Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1999-03-25
2001-02-20
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S202000, C333S204000
Reexamination Certificate
active
06191666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multi-layer ceramic lowpass filters particularly for use in mobile communication instruments such as portable telephones and cordless telephones.
2. Description of Related Art
The following references provide background information relating to multi-layer ceramic lowpass and are hereby incorporated by reference in their entireties:
[1] D. Swanson, “Thin-Film Lumped-Element Microwave Filters,” 1989 IEEE MTT-S Digest, pages 671-674, 1989.
[2] J. Helszajn, “Microwave Planar Passive Circuits and Filters,” Chapter 15, John Wiley & Sons, 1994.
[3] M. Miyazaky, et al., “A Broad Band Dielectric Diplexer Using a Snake Strip-Line,” 1991 IEEE MTT-S Digest, pages 551-554, 1991.
[4] U.S. Pat. No. 5,357,227 to Tinegawa, et al., 1994.
Depending on their implementation, filters can be grouped into three types: lumped-type, distributed-type and semi-lumped-type (which are constructed by lumped elements and distributed elements). The size of the distributed element, which is usually a section of transmission line, is determined by the signal wavelength associated with the operational frequencies. As a result, at frequencies lower than 200 MHz, the implementation of a filter by semi-lumped type or distributed-type elements is impractical due to the unacceptably large size of the distributed elements. However, in the higher microwave frequency region or millimeter wave frequency region, the distributed-type filters are always used due to acceptable size and better performance than the lumped-type filters. As discussed in reference [1], the reasons why the lumped elements are not suitable to be used in filter designs at frequencies higher than several hundred MHz are that the lumped inductors are too lossy and the parasitic effects make the control of filter performance more difficult. Although the problem of losses in lumped elements can be improved in superconductor applications, such applications are limited.
At frequencies between several hundred MHz to several Ghz, the size of the distributed elements is not appreciated in mobile communication instruments. The trends in developing mobile communication instruments are miniaturization and power saving. There are two ways to design a miniaturized filter with high performance; one is to employ a semi-lumped configuration and the other is to use a high dielectric constant structure. The semi-lumped type filters usually employ chip capacitors, inter-digital type capacitors or metal-insulation-metal (MIM) capacitors, which are not as lossy as lumped inductors, and some sections of distributed transmission lines, which are usually much shorter than ¼ signal wavelength. In addition to the capacity with regard to miniaturization of the semi-lumped configuration, this type of filter also has the ability to suppress periodic spurious signals from which the distributed type filters usually suffer. Recently, the high dielectric constant ceramic filters, such as coaxial type or mono-block type, are very popular in the frequency region of several hundred MHz to several Ghz due to high performance and small size. The high performance results from the shape of the cross-section of the transmission line which is usually round or smoothly curved, yielding lower conductor losses. The small size is enabled because the dielectric constant of ceramic materials is very high, resulting in a reduction in the signal wavelength. Yet, such kinds of filters are almost always used with regard to applications of bandpass filters or bandstop filters.
FIG. 1
is a perspective view of a conventional high frequency lowpass filter as described in reference [2]. In
FIG. 1
, the narrow microstrip transmission line sections
4
a,
4
b
are used as equivalent series inductors. The wide transmission line sections
5
a,
5
b,
5
c
are used as equivalent capacitors connected to ground plane
3
. More particularly, a first capacitive open-circuited stub electrode
5
a
forming a part of a first capacitor and an input electrode
6
a
forming an input terminal extend from the end of microstrip line electrode
4
a.
A second capacitive open-circuited stub electrode
5
b
forming part of a second capacitor extends from the other end of microstrip line electrode
4
a
and one end of the other end of the other microstrip line electrode
4
b.
A third capacitive open-circuited stub electrode
5
c
forming part of a third capacitor and an output electrode
6
b
as an output terminal extend from the other microstrip line electrode
4
b.
The equivalent circuit is as shown in FIG.
2
.
The drawbacks of this filter are as follows. The filter order is high in general applications resulting from the attenuation poles of the filter all being located at infinite frequency. This in turn results in a larger circuit size. The filter order is defined from the numbers of the branches in the equivalent circuit. One branch is defined as an inductor, a capacitor, a series-connected capacitor and inductor or a shunt-connected capacitor and inductor. Since the branches in
FIG. 2
are only single-capacitor or single-inductor, these branches contribute attenuation poles at infinite frequency. The attenuation pole means that all of the signal can not pass the filter. A series inductor is equivalent to an open circuit to infinite frequency (j&ohgr;L→∞, if &ohgr;→∞). A capacitor connected to ground is equivalent to a short circuit at infinite frequency (1/j&ohgr;C→0, if &ohgr;→∞). When a signal meets an open circuit or a short circuit, the total signal will be reflected back to the feed and no pass occurs. The number of attenuation poles at infinite frequency determine the slope of the rejection curve at the stopband. The larger the number is, the sharper the slope will be. Additionally, if the filter in
FIG. 1
is used in the RF band, the capacitors in
FIG. 2
usually have large values. This results in the areas occupied by the wide line sections
5
a,
5
b,
5
c
in
FIG. 1
being necessarily large, since in general the substrate thickness can not be too thin due to requirements for the supporting strength of the total circuit. Therefore, this kind of filter is too large in size for low frequency applications. Also, if the lengths of the wide line sections
5
a,
5
b,
5
c
in
FIG. 1
are decreased with the capacitor values being kept unchanged, then the widths of the wide line sections
5
a,
5
b,
5
c
must become even greater. This tradeoff means that it is quite difficult for this type of filter to be miniaturized. Further, because the size of the wide line sections
5
a,
5
b,
5
c
in
FIG. 1
is too large, these pads (the wide line sections
5
a,
5
b,
5
c
) are not much smaller than the wavelength, and the resonance phenomena will occur in the higher frequency range. This results in spurious response in the higher frequency range and degradation of the stopband performance.
FIG. 3
shows another example from reference [2]. The equivalent circuit of this filter is shown in FIG.
4
. In
FIG. 3
, narrow microstrip lines
34
a,
34
b
are used as equivalent series inductors. Wide transmission line sections
35
a,
35
b,
35
c
are used as equivalent capacitors connected to ground plane
3
. Narrow microstrip lines
34
c,
34
d,
34
e
are used as equivalent inductors in series with equivalent capacitors
35
a,
35
b,
35
c,
respectively. Input electrode
36
a
functions as an input terminal, and output electrode
36
b
functions as an output terminal. Comparing the equivalent circuit of
FIG. 2
with that of
FIG. 4
, the only difference is the capacitor branch is changed into a series-connected capacitor and inductor. The branch formed of the series-connected capacitor and inductor elements can contribute an attenuation pole at the resonant frequency of these elements. This branch is equivalent to a short circuit to ground at resonant frequency. This indicates that this type of filter can improve the high fil
Glenn Kimberly E
Industrial Technology Research Institute
Lee Benny
Stevens Davis Miller & Mosher L.L.P.
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