Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-04-26
2003-05-06
Ham, Seungsook (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S219000
Reexamination Certificate
active
06559741
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distributed element filter used in the RF (radio frequency) stage, etc. for mobile communication equipment as a bandpass filter to suppress noise and interfering signals, and more particularly to a distributed element filter which has flat amplitude characteristics and a flat group delay time in the passband, and transmission zeros in the stopbands, and is simplified in configuration so as to reduce losses for the improvement in performance so as to be advantageously used as a band pass filter.
2. Description of the Related Art
In high frequency circuit sections such as the RF stage of transmitter and receiver circuits for mobile communication system represented by analog or digital portable telephones or wireless telephones are often used bandpass filters (BPFs), for example, to attenuate harmonics radiation which are caused by the nonlinearity in amplifier circuits, or to eliminate undesired signal waves such as interfering waves, sidebands, etc. from the desired signal waves, or when using a common antenna for both the transmitter and the receiver circuits, to separate out the transmitter frequency band and the receiver frequency band that is different from the transmitter frequency band.
Generally, an ideal filter should have characteristics to pass desired signals without producing any distortion and to sufficiently attenuate interfering signals outside the passband. As shown in the diagrams of
FIGS. 16A and 16B
depicting the filter amplitude and the group delay time, the ideal filter characteristics must have a flat amplitude
17
as well as a flat group delay
18
throughout the passband, while at the same time, realizing attenuation poles
19
,
20
, i.e., transmission zeros, in the stopbands. In the prior art, complex circuit design has been required for the realization of such a filter.
Techniques for directly realizing a bandpass filter having such characteristics, based on a clear design procedure, are not known in the prior art, and it is common practice to construct filters empirically by mixture of various known techniques.
On the other hand, band pass filters for such communication applications are generally realized and constructed as filter circuits having the desired passband/stopband characteristics by connecting series or parallel resonant circuits constructed with various circuit elements in a plurality of stages. In many cases, filter circuit blocks are constructed by unbalanced distributed constant transmission lines such as coupled microstrip lines or patch resonators, because they have good electrical characteristics for high frequency circuits, and are small in size as circuit elements, and so on.
In fact, using coupled microstrip lines, band pass filters with characteristics having no attenuation poles can be easily realized. Conventional filters composed of a plurality of coupled resonators by quarter wavelength &lgr;/4 (&lgr; is the wavelength) coupled microstrip lines have uniformized coupling structure and generally allow little freedom in design, for example the sign, positive or negative, of each coupling reactance element cannot be chosen freely as described hereinafter. Consider the prior art example shown in
FIGS. 17A and 17B
. A ladder network with parallel and series resonators in
FIG. 17A
is transformed using imaginary gyrators to a circuit in
FIG. 18
which is compose of only parallel resonators that are easy to realize.
FIG. 17A
is an original circuit an example of a third order filter, and
FIG. 17B
is its strictly equivalent circuit that is derived using imaginary gyrators
21
,
22
.
In this case, for a strict transformation from the filter of
FIG. 17A
to the equivalent filter of
FIG. 17B
, the two imaginary gyrators
21
,
22
must be made opposite in sign. That is, strictly, the coupling reactance elements must have both signs, positive and negative. In practice, it is difficult to realize a coupling structure by &lgr;/4 coupled microstrip lines to achieve this.
On the other hand, in the case of a filter with simple characteristics having no attenuation poles, since no cross coupling is required in the filter circuit, there is no need to strictly control the positive and negative signs of the coupling; consequently, the imaginary gyrators may have only the positive or the negative sign, or the positive and the negative signs may be interchanged. As a result, the filter circuit can be realized without any problem, even with a structure in which a plurality of resonators formed by &lgr;/4 coupled microstrip lines are sequentially coupled in the same manner.
By contrast, in the case of a filter with complex characteristics that have attenuation poles or that need controlling the group delay and amplitude characteristics, a cross coupling structure is needed in the filter circuits, and the positive and negative phases of the coupling characteristics must be controlled strictly. As a result, &lgr;/4 coupled microstrip lines cannot arbitrarily give the positive and negative phases of the coupling characteristics, and it is difficult to use them as circuit elements for a filter circuit, and hence, it is difficult to create desired attenuation poles or to get prescribed amplitude and group delay characteristics by filter elements by &lgr;/4 coupled microstrip lines.
Multi-resonator filters constructed by connecting such &lgr;/4 coupled microstrip lines in multiple stages usually use straight microstrip lines; on the other hand, so-called hairpin-type multi-resonator filters constructed with microstrip resonators formed from bent microstrip lines called hairpin transmission lines are also used. Examples are shown in
FIGS. 18A and 18B
;
FIG. 18A
is a plan view showing an example of a multi-resonator filter of straight line type constructed by sequentially coupling four microstrip resonators
1
to
4
formed from straight microstrip lines, and
FIG. 18B
is a plan view showing an example of a multi-resonator filter of hairpin type constructed by sequentially coupling four microstrip resonators
5
to
8
formed from hairpin microstrip lines.
The hairpin-type multi-resonator filter, however, has the same problem as described above.
To solve the above problem, the inventor has previously proposed distributed element filters constructed with microstrip resonators in multiple stages formed by sequentially cascading quarter wavelength of the center frequency of the passband coupled microstrip lines. In these distributed element filters, a resonator sequentially coupling method that allows to align accurately the phase of the transmission characteristics is employed assuming by adding a cross coupling circuits to the sequentially coupled resonators it becomes possible to form attenuation poles and control the amplitude characteristic as well as group delay time.
However, the problem to he solved with these distributed element filters is how the cross coupling circuit is connected to the sequentially coupled microstrip resonators formed on the same plane. More specifically, when forming a cross coupling circuit for realizing the desired characteristics, and when the number of resonators to be cross coupled is an even number equal to or more than 4; as a result, if the cross coupling is to be made among the quarter wavelength coupled microstrip lines or quarter wavelength coupled hairpin microstrip lines formed on the same plane, as shown, for example, in the plan views of
FIGS. 19A
to
19
D depicting a filter configuration example, the cross coupling circuit
11
,
14
inconveniently has to cross the resonator pattern indicated at
1
to
10
,
12
,
13
. It is therefore required that the cross coupling circuit be formed in a three dimensional structure; that is, in an air bridge structure, for example, through the space over the resonator pattern. This, in turn, leads to the drawback that the advantage that this distributed element filter is a planar circuit is lost.
In the filters shown in
FIGS. 19A and 19B
, of the four coupled straight m
Ham Seungsook
Hogan & Hartson LLP
Kyocera Corporation
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