Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-04-26
2004-05-04
Callahan, Timothy P. (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S198000, C333S135000
Reexamination Certificate
active
06731185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave devices and communication apparatuses including such surface acoustic wave devices. More particularly, the present invention relates to a surface acoustic wave device that is advantageously used as a surface acoustic wave branching filter and to a communication apparatus including such a surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices (hereinafter referred to as “SAW devices”) include a surface acoustic wave element utilizing surface acoustic waves (SAWs) that propagate along the surface of a piezoelectric element, and are used in delay lines, filters, resonators, and other apparatuses. Since SAWs have a shorter wavelength than electromagnetic waves, they offer advantages in that, for example, SAW devices can be easily miniaturized. Thus, in the field of portable telephones and other electronic apparatuses, SAW devices are often used as filters in radio frequency circuits.
In recent years, particularly in the field of mobile communication, including portable telephones, there has been an increasing demand for further miniaturization and lower profile. Thus, SAW devices have also been required to be reduced in size and height. In particular, a portable telephone performs both transmission and reception in different frequency bands with a single antenna. Therefore, the demands on the use of SAW devices as branching filters are becoming greater.
Examples of technologies for such branching filters include a SAW branching filter disclosed in Japanese Unexamined Patent Application Publication No. 5-167388. This technology uses a plurality of SAW resonators (hereinafter simply referred to as “resonators”), as shown in
FIG. 10
, to constitute two SAW ladder filters (hereinafter may be referred to as “filters”). The filters are connected in parallel to define a branching filter.
More specifically, the branching filter uses ladder filters F
i
and F
ii
having a configuration in which series resonators S and parallel resonators P are alternately connected, and the filters F
i
and F
ii
are connected in parallel to a common terminal T
0
. Among the resonators S and P that constitute the filters F
i
and F
ii
, resonators S
i
and S
ii
that are directly connected to the common terminal T
0
are connected in series. When the filters F
i
and F
ii
are compared, the filter F
ii
has a relatively high bandpass frequency, and is used as a reception filter when used in a communication apparatus. On the other hand, the filter F
i
is used as a transmission filter.
As described above, with the branching filter being constituted by two SAW filters, and with respect to the filter F
i
, the following filter characteristic (an impedance characteristic) is required. That is, the filter F
i
needs to have an impedance that is close to the impedance of the entire circuit in the passband of the filter F
i
, and needs to have an impedance that is significantly larger than the impedance of the entire circuit in a rejection band that is a passband of the filter F
ii
. When typical transversal SAW filters are used, it is not easy to achieve such a filter characteristic, resulting in a complicated circuit configuration of the entire branching circuit.
In contrast, the technology of Publication No. 5-167388 discussed above uses series resonators in which the resonators S
i
and S
ii
that are in closest proximity to the common terminal T
0
are connected in series. Thus, the series resonators allow matching of impedance characteristics of the filters. The series resonators can also be used for phase adjustment in the configuration of the entire branching filter. Consequently, the impedance in the rejection band outside the range of the passband becomes quite high relative to the circuit impedance, thus making it possible to achieve the impedance characteristic described above.
In addition, Japanese Unexamined Patent Application Publication No. 11-068512 discloses a branching filter that uses SAW filters having a configuration other than a ladder filter. This technology utilizes, in the same manner as Publication No. 5-167388 discussed above, series resonators as elements for phase adjustment for a branching filter configuration.
In the case of a configuration in which SAW ladder filters are used to constitute a branching filter as discussed above, when resonators that are directly connected to a common terminal are connected in series in each SAW filter, the resonators (series resonators) can function as elements for phase adjustment. The series-connected resonators can also be used for phase adjustment in a bandpass filter other than a SAW ladder filter.
In order to prevent the deterioration of the impedance characteristics of each SAW filter, the impedance and frequency of the series resonators are critical parameters that affect the characteristics of the entire branching filter. Thus, in the technology disclosed in Publication No. 11-068512, the frequency of the series resonators, and the resonant frequency in particular are designed to match the center frequency of the passband of the SAW filter. This makes it possible to effectively prevent a decrease in the impedance characteristics of the SAW filter including the series resonators.
In the technology of Publication No. 5-167388, the resonant frequency of the series resonators is not particularly limited. With a ladder filter, however, in order to prevent a decrease in the impedance characteristics, it is still preferable to match the resonant frequency of the series resonators to the center frequency of the passband of the SAW filter including the series resonators.
In the technologies of the related art, when the resonant frequency is matched to the center frequency of the passband of the SAW filter under the condition that the impedance of the series resonators is set to be high for phase adjustment, this causes a problem in that the passband width is reduced.
That is, since the series resonators are used for phase adjustment, there is a need to restrict the impedance to a high range. Consequently, changes in the impedance of the resonators become considerably sharp relative to changes in the frequency. In particular, the frequency dependence of the impedance is maximized between the resonant frequency and the anti-resonant frequency.
When the resonant frequency of the series resonators is set to be the same as the center frequency of the passband of the SAW filter, there is no particular problem in a frequency band lower than the center of the passband since the impedance of the series resonators is low. In contrast, in a frequency band higher than the center of the passband, a sharp change in the impedance causes the impedance of the series resonators to become high. This results in an unsatisfactory matching in the high band, so that the high band, which is supposed to be a pass band, effectively becomes a rejection band (elimination band), thereby reducing the passband width.
When the band width ratio is small, even matching the resonant frequency of the series resonators to the center frequency of the passband does not substantially reduce the passband width. However, when the band width ratio is large, there is a problem in that sufficient matching between the SAW filters cannot be achieved.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a SAW device, constituted by a plurality of SAW filters, that achieves an improved matching characteristic and that prevents a reduction in passband even in the case of a large band width ratio of each SAW filter when used as a branching filter. In addition, preferred embodiments of the present invention provide a communication apparatus including such a novel SAW device.
According to a preferred embodiment of the present invention, a SAW device includes a first SAW filter that has a relatively low bandpass frequency, a second SAW filter that has a relatively high bandpass
Callahan Timothy P.
Keating & Bennett LLP
Luu An T.
Murata Manufacturing Co. Ltd.
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