / d) k2 of the stripe width c of the reflector and the stripe interval d became almost equal. Further, as a surface acoustic wave bandpass filter having a wide pass band, for example, there is a filter described in JP-A-7-38369, but the relationship between k1 and k2 is not considered.

[0004]

In particular, in recent years, a filter used in a mobile communication terminal has been required to realize a filter having a wide band and a small loss in a pass band. According to the results examined by the inventor, at high frequencies, as the electrode finger becomes thinner, the resistance loss increases, and the loss of the filter also increases. Therefore, the thicker electrode finger is advantageous for reducing the loss.

[0005] On the other hand, the condition that the reflection area of the reflector becomes the widest is when the stripe width and the stripe interval are equal. Therefore, both optimum conditions cannot be satisfied at the same time. Therefore, the conventional configuration cannot realize a surface acoustic wave filter having a wide band and low loss.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave bandpass filter which solves the above-mentioned drawbacks of the prior art and has a wide band and a small loss in a pass band.

[0007]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides an input and output acoustoelectric transducer divided on a substrate that propagates a surface acoustic wave in the direction of propagation of the surface acoustic wave. It is intended for a surface acoustic wave bandpass filter constituting a reflector.

The ratio a / b of the metal electrode finger width a to the electrode interval b of the acoustoelectric transducer is k1, and the ratio c / d of the reflector metal stripe width c to the stripe interval d is k.
2, k1 ≧ 1.0 0.8 ≦ k2 ≦ 1.2, and k2 <k1 <(d / b) k2 + (d / b−1).

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave bandpass filter according to the present invention comprises an input and output acoustoelectric transducer and a reflector which are divided into a plurality in the direction of propagation of a surface acoustic wave. First, the configuration of each transducer and the reflector itself will be briefly described.

FIG. 1A is a diagram showing the configuration of the input and output acoustoelectric transducers. The comb-shaped electrode fingers 2 are formed of parallel thin wires having one end commonly connected by a bus bar 1.
Are configured such that the teeth are interleaved with each other. The number of teeth of the comb determines the frequency bandwidth, and the period of the electrode finger 2 determines the center frequency of the pass band, and is generally set to be the wavelength of the surface acoustic wave excited at the center frequency.

FIG. 2B is a configuration diagram of the reflector, in which both ends of a row of metal stripes 3 are electrically connected in common by a bus bar 1.

The electrode finger 2 of the acoustoelectric transducer
The ratio a / b of the width a of the electrode finger 2 to the distance b between the electrode fingers 2 is k1, and the ratio c / d of the width c of the reflector stripe 3 to the distance d of the stripe 3 of the reflector.
Is set to k2, k1 and k2 are optimally set so as to minimize the loss, so that the device has excellent frequency characteristics as described later as compared with a conventional surface acoustic wave bandpass filter, and A wideband and low-loss filter was realized without increasing the size.

FIG. 2 is a schematic configuration diagram of a surface acoustic wave bandpass filter according to an embodiment of the present invention. This surface acoustic wave bandpass filter is formed by depositing a thin film such as metal vapor deposition, sputtering, or ion plating on a substrate (not shown) that can propagate a surface acoustic wave such as lithium niobate, lithium tantalate, or quartz. A conductive fine line pattern as shown in the figure is formed by the forming technique. The input side electroacoustic transducer 7-
1, 7-2, output-side electroacoustic transducers 8, 9-
1 and 9-2 and reflectors 10-1 and 10-2.

The input transducers 7-1 and 7-2 are connected by a bus bar, and are electrically connected to a signal power supply 4 via an input load 6. The output-side electroacoustic transducers 8, 9-1, and 9-2 are connected to each other by bus bars, and are commonly connected to the load circuit 5. Then, along the propagation direction of the surface acoustic wave on the substrate, the reflector 10-
1, an output-side electro-acoustic transducer 9-1, an input transducer 7-1, an output-side electro-acoustic transducer 8, an input transducer 7-2, an output-side electro-acoustic transducer 9-2, and a reflector 10-2. .

The input transducers 7-1 and 7-2
The surface waves emitted from the reflector and the reflected waves from the reflectors 10-1 and 10-2 are output from the output transducers 8, 9-1, 9-.
The signal is converted into an electric signal by the second circuit 2 and applied to the load circuit 5 as an output signal of the band-pass filter.

K1 which is the ratio a / b between the electrode finger width a and the electrode finger interval b of the transducer, and k which is the ratio c / d between the metal stripe width c and the stripe interval d of the reflector.
The optimization of each of No. 2 was experimentally examined. As a result, k1 needs to be as large as possible, that is, k1 ≧ 1, but if k1 is too large, when a conductive fine wire pattern is formed on the piezoelectric substrate, the number of defective accidents in which input and output are short-circuited increases. Therefore, care must be taken.

