JPH11317643A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resonator coupled surface acoustic wave(SAW) filter which is capable of reduce the group delay time deviation, while maintaining the pass band width of an intermediate band by setting the electrode pitches to two reflectors different to each other. SOLUTION: A primary-secondary cascaded double mode SAW filter includes the IDT electrodes 2 and 3, which are placed on a piezoelectric substrate 1 of tetraborate lithium in the transmitting direction of a surface wave. Then the grating reflectors 4a and 4b are placed at both ends of the electrodes 2 and 3. Each of electrodes 2 and 3 consists of a pair of plural comb-line electrode fingers inserted with each other, and the different electrode finger pitches Lr1 and Lr2 are defined as the distances set between the adjacent electrode finger centers of both reflectors 4a and 4b. In order to keep the filter loss small, the reflector electrode finger pitch ratio (Lr1/Lr2) is preferable to set it at >=0.944.

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 filter, and more particularly to a surface acoustic wave filter having improved group delay time characteristics.

[0002]

2. Description of the Related Art In recent years, there has been an ever-increasing competition for miniaturization of portable telephone terminals, and there is always a strong demand for smaller and lighter devices to be used.
Similarly, the size of the resonator-coupled SAW filter for the IF filter, which is often used in the mobile phone terminal and the like, has been reduced.
There is a strong demand for lighter weight and lower cost, and various improvements have been made to respond to this.

[0003] Resonator-coupled SAW filters are roughly classified into a longitudinally-coupled multi-mode SAW filter suitable for a relatively wide band and a laterally-coupled multi-mode SAW filter suitable for a narrow band. The passband width is also greatly affected by the electromechanical coupling coefficient of. For example, when an ST-cut quartz substrate with a small electromechanical coupling coefficient is used as a piezoelectric substrate, a narrow band (fractional bandwidth is about 0.05%) SA
W filter is obtained, and the electromechanical coupling coefficient is relatively large.
It is well-known that a filter having a wide band (fractional bandwidth of about 2 to 3%) can be formed by using 36 ° Y-cut lithium tantalate.

Recently, there has been a demand for a small SAW filter having a middle band (fractional bandwidth of about 0.3%) as an IF filter for a cellular phone or the like, and an ST cut quartz substrate and a 36 ° Y cut lithium tantalate are almost required. Attention has been focused on 45 ° X cut lithium tetraborate with intermediate electromechanical coupling coefficients. The use of the 45 ° X-cut lithium tetraborate substrate to form a longitudinally coupled dual mode SAW filter can realize a required mid-band IF filter.

[0005]

However, in the new communication system of the portable telephone, the modulation / demodulation of the conventional analog system is changed to the modulation / demodulation of the digital system, and the delay time characteristic after the IF filter is becoming an important factor. Figure 6 shows 45
従 来 Conventional two-dimensional structure on X-cut lithium tetraborate substrate
FIG. 9 is a diagram illustrating a passband characteristic α and a group delay time characteristic β of a cascaded double-mode SAW filter. Here, Δτ is the difference between the maximum value and the minimum value of the group delay time characteristic, and is called the maximum group delay time deviation. As is well known, dual mode SA
The W filter is a resonator type SA having two IDT electrodes and reflectors arranged on both sides thereof along the propagation direction of the surface wave.
In the W filter, a plurality of surface waves are confined between two reflectors, and two lower-order modes therein are strongly excited to operate as a SAW filter. As is clear from FIG. 6, the required bandwidth is 300 kHz.
(Including frequency temperature, etc.), but the maximum group delay time deviation was 2.2 μs, and there was a problem that the requirement of 1 μs per filter stage could not be satisfied. The present invention has been made to solve the above problem, and has as its object to provide a resonator-coupled SAW filter in which a group delay time deviation is reduced while maintaining a pass band of a middle band.

