JP2002057551A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce group delay time while maintaining linear phase characteristics and to reduce a group delay ripple. SOLUTION: A distance between the electrode centers of input side interdigital electrodes 12 and 14 and output side interdigital electrodes 16 and 18 is formed to decrease in proportion to the frequency of a surface acoustic wave(SAW) to be propagated on a channel so that a distance Lh between electrode centers on the channel to propagate the SAW of a highest frequency in a passband can be shorter than a distance L0 between electrode centers on a channel to propagate the SAW of a lowest frequency in the passband. As a result, phase characteristics become linear phase characteristics. Further, since the group delay time is reduced, the group delay ripple becomes small.

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 an input interdigital transducer having a plurality of oblique electrode fingers and a plurality of oblique electrodes arranged at a predetermined distance from the input interdigital transducer. The present invention relates to a surface acoustic wave filter having an output interdigital electrode having a finger and having a surface acoustic wave propagation region that varies depending on the frequency of an input signal.

[0002]

2. Description of the Related Art Conventionally, as a surface acoustic wave filter of this type, the distance between the electrode centers of the input-side IDT and the output-side IDT is constant, and the phase is linear with respect to the frequency of the input signal. The one having the following linear phase characteristic is used. In this conventional surface acoustic wave filter, the group delay time is determined by the wavelength of the signal having the lowest frequency among the passing frequencies, the logarithm of the oblique electrode fingers of the input interdigital electrode, the logarithm of the oblique electrode fingers of the output interdigital electrode, It is determined by the propagation distance of the surface acoustic wave between the input interdigital transducer and the output interdigital transducer.

[0003]

However, in such a surface acoustic wave filter, as the group delay time increases, the group delay ripple increases and the output signal is distorted.
It is necessary to reduce the group delay time and the group delay ripple. Therefore, a method of reducing the group delay time by reducing the number of diagonal electrode fingers can be considered, but this method deteriorates the cutoff characteristics of the surface acoustic wave filter. Also,
A method of shortening the propagation distance of the surface acoustic wave between the input-side interdigital electrode and the output-side interdigital electrode, that is, shortening the distance between the input-side interdigital electrode and the output-side interdigital electrode is also conceivable. . However, the distance between the input interdigital transducer and the output interdigital transducer cannot be shorter than the electrode width in the propagation region of the lowest frequency. Therefore, it is difficult to obtain a desired group delay time by this method. There are cases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to reduce the group delay time and the group delay ripple in a surface acoustic wave filter exhibiting linear phase characteristics.

[0005]

A surface acoustic wave filter according to the present invention comprises an input interdigital transducer having a plurality of oblique electrode fingers, and a plurality of input interdigital transducers arranged at a predetermined distance from the input interdigital transducer. An output interdigital electrode having oblique electrode fingers;
A surface acoustic wave filter in which the propagation area of the surface acoustic wave varies depending on the frequency of the input signal, wherein the input-side interdigital transducer and the output-side interdigital transducer are the input-side interdigital transducer in each of the propagation areas. The distance between the electrode center of the electrode and the output-side interdigital transducer is reduced in proportion to the frequency of the surface acoustic wave propagating in each of the propagation regions.

In the surface acoustic wave filter of the present invention, the distance between the electrode centers of the input and output interdigital transducers in each propagation region is proportional to the frequency of the surface acoustic wave propagating in each propagation region. It is formed to decrease. Such a surface acoustic wave filter exhibits a linear phase characteristic because the distance between the electrode centers is proportional to the frequency. On the other hand, since the electrode center distance in the high frequency band is shorter than the electrode center distance in the low frequency band, the change in phase becomes smaller as the frequency band becomes higher as compared with the conventional surface acoustic wave filter. Since the inclination of the entire phase is small, the group delay time is small. Therefore, the group delay time and the group delay ripple can be reduced while maintaining the linear phase characteristic.

[0007]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 is a plan view schematically showing the configuration of a surface acoustic wave filter 100 according to this embodiment. The surface acoustic wave filter 100 includes a piezoelectric substrate 1 made of a material exhibiting piezoelectricity.
0, input interdigital electrodes 12 and 14 having a plurality of oblique electrode fingers and receiving signals, and input interdigital electrodes 1
Output interdigital electrodes 16, 1 which are arranged at a predetermined distance from 2, 2 and 14 and have a plurality of oblique electrode fingers and output signals.
8 is provided. Each oblique electrode finger is formed such that the center interval between adjacent electrode fingers changes in a direction (Y direction in the figure) orthogonal to the propagation direction of the surface acoustic wave (X direction in the figure). The surface acoustic wave excited by is propagated through different propagation regions according to the frequency of the input signal. FIG.
5 is a graph showing frequency characteristics of insertion loss of the surface acoustic wave filter 100. As described above, the surface acoustic wave filter 100 has a broadband pass characteristic of passing a signal having a frequency of about 37 [MHz] to 43 [MHz].

