Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-11-02
2003-09-23
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S193000
Reexamination Certificate
active
06624725
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lattice filter using a resonator consisting of a crystal vibrator or an elastic surface wave vibrator or the like.
2. Description of the Related Art
FIG. 4
shows structure of a prior art lattice filter. In
FIG. 4
, between a first input end
11
and a first output end
12
, and between a second input end
13
and a second output end
14
, there are provided first resonators
15
,
15
, each having the same characteristic respectively. Also, between the first input end
11
and the second output end
14
, and between the second input end
13
and the first output end
12
, there are provided second resonators
16
,
16
, each having the same characteristic respectively.
In this case, the first resonator
15
has a series resonance frequency f
1s
and a parallel resonance frequency f
1p
higher than the series resonance frequency f
1S
, and the second resonator
16
has also a series resonance frequency f
2s
and a parallel resonance frequency f
2p
higher than the series resonance frequency f
2S
. Also, the parallel resonance frequency of one or the other of the first and second resonator, for example f
2p
of the second resonator
16
, is substantially equal to the series resonance frequency f
1s
of the first resonator
15
, which is the other resonator. Accordingly, reactance curves of these resonators
15
,
16
become as shown in FIG.
5
.
Further, in these two types of resonators
15
,
16
, as shown in
FIG. 5
, their reactance curves have such relationship that they cross each other in a region of lower frequencies than the series resonance frequency f
2s
of the second resonator
16
, and that they cross each other in a region of higher frequencies than the parallel resonance frequency f
1p
of the first resonator
16
. Such crossing relationship can be obtained by setting physical dimensions within the respective resonators.
As a result, a transmission characteristic when a signal is inputted between the first input end
11
and the second input end
13
becomes a characteristic of a band pass filter which has between the series resonance frequency f
2s
of the second resonator
16
and the parallel resonance frequency f
1p
of the first resonator
16
as the passing band as shown in FIG.
6
.
Also, at frequencies f
L
and f
H
at which each other's reactance curves cross, phases at two output ends
12
and
14
become the same and therefore, no signal is generated between these two output ends
12
and
14
, but a characteristic which attenuates as shown in
FIG. 6
is obtained.
Therefore, in order to widen the passing band, a difference (&Dgr;1=f
1p
−f
1s
, &Dgr;2=f
2p
−f
2s
) between the series resonance frequency and the parallel resonance frequency of each resonator
15
,
16
is made larger, or the parallel resonance frequency f
2p
of the second resonator
16
is made lower than the series resonance frequency f
1s
of the first resonator
15
, whereby the series resonance frequency f
1s
of the first resonator
15
can be spaced apart from the parallel resonance frequency f
2p
of the second resonator
16
.
However, the difference between the series resonance frequency and the parallel resonance frequency in each resonator cannot be made much large because of constraints in the physical dimensions, but also for the similar reason, it becomes impossible to freely set a frequency at which the reactance curves cross each other as well.
Also, there is the problem that when the series resonance frequency of the first resonator is spaced apart from the parallel resonance frequency of the second resonator that is higher than the series resonance frequency, a ripple occurs in the transmission characteristic of the passing band.
SUMMARY OF THE INVENTION
Thus, in a lattice filter according to the present invention, even when the difference between the series resonance frequency and the parallel resonance frequency of each resonator cannot be made large, it is an object to make it possible to widen the passing band without causing any ripple in the passing band.
As means for solving the above-described problem, there is provided a lattice filter comprising: two first resonators which are substantially equal to each other in the first series resonance frequency and are substantially equal to each other in the first parallel resonance frequency, that is higher than the first series resonance frequency; two second resonators which are substantially equal to each other in the second series resonance frequency and are substantially equal to each other in the second parallel resonance frequency, that is higher than the second series resonance frequency; two first inductance elements which are substantially equal to each other in the inductance; and two second inductance elements which are substantially equal to each other in the inductance, wherein one of each of the first resonator and the first inductance element is connected to each other in series, and is inserted between the first input end and the first output end while the other of each of the first resonator and the first inductance element is connected to each other in series, and is inserted between the second input end and the second output end; and one of each of the second resonator and the second inductance element is connected to each other in series and is inserted between the first input end and the second output end while the other of each of the second resonator and the second inductance element is connected to each other in series and is inserted between the second input end and the first output end, whereby the parallel resonance frequency of the second resonator is caused to substantially coincide with the series resonance frequency based on the first resonator and the first inductance.
Also, a reactance curve based on the first resonator and the first inductance element and a reactance curve based on the second resonator and the second inductance element are caused to cross each other in a region of lower frequencies than a series resonance frequency based on the second resonator and the second inductance element.
Also, the reactance curve based on the first resonator and the first inductance element and the reactance curve based on the second resonator and the second inductance element are caused to cross each other in a region of higher frequencies than the parallel resonance frequency of the first resonator.
REFERENCES:
patent: 2045991 (1936-06-01), Mason
patent: 5847626 (1998-12-01), Taguchi et al.
patent: 5933062 (1999-08-01), Kommrusch
patent: 2001-223559 (2001-08-01), None
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Summons Barbara
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