Surface acoustic wave filter

• US 5,770,985 A
• Filed: 06/27/1996
• Issued: 06/23/1998
• Est. Priority Date: 07/08/1993
• Status: Expired due to Term

• Current Assignee: Murata Manufacturing Co Limited
• Original Assignee: Murata Manufacturing Co Limited
• Inventors: Ieki, Hideharu, Ushiroku, Tadamasa
• Primary Examiner(s): GAMBINO, DARUIS
• Application Number:
  US08/671,086
• Time in Patent Office: 726 Days
• Field of Search: 333/193-196, 310/131 R, 310/313 B
• US Class Current: 333/193
• CPC Class Codes: H03H 2250/00 Indexing scheme relating to dual- or multi-band filters
• CPC Class Codes: H03H 9/1455 constituted of N parallel or series transducers
• CPC Class Codes: H03H 9/6426 Combinations of the characteristics of different transducers
• CPC Class Codes: H03H 9/6436 having one acoustic track only

Abstract

A surface acoustic wave (SAW) filter includes a plurality of interdigital transducers located on a piezoelectric substrate along a surface wave propagation direction, at least a single one-port SAW resonator being connected in parallel with an output side of the SAW filter, while the resonance frequency (f₀) of the SAW resonator is set in frequency region level which is lower than the pass band of the SAW filter.

Claims

1. A surface acoustic wave filter having an input end and an output end, said filter comprising:

a piezoelectric substrate;

a plurality of interdigital transducers located on said piezoelectric substrate and arranged along a surface wave propagation direction in a surface wave propagating region, said plurality of interdigital transducers forming a surface acoustic wave filter part having a pass band and having input and output sides directly coupled to said input and output ends of said surface acoustic wave filter, respectively; and

a one-port SAW resonator, having at least one interdigital transducer and being connected at least between one of said input and output sides of said surface acoustic wave filter part and a ground potential, said one-port SAW resonator being located in a region of the piezoelectric substrate other than said surface wave propagating region,

a resonance frequency of said one-port SAW resonator being set at a frequency region lower than the pass band of said surface acoustic wave filter part.

2. A surface acoustic wave filter in accordance with claim 1, wherein said one-port SAW resonator is formed on said piezoelectric substrate.

3. A surface acoustic wave filter in accordance with claim 1, wherein said piezoelectric substrate is formed by a 36°

Y cut-LiTaO₃ substrate,

said interdigital transducer of said one-port SAW resonator being so structured that the following equation (1) is satisfied when said SAW filter is provided with a single said one-port surface acoustic wave resonator;

##EQU3## while the sum of (N×

A)/f₀ of respective said resonators is not more than 30 when a plurality of said one-port SAW resonators are connected in parallel with each other, assuming that f₀ (MHz) represents the resonance frequency of said one-port SAW resonator, N represents the number of pairs of electrode fingers, and A (μ

m) represents an overlap length.

4. A surface acoustic wave filter in accordance with claim 1, wherein said piezoelectric substrate consists of 64°

Y cut-LiNbO₃,

said interdigital transducer of said one-port SAW resonator being so structured that the following equation (2) is satisfied when said SAW filter is provided with a single said one-port surface acoustic wave resonator;

##EQU4## while the sum of (N×

A)/f₀ of respective said resonators is not more than 45 when a plurality of said one-port SAW resonators are connected in parallel with each other, assuming that f₀ (MHz) represents the resonance frequency of said one-port SAW resonator, N represents the number of pairs of electrode fingers, and A (μ

m) represents an overlap length.

5. A surface acoustic wave filter in accordance with claim 1, wherein said one-port SAW resonator is connected to said surface acoustic wave filter part to be in parallel with said input or output side of said surface acoustic wave filter part.

6. A surface acoustic wave filter in accordance with claim 1, wherein more than two said one-port SAW resonators are connected in parallel with each other to be connected in parallel with said input or output side of said surface acoustic wave filter part.

7. A surface acoustic wave filter in accordance with claim 1, wherein said one-port SAW resonator is connected in parallel with each of said input and output sides of said surface acoustic wave filter part.

8. A surface acoustic wave filter in accordance with claim 1, wherein said surface acoustic wave filter part is formed by a interdigited interdigital type SAW filter.

9. A surface acoustic wave filter in accordance with claim 1, wherein said surface acoustic wave filter part is formed by a two IDTs type SAW filter.

10. A surface acoustic wave filter in accordance with claim 1, wherein said surface acoustic wave filter part is formed by a three IDTs type SAW filter.