Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2003-03-06
2004-06-01
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S195000, C310S31300R
Reexamination Certificate
active
06744333
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter having an impedance that is different between the input and output sides and which has an unbalance-to-balance conversion capability.
2. Description of the Related Art
In recent years, the size and the weight of portable telephones are becoming increasingly smaller. To this end, in addition to the reduction in the number of component parts and the reduction in the size thereof, development of components having a plurality of functions is being pursued.
In view of the above situation, research is being pursued to produce a surface acoustic wave filter having a balance-unbalance conversion capability called balun capability for use in the RF stage of portable telephones, and some surface acoustic wave filters are used mainly in GSM.
In portable telephones, a portion extending from an antenna to a bandpass filter is generally arranged in an unbalanced fashion and has a characteristic impedance of 50 &OHgr;. On the other hand, amplifiers or other components following the filter are generally arranged in a balanced manner and have an impedance of 150 &OHgr; to 200 &OHgr;. In view of the above, it has been proposed to form a bandpass filter using a surface acoustic wave filter having a capability of converting 50 &OHgr; unbalanced impedance to 150-200 &OHgr; balanced impedance.
In a technique disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, four surface acoustic wave filter elements are used to realize conversion from an unbalanced input to a balanced output.
FIG. 28
shows the structure of the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123 cited above. In this surface acoustic wave filter device, a first surface acoustic wave filter unit
203
is constructed by cascading two surface acoustic wave filter elements
201
and
202
, and a second surface acoustic wave filter unit
206
is defined by cascading a surface acoustic wave filter element
204
and a surface acoustic wave filter element
205
having a transmission phase characteristic that is different by about 180° from that of the surface acoustic wave filter element
204
. The input/output terminals of the respective surface acoustic wave filter units
203
and
206
are connected in parallel or in series so that the parallel-connected terminals define unbalanced terminals and the series-connected terminals define balanced terminals.
FIG. 29
illustrates a surface acoustic wave filter device
211
having three IDTs, disclosed in Japanese Unexamined Patent Application Publication No. 6-204781. In this surface acoustic wave filter device
211
, two output-side IDTs
212
and
213
are disposed on respective opposite sides such that phases become opposite to each other, and the output terminals of the respective IDTs
212
and
213
define balanced terminals. One end of an input-side IDT
214
disposed at a central location defines an unbalanced terminal. In this structure, the input impedance may be set to 50 &OHgr;, and the output impedance may be set to 150-200 &OHgr;.
In the surface acoustic wave filters having the balance-unbalance conversion capability, expansion of the passband is desired to meet the requirement of expansion of passbands in portable telephone systems. In surface acoustic wave filters having the balance-unbalance conversion capability, it is required that the transmission characteristics from the unbalanced terminal to the balanced terminals should be equal in amplitude but different by 180° in phase, over the passband. That is, an improvement in the degree of balance is desired.
However, in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, the expansion of the passband causes the impedance of the surface acoustic wave filter elements to become capacitive. This resultant capacitance and the parasitic capacitance present between the two cascaded stages cause an impedance mismatch between surface acoustic wave filter units. This makes it difficult to achieve expansion of the passband.
Furthermore, because as many as four surface acoustic wave filter elements are used, complicated interconnections are required, and the complicated interconnections result in an increase in parasitic capacitance which in turn results in degradation in the degree of balance. Furthermore, the use of the large number of surface acoustic wave filter elements results in an increase in size which makes it difficult to obtain a small device size. Furthermore, the use of the large number of surface acoustic wave filter elements results in a reduction in the number of surface acoustic wave filter devices obtained from each wafer, and thus results in an increase in cost.
On the other hand, in the surface acoustic wave filter device disclosed Japanese Unexamined Patent Application Publication No. 6-204781, the two IDTs
212
and
213
have different structures so as to achieve the balanced terminals, and the locations of the IDTs
116
and
117
with respect to the location of the central IDT
214
are different from each other. Such differences in structure or location often cause degradation in the degree of balance. Furthermore, the series connection of the IDTs
212
and
213
on the balanced terminal side results in an increase in loss due to the resistance of the electrode fingers, which results in an increase in insertion loss in the passband.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter device which has unbalanced/balanced input/output terminals and which has a wide passband and a high degree of balance.
In the surface acoustic wave filter device according to a first preferred embodiment, the second and third surface acoustic wave filter elements are arranged such that they are substantially equal in transmission amplitude characteristic within the band but different in transmission phase characteristic by about 180°, and at least one IDT of the second surface acoustic wave filter element and at least one IDT of the third surface acoustic wave filter element are connected to at least one IDT of the first surface acoustic wave filter element. Thus, the surface acoustic wave filter device can achieve the balance-unbalance conversion capability by using the electrode connected to the first surface acoustic wave filter element as an unbalanced terminal and using the terminals connected to respective second and third surface acoustic wave filter elements as balanced terminals. In this surface acoustic wave filter device according to the first preferred embodiment of the present invention, unlike the conventional surface acoustic wave filter device in which four surface acoustic wave filter elements are used, the balance-unbalance conversion capability is achieved using only three surface acoustic wave filter elements. As a result, reductions in the size and the cost of the surface acoustic wave filter device having the balance-unbalance conversion capability are achieved.
Furthermore, the reduction in the number of surface acoustic wave filter elements results in a reduction in parasitic capacitance, which suppresses degradation in the degree of balance and which makes is easy to expand the passband.
In the surface acoustic wave filter device according to a second preferred embodiment of the present invention, the second and third surface acoustic wave filter elements are arranged such that they are substantially equal in transmission amplitude characteristic within a band but different in transmission phase characteristic by about 180°, a second IDT of the first surface acoustic wave filter element is connected to the second surface acoustic wave filter element, and a third IDT of the first surface acoustic wave filter element is connected to an IDT of the third surface acoustic wa
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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