[0006]

According to a first aspect of the present invention, there is provided a surface acoustic wave filter according to the present invention.
In a longitudinally coupled multiple mode in which a DT electrode and grating reflectors are arranged on both sides thereof, an electrode pitch of the two reflectors is different from each other. The invention according to claim 2 is characterized in that, when the electrode pitches of the two reflectors are Lr1 and Lr2, respectively, the reflector electrode pitch ratio Lr1 / Lr2 is 0.944 <Lr1 / Lr2 ≦ 0.994. It is a surface acoustic wave filter described.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 shows one embodiment of the present invention.
FIG. 2 is a plan view showing a configuration of a secondary-secondary longitudinally-coupled dual-mode SAW filter (hereinafter, referred to as a dual-mode SAW filter), showing an IDT on a lithium tetraborate piezoelectric substrate 1 along a surface wave propagation direction. Electrodes 2 and 3 are arranged, and grating reflectors 4a and 4b (hereinafter, referred to as reflectors) are arranged on both sides thereof. Each of the IDT electrodes 2 and 3 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. One of the IDT electrodes 2 is connected to the input terminal IN, and the other is connected to the input terminal IN. Ground and IDT electrode 3
One of the comb electrodes is connected to the output terminal OUT, and the other comb electrode is grounded.

[0008] A characteristic configuration of the present invention is a dual mode SAW.
In order to improve the group delay time characteristic of the filter, the electrode finger pitches Lr1 and Lr2, which are the intervals between the centers of adjacent electrode fingers of the two reflectors 4a and 4b, are different from each other. FIG. 2 shows the electrode pitch Lr1 of the reflector 4a and the electrode pitch L of the reflector 4b.
This figure shows the experimental results obtained by changing r2 in various ways, and shows the ratio Lr1 / Lr2 of the electrode finger pitches (hereinafter referred to as the reflector electrode pitch ratio) and the maximum within the 3 dB bandwidth of the dual mode SAW filter. The relationship with the group delay time deviation Δτ is shown. That is, if the horizontal axis is the reflector electrode pitch ratio Lr1 / Lr2 and the vertical axis is the maximum group delay time deviation Δτ, the reflector electrode pitch ratio Lr
As 1 / Lr2 increases, the maximum group delay time deviation Δτ monotonically increases. Therefore, when the curve of the maximum group delay time deviation Δτ in FIG. 2 is approximated by a polynomial using the reflector electrode pitch ratio Lr1 / Lr2, the following expression can be obtained. Δτ = 141.79 × (Lr1 / Lr2) ² −262.55 × (Lr1 / Lr2) +121.88 (1) For example, if the reflector electrode pitch ratio Lr1 / Lr2 is set to 0.994 or less, per section of the dual mode SAW filter Can be reduced to 1 μs or less. The maximum group delay time deviation .DELTA..tau. When the double mode SAW filters are cascaded in two stages may be obtained by doubling .DELTA..tau. In FIG.

On the other hand, the stop bands formed by the reflectors 4a and 4b are shifted on the frequency axis by the difference in the reflector electrode pitch as shown in FIG. If the passband of the dual mode SAW filter is within the band W of the overlapped portion of the stop band shown in FIG. 3, sufficient reflection can be obtained, and the insertion loss of the filter can be sufficiently reduced. FIG. 4 is a diagram showing the relationship between the bandwidth W of the portion where the stop bands formed by the reflectors 4a and 4b overlap and the reflector electrode pitch ratio Lr1 / Lr2. 4, the horizontal axis takes the electrode pitch ratio Lr1 / Lr2, the vertical axis using the normalized values SB (% indication) the bandwidth W at the center frequency F _0. Referring to FIG. 4, the relationship between the reflector electrode pitch ratio Lr1 / Lr2 and SB is expressed by the following equation as an approximate equation. SB = 10.079 × (Lr1 / Lr2) −9.513 (2) When the value of SB is less than 0, the surface wave vibration mode excited by the IDT electrodes 2 and 3 is not bound between the reflectors 4a and 4b, and resonance occurs. The mode no longer exists.

In order to keep the insertion loss of the dual mode SAW filter shown in FIG. 1 small, the bandwidth W of the overlapping portion of the respective stop bands formed by the reflectors 4a and 4b.
= SB × F ₀ must be set at least approximately equal to the pass bandwidth of the filter. Therefore, even if a narrow band dual mode SAW filter is designed, the reflector electrode pitch ratio Lr1 / Lr2 needs to be larger than 0.944.