The input interdigital transducers 12 and 14 and the output interdigital transducers 16 and 18 are connected to the center of the input interdigital transducers 12 and 14 and the output interdigital transducer 1 in each propagation region.
The distances from the centers of the electrodes 6 and 18 (hereinafter referred to as electrode center distances) are formed to slightly decrease in proportion to the frequency of the surface acoustic wave propagating in each propagation region. FIG.
3 is a graph showing a relationship between a distance between electrode centers in each propagation region and a frequency of a surface acoustic wave propagating in the propagation region. The input-side interdigital electrodes 12, 14 and the output-side interdigital electrodes 16, 18 are formed such that the distance between the electrode centers is proportional to the frequency of the surface acoustic wave with a slight negative slope. Here, the highest frequency fh in the pass frequency band
(= 43 [MHz]), the distance between the electrode centers in the area where the surface acoustic wave propagates is Lh, the lowest frequency in the pass frequency band is f0 (= 37 [MHz]), the center frequency is fc, and the wavelength at the center frequency fc. Is defined as λ0, and the distance L0 between the electrode centers in the region where the surface acoustic wave of wavelength λ0 propagates. FIG.
Electrode center distance and electrode center distance L in each propagation region
0 and the weighted distance ΔW normalized by the wavelength λ0
7 is a graph showing a relationship between the center frequency in each propagation region. As shown in FIG. 4, the weighting distance at the frequency fh is ΔW = 4, and the electrode center distance Lh is shorter than the electrode center distance L0 by about 4 × λ0.

Next, the surface acoustic wave filter 1 of the present embodiment
The phase characteristic of 00 and the frequency characteristic of the group delay time (hereinafter referred to as the group delay characteristic) will be described. In general, in a surface acoustic wave filter having oblique electrode fingers, there are many sub-filters having different pass frequency bands for each of small areas Δy0, Δy1... Δyj divided in the Y direction in FIG. It is possible to explain the characteristics of the surface acoustic wave filter by assuming that these sub filters exist and are connected in parallel to form a surface acoustic wave filter. Assuming that the center frequency of the pass frequency band of the sub-filter in the j-th channel is f _j , the distance Lj between the electrode centers in the j-th channel is proportional to the center frequency fj with a negative slope.

L _j = L 0 −αf _j (α> 0)

ΔL _j = −αΔf _j (1)

FIG. 5 is a conceptual diagram showing the relationship between the insertion loss A _j (f), the phase θ _j (f), and the frequency of the sub-filter of the j-th channel. the phase θ of the j-th channel
_{The j} (f) and the phase θ _{j + 1} (f) of the ( _{j + 1} ) th channel have linear phase characteristics having almost the same slope as shown in FIG. 5, but have a slight phase difference. Considering the phase gradient when the sub-filters are combined, if the wavelength in the j-th channel is λ _j , the phase difference Δθ _j between the j-th channel and the (j + 1) -th channel is

[Number 3] _{Δθ j = 2πL j + 1 /} λ j + 1 -2πL j / λ j = 2π (L j + ΔL j) / λ j + 1 -2πL j / λ j = 2π (L j / λ j + ₁₋ L _j / λ _j ) + 2π (ΔL _j / λ _{j + 1} ) = 2π · (L _j / V) Δf _j + 2π · (ΔL _j / V) · f _{j + 1} FIG. 6 is a graph showing the relationship between the combined phase θ (f) and the frequency. The phase θ (f) shows a linear phase characteristic that is proportional to the frequency within the pass frequency band.

The group delay time τ of the surface acoustic wave filter 100
Is the frequency derivative of the phase,