FIG. 3 is a characteristic diagram showing the result of studying the relationship between k2 and the 3.5 dB bandwidth. As is apparent from this figure, even if k2 is less than 0.8 or exceeds 1.2, the 3.5 dB bandwidth is narrow. On the other hand, k2 is 0.8
≦ k2 ≦ 1.2, preferably 0.85 ≦ k2 ≦
It has been found that a bandpass filter having a wide band and low loss can be obtained by setting the ratio to the range of 1.1.

When the values of k1 and k2 are changed, the average propagation speed of the surface acoustic wave changes between the electrode portion and the reflector portion. Therefore, the period of the electrode portion (a + b) or the period of the reflector portion (c + d) ) Was changed to optimize the center frequency. This optimization is b +
This is achieved by setting a <c + d. That is, considering the above relationship, the relative relationship between k1 and k2 is k2 <k1 <
It is better to set (d / b) k2 + (d / b-1).

FIG. 4 is a diagram showing a configuration of a surface acoustic wave bandpass filter according to another embodiment of the present invention. As shown in the figure, the surface acoustic wave band-pass filters A and B are formed substantially vertically symmetrically with respect to the center.

The surface acoustic wave bandpass filter A includes input transducers 11, 12-1, 12-2 and output transducers 13-1, 13-2, 14-1 inserted between and outside the respective input transducers. ,
14-2, and reflectors 19-1 and 19-2 are formed outside the transducers at both ends. Also,
Transducers 13-2, 14-2 and 13-1,
14-1 are connected to each other on the center line of the filter.

The surface acoustic wave bandpass filter B has a configuration in which the conductive thin wire of the surface acoustic wave bandpass filter A is folded with respect to the center broken line. Here, the upper and lower transducers and reflectors are structured,
It is not necessary that the logarithm, the dimensions of each part, and the like match exactly. The ratio k1 between the electrode finger width and the electrode finger interval and the ratio k2 between the metal stripe width and the stripe interval are set so as to have a wide band and a small loss as described above.

FIG. 5 is a frequency characteristic diagram when the surface acoustic wave bandpass filter according to the embodiment of FIG. 4 is configured under the following specific conditions. The constituent conditions of each part of this filter are as follows.

Center frequency 942.5 MHz Piezoelectric substrate material 64 ° rotation Y-cut X propagation LiNbO ₃ dimensions 1.3 mm × 1.7 mm Electrode / strip material Aluminum Input / output transducer configuration Number of electrode fingers 17 pairs × 2 16 pairs × 4 interval 4.7 μm Transducer configuration (A-B connection) Number of electrode fingers 20 pairs × 2 Distance 4.7 μm Number of electrode fingers 8 pairs × 4 Distance 4.625 μm Configuration of reflector Number of stripes 200 × 4 Spacing 2.375 μm electrode finger width a / electrode finger spacing b k1 ≒ 1.6 stripe width c / stripe spacing d k2 ≒ 0.9 As is clear from the characteristic diagram of FIG.
B, 3.5 dB, and a bandwidth of 46 MHz, which realize a wideband and low-loss characteristic. From experiments, 0.8 ≦ k
It was found that the characteristic was the widest in the case of 2 ≦ 1.2.

[0024]

According to the present invention, a surface acoustic wave band-pass filter having the above-mentioned structure and having a wide band and low loss and applicable to, for example, a high-frequency filter for analog or digital mobile communication can be provided. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an acoustoelectric transducer and a reflector according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a surface acoustic wave bandpass filter according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a relationship between K2 and a 3.5 dB bandwidth.

FIG. 4 is a diagram showing a configuration of a surface acoustic wave bandpass filter according to another embodiment of the present invention.

FIG. 5 is a frequency characteristic diagram of the surface acoustic wave bandpass filter according to the embodiment of FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Bus bar 2 Electrode finger 3 Metal stripe 4 Signal source 5 Output load 6 Power load 8, 9, 15, 16 Output transducer 7, 11, 12 Input transducer 13, 14, 17, 18 Transducer 10, 19, 20 Reflector

Claims (2)

[Claims] 1. A surface acoustic wave band-pass filter comprising a plurality of input and output acoustoelectric transducers and reflectors divided into a plurality of parts in a propagation direction of the surface acoustic wave on a substrate that propagates the surface acoustic wave. The ratio a / b of the metal electrode finger width a to the electrode interval b of the electric transducer is k1, and the ratio c / d of the metal stripe width c and the stripe interval d of the reflector is k2, k1 ≧ 1.0 0.8 ≦ k2 ≦ 1.2 and k2
<K1 <(d / b) k2 + (d / b-1) 2. The method according to claim 1, wherein k2 is equal to 0.
A surface acoustic wave bandpass filter, wherein 85 ≦ k2 ≦ 1.1.