Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1997-12-24
2002-03-19
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S104000, C333S127000, C333S134000, C333S202000, C333S206000, C333S204000
Reexamination Certificate
active
06359529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a filtering device used in a high-frequency device for use in a mobile communication system or the like.
2. Description of the Related Art
As a result of recent introduction of the TMDA technique into portable telephone systems, the communication scheme of intermittent transmission/reception in units of time slots has become widely used instead of the concurrent transmission/reception technique. As a result of the change in the communication scheme, the microwave filter which is located at the first stage of a radio communication device and which is used in common in transmission and reception has been changed from a combination of transmission and reception filters to a switching type filter in which a transmission filter and a reception filter are switched from time to time.
In general, when a transmission filter and a reception filter are switched from each other by a switch, isolation of the switching circuit makes it possible to reduce signal leakage from a transmission circuit to a reception circuit to a lower level than can be achieved by a single filter. Therefore, requirement of the attenuation characteristic for a filter of the transmission-reception switched type is less severe than that for a filter of the combined transmission-reception type. This makes it possible to realize a smaller-sized filter at a lower cost.
FIG. 31
illustrates a typical transmission-reception switched type filter. In
FIG. 31
, diodes D
1
and D
2
are used as switching devices for switching a transmission filter and a reception filter from each other. If a switching control current is applied so as to turn on both diodes D
1
and D
2
into a closed state, a transmission signal is passed through the transmission filter to an ANT terminal. However, because the transmission signal is shunted to ground by the diode D
2
, the transmission signal cannot reach the reception filter. On the other hand, when the switching control signal is given in such a manner as to turn off both diodes D
1
and D
2
into an open state, a reception signal is passed through the reception filter. In
FIG. 31
, L
3
is a high-frequency choke coil and C
2
is a high-frequency signal shunting capacitor. The combination of L
3
and C
2
prevents ingress of the RF signal to a control circuit which generates the switching control signal.
To improve the isolation of the switching circuit using diodes, it is more desirable to dispose the diodes in a shunted fashion. If the diodes are disposed in a series fashion, leakage of signal occurs due to residual capacitance when the diodes are in an off-state, which results in degradation in isolation between reception and transmission filters.
However, in the switching circuit of the type in which a switching device is turned on into a closed state so as to shunt the circuit, it is required that the impedance of the switching device seen from the antenna terminal should be as high as can be regarded as open-circuited thereby eliminating the influence of the closed switching device on the filter used. One known technique of achieving the above requirement is to add an LC phase shift circuit consisting of L
1
, L
2
, and C
1
to the switching device as shown in FIG.
31
. Another technique is to insert a &lgr;g/4 transmission line so that the impedance seen from the transmission filter becomes as high as can be regarded as substantially open-circuited.
Thus, it is an object of the present invention to provide a filtering device of the transmission-reception switched type which can be constructed in a form with a reduced size at a low cost without having to use circuit elements such as a capacitor and a coil forming a phase shift circuit which are not essential to the filtering device.
SUMMARY OF THE INVENTION
To achieve the above requirement of reducing the device size and the production cost without using a conventional phase shift circuit, the present invention provides a filtering device according to any aspect described below. According to a first aspect of the present invention, there is provided a filtering device comprising: a plurality of filters each having a distributed-parameter resonance line at least one end of which is open-circuited; and a coupling line, a coupling electrode, or a coupling element coupled to at least one distributed-parameter resonance line included in each filter, wherein a switch is connected to the above-described at least one distributed-parameter resonance line so that the open-circuited end of the above-described at least one distributed-parameter resonance line is short-circuited when the switch is operated.
FIG. 1
illustrates a specific example of the circuit configuration of the filtering device according to the above aspect of the invention. As shown in
FIG. 1
, the filtering device comprises: distributed-parameter resonance lines R
11
, R
12
, R
13
, R
21
, R
22
, and R
23
whose one end is open-circuited; and coupling reactances k
11
, k
12
, k
13
, k
14
, k
21
, k
22
, k
23
, and k
24
located between adjacent distributed-parameter resonance lines or between an input or output port and a first- or final-stage line. In this specific example, a filter
1
is formed between port
1
and port
3
and a filter
2
is formed between port
3
and port
2
. Diode switches (hereinafter referred to simply as switches) D
1
and D
2
are connected between the open-circuited ends of the distributed-parameter resonance lines R
13
and R
21
and ground. Although a bias circuit for supplying a bias voltage to the switches D
1
and D
2
are needed, it is not shown in FIG.
1
. The direction of the switches D
1
and D
2
is not limited to that shown in
FIG. 1
, but the direction may be determined in different manners depending on the configuration of the bias circuit used to supply a bias voltage to the switches D
1
and D
2
.
In
FIG. 1
, when the switch D
2
is in an open state and the switch D
1
is in a closed state, the distributed-parameter resonance line R
13
is short-circuited at its both ends, and thus it acts as a &lgr;/2 resonator. In this state, the other distributed-parameter resonance lines act as &lgr;/4 resonators and therefore they have a resonance frequency twice the signal frequency. As a result, the distributed-parameter resonance line R
13
acts as a very high impedance (very low admittance) at frequencies in the signal frequency band. In this state, on the other hand, the coupling reactance k
14
between the distributed-parameter resonance line R
13
and the port
3
acts as an impedance directly connected to ground via the switch D
1
. Therefore, when seen from the port
3
, the filter
1
is not short-circuited but it is seen as a circuit having a certain reactance. If the filter
2
is designed taking into account this reactance, the filter
2
can have desired characteristics independent of the filter
1
. In the case where the filter
2
operates using the port
3
as an input port and the port
2
as an output port, when the switch D
1
is in a closed state, a signal input to the port
3
is passed through the filter
2
and output to the port
2
but no signal is output to the port
1
. On the other hand, in the case where the filter
2
operates using the port
2
as an input port and the port
3
as an output port, when the switch D
1
is in a closed state, a signal input to the port
2
is passed through the filter
2
and output to the port
3
, but no signal is input to the filter
1
.
Conversely, if the switch D
1
is in an open state and the switch D
2
is in a closed state, the filter
1
can be used without being affected by the filter
2
.
In the design of the filter, when the filter
2
is designed first so that the filter
2
has desired characteristics taking into account the effects of k
14
. This can be achieved by performing a simulation repeatedly on the filter
2
taking into account the reactance k
14
while varying parameters of the respective elements in the filter
2
by small amounts at a time until desired charac
Tada Hitoshi
Tsunoda Kikuo
Murata Manufacturing co., Ltd.
Ostrolenk, Faber, Gerb & Sofen, LLP
Pascal Robert
Summons Barbara
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