Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-08-29
2004-08-03
Young, Brian (Department: 2819)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S193000, C310S31300R, C310S31300R
Reexamination Certificate
active
06771145
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device especially having a balance-unbalance conversion function.
2. Description of the Related Art
Remarkable technical progress has been made in recent developments of small-sized and light-weight portable telephones. In order to produce such portable telephones, parts with a combination of plural functions have been made as well as the number of components has been reduced and the size of the components has been decreased. In these situations, in recent years, an extensive investigation of surface acoustic wave filters for use in the RF stages of portable telephones, having a balance-unbalance conversion function, a so-called balun function, had been made, and such surface acoustic wave filters have been used mainly in the GSM system (Global System for Mobile Communications).
In the case in which a balanced line such as a twin-lead type feeder and an unbalanced line such as a coaxial cable are connected directly to each other, unbalanced current flows, and undesirably, the feeder itself functions as an antenna. Thus, the team “balun” means a circuit for preventing the unbalanced current and matching the balanced line and the unbalanced line to each other.
Referring to conversion between a balanced signal and an unbalanced signal, in some cases, the input impedance (e.g., 75&OHgr; for an unbalanced signal) and the output impedance (e.g., 300&OHgr; for a balanced signal) are different from each other by approximately a factor of four.
As the basic configuration of a surface acoustic wave device having the above-described balance-unbalance conversion function, the configuration shown in
FIG. 14
is widely used. Longitudinally coupled resonator type surface acoustic wave elements
101
and
102
are contained in the configuration shown in FIG.
14
.
The surface acoustic wave element
101
contains interdigital electrode portions referred to as an interdigital transducer, hereinafter referred to as IDT,
104
,
103
, and
105
, and has reflectors
106
and
107
arranged so as to sandwich the IDTs.
The surface acoustic wave element
102
contains three IDTs
109
,
108
, and
110
arranged along the propagation direction of a surface acoustic wave and, moreover, reflectors
111
and
112
arranged so as to sandwich these IDTs.
The above-described configuration contains an unbalanced terminal
113
which causes the terminals on one side of the surface acoustic wave elements
101
and
102
to be electrically connected in parallel to each other, and balanced terminals
114
and
115
which are connected in series with the terminals on the other side of the surface acoustic wave elements
101
and
102
.
The surface acoustic wave element
101
and the surface acoustic wave element
102
are different from each other in that the IDT
103
and the IDT
108
are inverted with respect to each other, and thereby, signals output from the balanced terminal
114
and the balanced terminals
115
are 180° out-of-phase. Thus, an unbalanced signal input through the unbalanced terminal
113
is converted to balanced signals which are output from the balanced terminals
114
and
115
.
In the above-described configuration, the output impedance is about four times of the input impedance. By exchanging the input terminal and the output terminals in the configuration of
FIG. 14
, a surface acoustic wave device in which the input impedance is about four times of the output impedance, and balanced inputs and an unbalanced output signal are applied can be provided.
Furthermore, for the surface acoustic wave device having a balance-unbalance conversion function, an excellent attenuation characteristic presented out of the pass band is required. Japanese Unexamined Patent Application Publication No. 6-177697 discloses a method of improving the above-described out-band characteristics. In this Patent Application, as shown in
FIG. 15
, the configuration is used in which surface acoustic wave resonators
202
and
203
are electrically connected in series with the input and output terminal sides of the surface acoustic wave element
201
, respectively.
FIG. 16
shows an example of a surface acoustic wave device having a balance-unbalance conversion function which adopts the above-described configuration. Surface acoustic wave resonators
303
and
304
are connected in series with the input sides of the surface acoustic wave elements
301
and
302
, respectively, and surface acoustic wave elements
305
and
306
are connected in series with the output sides, respectively. With this configuration, high attenuation and balance-unbalance conversion can be realized.
Referring to the transmission characteristics in the pass bands caused between the unbalanced signal terminal and the balanced signal terminals of the surface acoustic wave device having a balance-unbalance conversion function, it is required that the amplitude characteristics should be the same, and the phases should be reversed from each other by 180°. These characteristics are expressed in terms of an amplitude balancing degree and a phase balancing degree.
Referring to the amplitude and phase balancing degrees, the surface acoustic wave device having a balance-unbalance conversion function is assumed as a three port device, and for example, the unbalance input terminal is referred to as a first port, and the balanced output terminals are referred to as second and third ports, respectively. In this case, the amplitude balancing degree [A] is defined by A=[20log(S
21
)]−[20log(S
31
)], and the phase balancing degree [−180] is defined by B=[<S
21
−<S
31
], in which S
21
represents a transfer coefficient for the transfer from the first port to the second port, and S
31
represents a transfer coefficient for the transfer from the first port to the third port. The symbol [ ] represents the absolute value. Regarding the balancing degrees, the amplitude balancing degree is zero dB, and the phase balancing degree in the pass band of a surface acoustic wave device is zero degree in the ideal condition.
However, in practice, deviations from the balancing degrees are caused in the configuration of FIG.
16
. The values are on such a large level as to become problems in practical use. The reason lies in that in the configuration of
FIG. 16
, the electrode fingers of the IDT
307
adjacent to the IDTs
308
and
309
are grounding electrode fingers, while the electrode fingers of the IDT
312
adjacent to the IDTs
313
and
314
, respectively, are signal electrode fingers.
In the case in which the signal electrode and the grounding electrode are adjacent to each other in each of the IDT-IDT interval areas, the efficiency of conversion to electric current caused in the resonance mode having an intensity peak in each IDT-IDT interval area is improved. Thus, the insertion loss in the pass band, especially on the high frequency side, is decreased compared with the case in which the grounding electrode and the signal electrode are adjacent to each other. Moreover, the pass bandwidth is increased, and also, a deviation is caused in the phase-relationship.
FIG. 17
shows differences between the frequency characteristics of the surface acoustic wave filters
320
and
321
shown in FIG.
16
and those between the phase characteristics thereof (the matching is made at 100&OHgr;, and the characteristics are obtained in the configuration of FIG.
16
).
The difference between the frequency characteristics of the surface acoustic wave filters
320
and
321
becomes large especially on the high frequency side of the pass band. Moreover, the phase characteristics of the surface acoustic wave filters
320
and
321
are not completely inverted from each other, and some deviation from the complete inversion is present. If a surface acoustic wave device is formed using the surface acoustic wave filters
320
, and
321
, these differences will deteriorate the
Ouchi Minefumi
Yata Masaru
Nguyen John B
Young Brian
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