Surface acoustic wave filter

Details Surface acoustic wave filter Surface acoustic wave filter

1998-09-22

2001-09-18

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S133000, C333S194000, C310S31300R

Reissue Patent

active

RE037375

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to surface acoustic wave (SAW) filters, and more particularly to a ladder-type SAW filter suitable for an RF (Radio Frequency) filter provided in pocket and mobile telephones, such as automobile phone set and portable phones.
2. Description of the Prior Art
In Japan, an automobile phone or portable phone system has a specification in which a transmission frequency band is ±8.5 MHz about a center frequency of 933.5 MHz. The ratio of the above transmission band to the center frequency is approximately 2%.
Recently, SAW filters have been employed in automobile phone or portable phone systems. It is required that the SAW filters have characteristics which satisfy the above specification. More specifically, it is required that the pass band width is so broad that 1) the ratio of the pass band to the center frequency is equal to or greater than 2%, 2) the insertion loss is small and equal to 5 dB-2 dB, and 3) the suppression factor is high and equal to 20 dB-30 dB.
In order to satisfy the above requirements, SAW filters are substituted for conventional transversal filters. Generally, SAW elements are so connected that a ladder-type filter serving as a resonator is formed.
FIG. 1
is an equivalent circuit of a SAW filter disclosed in Japanese Laid-Open Patent Publication No. 52-19044. A SAW filter
1
shown in
FIG. 1
comprises a SAW resonator
3
in a series arm
2
, and a SAW resonator
5
in a parallel arm
4
. The equivalent parallel capacitance C
OB
of the resonator
5
in the parallel arm
4
is larger than the equivalent parallel capacitance C
OA
of the resonator
3
in the series arm
2
.
The SAW filter
1
shown in
FIG. 1
has a characteristic shown in
FIG. 2. A
curve
6
shows an attenuation quantity v. frequency characteristic of the SAW filter
1
. As indicated by arrows
7
shown in
FIG. 2
, the suppression factor increases as the equivalent parallel capacitance C
OB
increases. However, as the equivalent parallel capacitance C
OB
increases, the band width decreases, as indicated by arrows
8
, and the insertion loss increases, as indicated by an arrow
9
. Hence, the characteristic deteriorates, as indicated by a broken line
10
. When trying to obtain a suppression factor equal to or larger than 20 dB, the band width is decreased so that the ratio of the pass band to the center frequency is equal to or smaller than 1%, and does not satisfy the aforementioned specification of the 800 MHz-band radio systems.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a SAW filter in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a SAW filter having a large band width, a large suppression factor, and a small insertion loss.
The above objects of the present invention are achieved by a SAW filter comprising: a first SAW resonator (
21
, R
1
A, R
1
B) having a pair of terminals and a predetermined resonance frequency (f
rp
), the first SAW resonator being provided in a parallel arm (
24
) of the SAW filter; a second SAW resonator (
23
) having a pair of terminals and a predetermined resonance frequency (f
rs
) approximately equal to the predetermined antiresonance frequency of the first SAW resonator (f
ap
), the second SAW resonator being provided in a series arm (
24
) of the SAW filter; and an inductance element (
25
, L
1
) connected in series to the first SAW resonator.


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