FIG. 5 shows a dual mode SAW to which the present invention is applied.
It shows the filter characteristics of the filter. The piezoelectric substrate has 45 ° X cut lithium tetraborate, the logarithm of the IDT electrode 2 is 37.5, the logarithm of the IDT electrode 3 is 25.5, the reflectors 1 and 2,
And the number of reflectors is 50, and the reflector electrode pitch ratio Lr1 / Lr2
Was set to 0.980. ID of this dual mode SAW filter
A two-stage cascaded double-mode SAW filter is configured by cascade-connecting sides having the same number of T electrode pairs. As is apparent from FIG. 5, the maximum group delay time deviation Δ per one stage of the dual mode SAW filter while satisfying the 3 dB bandwidth of 300 kHz.
τ is 0.87 μsec., and even when two stages are cascaded, Δ
τ is 1.73 μsec., which is a digital mobile phone IF
Now applicable to filters.

The above description is the simplest dual mode SAW
Although the filter has been described, the present invention relates to a first-order to third-order longitudinally coupled dual-mode SAW filter, a first-order to third-order longitudinally-coupled triple-mode SAW filter, and a first-order to third-order to fifth-order longitudinally-coupled triple-mode SAW filter. It goes without saying that the present invention can be applied to a longitudinally coupled multi-mode SAW filter such as a filter. Further, although a lithium tetraborate single crystal piezoelectric substrate has been described as the piezoelectric substrate, the present invention can be applied to other piezoelectric substrates, for example, piezoelectric crystals such as quartz, lithium tantalate, lithium niobate, and langasite.

[0013]

The present invention is constructed as described above. In a longitudinally coupled multi-mode SAW filter comprising a plurality of IDT electrodes on a piezoelectric substrate and reflectors arranged on both sides thereof, a desired delay is obtained. If the reflector electrode pitch ratio is appropriately set in accordance with the time deviation, not only a desired group delay time deviation can be obtained, but also a vertically coupled multiple mode SA having a small insertion loss.
It has an excellent effect that a W filter can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing an electrode pattern of a longitudinally coupled dual mode SAW filter according to the present invention.

FIG. 2 is a diagram showing a relationship between a reflector electrode pitch ratio Lr1 / Lr2 and a group delay time deviation Δτ.

FIG. 3 is a diagram illustrating a bandwidth W overlapping a stop band of each of the reflectors 4a and 4b.

FIG. 4 shows a reflector electrode pitch ratio Lr1 / Lr2 and a reflector 4a;
FIG. 9 is a diagram showing a relationship between an overlapped bandwidth W of 4b and a fractional bandwidth SB (%) normalized by its center frequency.

FIG. 5 is a diagram showing a pass band characteristic A and a group delay time characteristic B of a filter in which two vertically coupled dual mode SAW filters according to the present invention are cascaded.

FIG. 6 is a diagram showing a pass band characteristic α and a group delay time characteristic β of a conventional filter in which two longitudinally coupled dual mode SAW filters are connected in cascade.

[Explanation of symbols]

1. Piezoelectric substrate 3. IDT electrodes 4a, 4b. Reflector Lr1, Lr2. Electrode pitch of reflectors 4a, 4b.DELTA..tau .. Group delay time deviation (.mu.s) W. Reflectors 4a, 4b. The bandwidth of the overlapped portion of the stop band SB: The above W divided by the center frequency of the stop band (% display)

Claims (2)

[Claims] In a longitudinally coupled multiplex mode in which a plurality of IDT electrodes are arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave and grating reflectors are arranged on both sides thereof, the electrode pitches of the two reflectors are set to each other. A surface acoustic wave filter characterized by being different. 2. The elasticity according to claim 1, wherein the electrode pitch ratio Lr1 / Lr2 of the two reflectors is set to 0.944 <Lr1 / Lr2 ≦ 0.994 when the electrode pitches of the two reflectors are Lr1 and Lr2, respectively. Surface wave filter.