Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2000-08-01
2002-08-20
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06437662
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use in a communication apparatus that is used in high frequency bands, and more particularly to a ladder type surface acoustic wave filter.
2. Description of the Related Art
Generally, as a surface acoustic wave filter designed to achieve good characteristics such as low loss and wide band characteristics, a ladder type surface acoustic wave filter that has one-port resonators alternately arranged on a series arm and parallel arms is known.
FIG. 1
is a schematic view of a ladder type surface acoustic wave filter as described above. In a surface acoustic wave filter
110
shown in
FIG. 1
, two surface acoustic wave resonators
111
a
and
111
b
are connected to define a series arm, and three surface acoustic wave resonators
112
a
,
112
b
, and
112
c
are connected to the series arm to define parallel arms, respectively. Each of these surface acoustic wave resonators
111
a
,
111
b
,
112
a
,
112
b
, and
112
c
includes an interdigital transducer
124
having a plurality of electrode fingers
125
, and a pair of reflectors
122
having a plurality of electrode fingers
123
which are provided on both sides of the interdigital transducer
124
.
In the field of communication apparatuses using the above-described surface acoustic wave filter, high-frequency requirements have been increasing, and development of new surface acoustic wave filters meeting the increased requirements are progressing. For example, Japanese Unexamined Patent Publication No. 9-167936 discloses a 38-to-46-degree Y-cut X-propagation LiTaO
3
substrate to meet the high-frequency requirements. Conventionally, as a substrate of the surface acoustic wave filter, a 36-degree Y-cut X-propagation LiTaO
3
substrate has conventionally been used because it produces a low propagation loss and has a large electromechanical coupling coefficient.
Although the propagation loss decreases where the thickness of an electrode film defining the interdigital transducers is negligibly small relative to the wavelength of a surface acoustic wave, the 36-degree Y-cut X-propagation LiTaO
3
has a problem in that, the propagation loss increases where the thickness of an electrode film is increased. Particularly, as the wavelength of the surface acoustic wave decreases in the high-frequency band, the thickness of the electrode film relative to the wavelength becomes so large that the propagation loss increases. On the other hand, when the influence of bulk waves and the increase in electrode resistance are taken into account, reduction in thickness of the electrode film is not preferable because it causes reduction in the characteristics.
In view of the foregoing problems, Japanese Unexamined Patent Publication No. 9-167936 discloses that even in a case where the thickness of the electrode film is increased in consideration of the influence of bulk waves and the increase in the electrode resistance, the propagation loss can be reduced by use of the 38-to-46-degree Y-cut X-propagation LiTaO
3
substrate as a substrate of the surface acoustic wave filter.
Conventionally, as a modulation method for cellular phones, a TDMA (time division multiple method) has been used. Recently, however, a CDMA (code division multiple method) is used to efficiently transmit an increasing amount of information. In ordinary cellular phone systems, the total-system frequency band is divided via channels into smaller bands. In this case, according to the TDMA method, the frequency width per channel is as small as several tens of kilohertz (kHz). However, according to the CDMA method, the frequency width is as large as 1 MHz or more.
Where very small ripples exist in the passbands, the difference in the frequency width per channel according to the aforementioned modulation methods becomes apparent with the difference in influence of the ripples. Specifically, according to the TDMA method, when very small ripples exist in the passbands, deviation in loss does not increase since the per-channel frequency width is relatively small. However, according to the CDMA method, the deviation in loss increases since the per-channel frequency width is relatively large. In the cellular phone system, a large amount of loss makes modulation difficult. With a large amount of the deviation in loss that diffuses the frequency for information, a problem also arises in that the CDMA method itself makes modulation difficult. Therefore, with the CDMA method, very small ripples occurring in the passbands become apparent as a problem while such ripples have not caused a problem in the TDMA method. In particular, the very small ripples are required to be reduced to be less than 0.7 dB.
Nonetheless, in the conventional ladder type surface acoustic wave filter, ripples of more than 0.7 dB have occurred in the passbands because of interference of reflection caused in the interdigital transducers and interference of reflection caused in the reflectors in the series arm surface acoustic wave resonators.
Hereinbelow, a description will be given regarding reasons why the ripples are caused in the passbands. The description will be provided referring to the reflector as an example, but the description can also be applied to the interdigital transducers.
Each of
FIGS. 2 and 3
shows frequency characteristics of the reflector.
FIG. 2
shows the characteristics where the number of the electrode fingers is 50, while
FIG. 3
shows the characteristics where the number of the electrode fingers is 100. In either of the figures, the center frequency is 800 MHz.
As shown in
FIGS. 2 and 3
, outside of the stopband, the minimum value to which the reflection coefficient becomes small is repeated. With these minimum values, since excitation efficiency decreases, in view of impedance characteristics of the surface acoustic wave resonator, locally-high-impedance portions occur, as shown in
FIG. 4. A
surface acoustic wave resonator having the characteristics in which the aforementioned locally-high-impedance portions occur is series-connected as shown in
FIG. 5
, and transmission characteristics relative to the frequency are measured. As a result, it is known that very small ripples as shown in
FIG. 6
occur. In
FIG. 6
, a graph indicated by B is an enlarged view of a graph indicated by A, and scale points thereof are indicated on the right side of the vertical axis (other characteristic views in this Specification are similarly presented). As shown in
FIG. 6
, when the ripples occur in the transmission characteristics of the series-connection configuration, ripples also occur in filter characteristics of a surface acoustic wave filter configured using the aforementioned surface acoustic wave resonator. That is, by the influence of the minimum values, ripples occur in filter characteristics of the surface acoustic wave filter.
Hereinbelow, a description will be given of frequencies having the aforementioned minimum values of the reflection coefficients.
Expression 1 shown below can be used to regulate frequencies f having the minimum values of the reflection coefficients by a center frequency f
0
.
f/f
0
=(1
−K
11
/k
0
)±{(
K
12
/k
0
)
2
+(
n
0
/N
)
2
}
½
In the above, K
11
and K
12
represent, respectively, a self-coupling coefficient (coefficient representing the coupling strength between surface acoustic waves proceeding in the same direction), which is uniquely determined according to factors such as substrate material and electrode-film thickness, and a mutual coupling coefficient (coefficient representing the coupling strength between surface acoustic waves proceeding in directions opposing each other); k
0
represents the number of waves in the center frequency; n
0
represents an integer larger than 0; and N represents the number of the electrode fingers.
As shown in the expression that expresses the frequency having the minimum value, if the number N of the electrode fingers is infinite, (n
0
/N)
2
=0; however, i
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