Τ (fj) = Δθ / (Δf _j · 2π) = L _j / V + 2π · (ΔL _j / V) · f _{j + 1} / (2π · Δf _j ) = L _j / V + (f _{j + 1} / v) (ΔL _j / Δf _j ) (2) From equation (1), since ΔL _j / Δf _j (= −α),
From the equation (2), the group delay time τ can be arbitrarily determined by changing ΔL _j / Δf _j . ΔL _j / Δ
By increasing f _j, that is, by reducing the center distance between electrode fingers on the high frequency band side, the group delay time τ
Can be reduced. FIG. 7 is a graph showing the group delay characteristics of the surface acoustic wave filter 100, and FIG. 8 is an enlarged graph of a part of the graph shown in FIG. Further, FIG. 9 shows a group delay characteristic of the surface acoustic wave filter electrode center distance L _j of each channel to the same extent as the surface acoustic wave filter 100 was inserted loss constant in each channel in the surface acoustic wave filter 100 A graph,
FIG. 10 is a graph obtained by enlarging a part of the graph shown in FIG. As shown in FIGS. 7 to 10, in the surface acoustic wave filter 100, the group delay time is about 1.22 [μs], and the electrode center distance Lj of each channel is constant in each channel. The group delay time is smaller by about 0.68 [μs] and the group delay ripple is smaller than that of.

[0013] As described above, in the surface acoustic wave filter 100, since the electrode center distance L _j in each channel is formed so as to be proportional to the center frequency f _j in each channel, while maintaining a linear phase characteristic, the group The delay time can be reduced, and the group delay ripple can be reduced.

In the surface acoustic wave filter 100, the group delay time can be changed by changing the slope of the distance between the electrode centers with respect to the frequency, that is, the slope of the weighted distance ΔW with respect to the frequency. FIG. 11 shows the weighted distance ΔW
Of the surface acoustic wave filter 100 to
12 is a graph showing the relationship between the weighted distance ΔW of the surface acoustic wave filter set to / 2 and the center frequency in each channel, FIG. 12 is a graph showing the group delay characteristics of this surface acoustic wave filter, and FIG. 12 is a graph obtained by enlarging a part of the graph shown in FIG. The group delay time is about 1.57 [μs] as shown in FIGS. 12 and 13, and is about 0.3 μm compared to the group delay time of the surface acoustic wave filter 100.
4 [μs], and the group delay ripple also increases. As described above, the group delay time of the surface acoustic wave filter can be set to various values by changing the inclination of the electrode center distance with respect to the frequency.

The present invention is not limited to these embodiments at all, and may be implemented in various forms without departing from the gist of the present invention, for example, in a form in which the number of electrode fingers is increased. Obviously you can get it.

[0016]

As described above, in the surface acoustic wave filter, by forming the distance between the electrode centers in each channel to be proportional to the center frequency in each channel, the group delay time can be reduced while maintaining the linear phase characteristic. This makes it possible to reduce the group delay ripple.

[Brief description of the drawings]

FIG. 1 is a plan view schematically showing a configuration of a surface acoustic wave filter 100 according to an embodiment.

FIG. 2 is a graph illustrating frequency characteristics of insertion loss of the surface acoustic wave filter 100.

FIG. 3 is a graph showing a relationship between a distance between electrode centers and a frequency.

FIG. 4 is a graph showing a relationship between a weighted distance ΔW and a center frequency in each channel.

FIG. 5 is a conceptual diagram showing a relationship between an insertion loss Aj (f), a phase θj (f), and a frequency of a sub-filter of a j-th channel.

FIG. 6 is a graph showing phase frequency characteristics.

FIG. 7 is a graph showing a group delay characteristic of the surface acoustic wave filter 100.

FIG. 8 is a graph obtained by enlarging a part of the graph shown in FIG. 7;

FIG. 9 is a graph showing a group delay characteristic of a surface acoustic wave filter in which a distance between electrode centers of each channel is constant in each channel.

FIG. 10 is a graph obtained by enlarging a part of the graph shown in FIG.

FIG. 11 is a graph showing the relationship between ΔW of a surface acoustic wave filter in which the slope of the weighting distance ΔW is 傾 き of that of the surface acoustic wave filter 100 and the center frequency in each channel.

12 is a graph showing a group delay characteristic of a surface acoustic wave filter in which a weighting distance ΔW is formed as shown in FIG.

FIG. 13 is a graph obtained by enlarging a part of the graph shown in FIG.

[Explanation of symbols]

12, 14 Input interdigital transducer, 16, 18 Output interdigital transducer, 100 surface acoustic wave filter.

Claims (1)

[Claims] An input interdigital transducer having a plurality of oblique electrode fingers, and an output interdigital electrode arranged at a predetermined distance from the input interdigital electrode and having a plurality of oblique electrode fingers. A surface acoustic wave filter in which the propagation area of the surface acoustic wave varies depending on the frequency of the input signal, wherein the input-side interdigital transducer and the output-side interdigital transducer are the input-side interdigital transducer in each of the propagation areas. A surface acoustic wave filter formed so that the distance between the center of the electrode and the output-side interdigital electrode decreases in proportion to the frequency of the surface acoustic wave propagating in each of the propagation regions.