JPH0460367B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is applicable to surface acoustic wave (SAW) filters.
IDT excitation type filter, especially group type unidirectional
Regarding IDT excitation type filter.

(Conventional technology) SAW filters are smaller than LC filters, etc.
It has become widely used because it is lightweight, does not require adjustment, is insensitive to vibration and shock, and is easy to mass produce, but it has the drawback of relatively high insertion loss. In particular, various unidirectional transducers such as three-phase type or reflector type have been proposed in order to reduce the bidirectional loss of 6 dB in general SAW filters, but among them, a 90-degree phase shifter is used. Filters using group-type unidirectional transducers are prized because they have relatively low loss.

However, even with a filter using a group type unidirectional transducer as described above, there is still a problem that the insertion loss is large and the filter characteristics are not satisfactory because the ohmic loss of the ground line (mian line) is not small. Ta.

(Objective of the Invention) The present invention has been made to eliminate the problems of the conventional IDT excitation filter using a unidirectional transducer as described above, and is a unidirectional IDT with significantly reduced insertion loss. The purpose is to provide an excitation type filter.

(Summary of the Invention) In order to achieve the above-mentioned object, in the present invention, the meander line is grounded at both ends and at a suitable location along the way, straddling a bus bar connecting IDT electrode fingers. be.

(Embodiments of the Invention) The present invention will be described in detail below with reference to embodiments shown in the drawings.

Before explaining the embodiments, a unidirectional transducer was first used to facilitate understanding of the present invention.
Let me briefly explain the SAW filter.

FIG. 2 is a conceptual diagram showing the general configuration of a SAW filter using a conventional group type unidirectional transducer.
A 90-degree phase IDT 3 is provided opposite to this, and the phase difference between the two is given by a phase shifter 4, and the amount IDT is
A meander line 5 is arranged between electrode fingers 2 and 3 and grounded to form a unidirectional input transducer. The meander line 5 has a line width of 1/2λ (λ
is the wavelength of the excited SAW), and is set to λ in the part where the phase changes from 90 degrees to zero, so that the SAW propagates in only one direction.

Incidentally, a transducer similar to the input transducer is generally arranged on the output side with a shield electrode 6 interposed therebetween.

As is clear from the above description, both of the meander lines 5 and 5' are routed through the input/output transducers over a considerable distance, and the grounding is performed only at the ends of the input/output transducers. As mentioned above, the ohmic loss becomes considerably large, which becomes the insertion loss of the filter and deteriorates the filter characteristics. Moreover, it goes without saying that this loss becomes more significant as the frequency of the excited SAW becomes higher.

In order to solve this problem, the present invention adopts the following configuration.

That is, as shown in FIG.
It is constructed so that it is extended at a suitable point in the middle to straddle each bus bar of the zero-degree phase IDT 2 and 90-degree phase IDT 3 and connect to the ground wires 7 and 8 disposed on the outside thereof, respectively. At this time, the connection between the meander line 5 and the ground wire is made by sputtering an insulating layer such as silicon oxide SiO ₂ on the surface of a required portion of the IDT bus bar, or by sputtering or the like.
After forming an insulating film by oxidizing with O ₂ gas, a conductive film is attached on the insulating part to connect the meander line 5 and the ground line 7 or 8.
Just connect it.

However, the method described above requires two photolithography steps, making the process complicated. Therefore, a pattern is used that exposes only the part where the ground wire straddles the bus bar, and the anode is connected to the IDT while passing current only to the bus bar. If oxidation is performed, only the part concerned will be oxidized, and if a conductor is vapor-deposited again on this insulating film and the meander line 5 and the ground line 7 or 8 are connected, this can be done in one photolithography process.

Above, the most common group type unidirectional SAW
Although only a filter has been described, the present invention is not intended to be applied only to this. For example, the present invention may be applied only to either one of the input and output transducers, or if the input transducer is bidirectional and has two filters on both sides. It will be obvious that the invention may also be applied to filters of the type with unidirectional output transducers.

Further, the present invention need not be limited to filters using SAW, but can be similarly applied to filters that utilize any wave that can be excited by IDT.

Finally, as a result of applying the present invention to a filter with 10 pairs of unidirectional input/output IDTs, a center frequency of 600 MHz, and a bandwidth of 40 MHz, the insertion loss of a conventional filter that was grounded only at both ends of the aminda line, which had an insertion loss of 8 dB, was reduced to meander. It was confirmed that the insertion loss was reduced to 3 dB by simply grounding at two additional points along the line, which was a significant effect.

(Effects of the Invention) Since the present invention is configured as described above, the present invention can be applied to a group type unidirectional surface acoustic wave filter that has been considered optimal for realizing a conventional low-loss IDT excitation filter, especially a surface acoustic wave filter. However, this effect more than compensates for the slight increase in manufacturing steps.

[Brief explanation of the drawing]

FIG. 1 is a partial plan view showing an embodiment of a unidirectional IDT excitation type filter according to the present invention, and FIG. 2 is a general configuration diagram of a conventional unidirectional SAW filter to which the present invention is applied. 1... Piezoelectric substrate, 2, 2' and 3, 3'... Input/output IDT, 5... Meander line, 6... Shield electrode.

Claims (1)

[Claims] 1. An input/output interdigital transducer (IDT) electrode is provided on the surface of the piezoelectric substrate, and one of the paired electrode fingers of the IDT electrode is in the zero-degree phase and the other is in the 90-degree phase, and the input/output IDT In a filter that arranges a meander line between one or both of the IDT electrode fingers and grounds the meander line, the meander line may be placed at a suitable location other than at both ends.
A unidirectional IDT excitation type filter characterized by reducing insertion loss of the filter by grounding it across a bus bar connecting IDT electrode fingers.