Interdigital transducer for use in a surface acoustic wave filter

An interdigital transducer for use in a surface acoustic wave filter has two busbars of an irregular shape to reduce the effect of internally reflected surface waves. The busbars will usually have an identical shape to one another and have stepped outer and inner edges. The distance between a portion of the outer edge and a corresponding portion of the inner edge can be constant for a particular busbar or that distance can vary. With previous busbars, the outer and inner edges are linear and each busbar has a rectangular shape.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an interdigital transducer for use in a surface 
acoustic wave (SAW) filter and in particular to transducers having busbars 
with edges of non-linear shapes. 
2. Description of the Prior Art 
Transducers formed from a thin film of metal pattern on a piezoelectric 
substrate are known. The pattern is shaped so that the transducer has two 
busbars with a plurality of electrodes extending between the busbars. The 
electrodes are parallel to one another and each electrode has a break. The 
transducer is connected within the filter so that when voltage is applied 
across the busbars, electric fields between the electrodes generate 
surface acoustic waves. The busbars have a rectangular shape. The surface 
waves experience reflections at any discontinuities, such as features in 
the patterns or substrate edges. These reflections produce spurious 
signals which distort the response of the transducer. Substrate edges are 
usually cut at special angles to reflect waves into harmless directions. 
An acoustic absorber is used at critical points to attenuate the unwanted 
waves. However, these techniques are not directly applicable to unwanted 
reflections occurring within the transducer itself. These reflections can 
occur at the boundaries between the free surface region, the region under 
the busbars and the region containing the electrodes. The surface wave 
propagation velocity is different in each region. Although reflections 
from the boundaries between propagation regions are very weak, they do 
produce significant effects, particularly in the stopband region on the 
high frequency side of a SAW filter passband. 
SUMMARY OF THE INVENTION 
An interdigital transducer for use in a surface acoustic wave filter has a 
thin film of metal pattern on a piezoelectric substrate. The pattern is 
shaped so that the transducer has two busbars with a plurality of 
electrodes extending between the busbars. The electrodes are parallel to 
one another. Each electrode has a break, the transducer being connected 
within the filter so that when voltage is applied across the busbars, 
electric fields between the electrodes generate surface acoustic waves. 
Each busbar has an inner edge and an outer edge, the inner edge extending 
between each of the electrodes. The outer edge has outermost portions and 
all outermost portions are parallel to said inner edge. The inner and 
outer edges have a non-linear shape and are shaped relative to one another 
to reduce the effect of internally reflected surface waves.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In FIG. 1, there is shown a prior art filter 2 having an input transducer 4 
and an output transducer 6. Each transducer 4, 6 has two busbars 8, 10 
with electrodes 12 extending therebetween. Each electrode has a break 14. 
The filter 2 has absorbers 15 at either end. The output transducer 6 has 
N+1 electrodes where N is an integer equal to or greater than zero. 
It can be seen that each busbar 8, 10 has a straight outer edge 16 that is 
parallel to an inner edge 18. The point at which the electrodes join the 
busbars are all in a straight line parallel to a central axis of the 
transducer. 
In FIG. 2, a busbar 20 has an outer edge 22 and an inner edge 24 that each 
have a non-linear shape. The outer edge 22 has outermost portions 26 and 
all outermost portions 26 are parallel to the inner edge 24. The outer 
edge 22 has a random shape and the inner edge is located so that the 
distance between each outermost portion 26 and the inner edge 
corresponding to that outermost portion is a constant distance L. 
In FIG. 3, a busbar 28 has an outer edge 30 with outermost portions 32 and 
an inner edge 34. The outer edge 30 has a sawtooth or stepped 
configuration that steadily increases and then steadily decreases by a 
fixed amount. The distance between each outermost portion 32 and that part 
of the inner edge corresponding to the outermost portion is constant. 
In FIG. 4, there is shown a busbar 36 having an outer edge 38 and outermost 
portions 40 with an inner edge 42. The outer and inner edges are randomly 
shaped and the distance between the outermost portions and a corresponding 
portion of the inner edge varies. 
In each of FIGS. 2, 3 and 4, only one busbar is shown, but the two busbars 
of the same transducer usually have an identical shape. Also, a transducer 
will usually have thousands of electrodes and, for ease of illustration, 
only a relatively small number of these electrodes is shown. A particular 
pattern of electrodes can be repeated throughout the busbar. 
Usually the busbars have an identical shape. With optical pattern generator 
technology, where the pattern is found by repeated flashes of a reticle, 
all four busbar edges must move together and it would therefore be 
extremely inefficient to make the busbars different from one another. 
However, with E-beam mask technology, for example, the four edges can be 
given independent shapes, without penalty. While there is no particular 
advantage of doing this and while it will increase the complexity of data 
preparation, the performance of the transducer will not be diminished. 
The thin film of metal pattern is usually aluminum. The piezoelectric 
substrate can be made of any suitable material, for example, quartz, 
lithium niobate or lithium tantalate. A masked pattern is formed using a 
reticle which would be moved in steps in the `x` direction to flash out an 
electrode gap at each step. By moving the reticle in the `y` direction at 
each step as well, arbitrary edge profiles can be formed. A second pass 
with a different reticle is used to define the electrode breaks. To 
provide independent patterns of all four edges (i.e. an inner and outer 
edge for each busbar) requires four times as many flashes which would 
usually be prohibitively costly in machine time. With other coventional 
mask generation technologies, there is no penalty in specifying 
independent edge patterns. Edge patterns could be random or sawtooth 
structures or a combination of random and sawtooth structures. The outer 
edge profiles shown in the drawings have a rectangular structure. 
Comparable random or sawtooth structures could be produced from 
non-rectangular structures, for example, a parallelogram. When a 
parallelogram is used, the outermost portions of the outer edge will still 
be parallel to the inner edge but a side edge extending to and from the 
outermost portion will be at an angle between 0.degree. and 90.degree. 
relative to the inner edge.