CELLULAR TELEPHONE FILTER

FILTRE DE RADIOTELEPHONE

English Abstract

ABSTRACT
A microstripline planar filter has two substrates,
the first substrate having a high dielectric constant and a
low loss and the second substrate being either air or
ceramic of high dielectric constant and low loss. The two
substrates together produce a very high average dielectric
constant of the medium resulting in an ultrashort resonator
length and an overall small filter size. The microstripline
is printed on the first substrate with a strip thickness
chosen to give a very high unloaded Q. The filter has an
input and output with wide impedance lines at input and
output tapping points. The impedance lines are fourteen
ohms and are printed inductors. The filter is particularly
suitable in cellular telephones. Since the filter is much
smaller than previous filters, telephones made using the
filter can also be made much smaller.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A microstripline planar filter comprising a
housing containing a first substrate medium and a second
substrate medium that are inhomogeneous from one another,
said first substrate medium having a high dielectric
constant and a low loss, said first medium having a
plurality of resonators formed by a hairline metal pattern
located thereon, said pattern being located between said
first medium and said second medium, said second medium
being selected from the group of air and ceramic of high
dielectric constant and low loss, said first and second
medium together producing a very high average dielectric
constant of the medium, said filter having an input and
output with wide impedance lines at input and output tapping
points, said filter having a high Q.
2. A filter as claimed in Claim 1 wherein the
impedance lines are printed inductors.
3. A filter as claimed in Claim 2 wherein the first
substrate is hard ceramic and the microstripline is printed
thereon with a strip thickness chosen to give a very high
unloaded Q.
4. A filter as claimed in Claim 3 wherein the input
and output are located on the two outermost resonators.
5. A filter as claimed in any one of Claims 2, 3 or 4
wherein the impedance lines are fourteen ohms.
6. A filter as claimed in any one of Claims 2, 3 or 4
wherein the filter is a 5-pole Chebyshev filter with five
coupled resonators resonating at the same TEM mode, energy
from the first resonator being coupled to the second
resonator, energy from the second resonator being coupled to
the third resonator, energy from the third resonator being
coupled to the fourth resonator and energy from the fourth
resonator being coupled to the fifth resonator, all of said
couplings occurring through capacitative coupling, energy
being coupled into the first resonator and out of the fifth
resonator through the input and output respectively.

Description

2a327~
This invention relates to a microstripline planar
filter having two substrate media that together produce a
very high average dielectric constant of the medium, said
filter having wide impedance lines at input and output
tapping points.
Filters for cellular telephones of around 800 MHz
are known. One type of filter presently being used is a
dielectric resonator filter. Unfortunately, these filters
can requirè complex fabrication techniques, resulting in
increased manufacturing costs. Also, these filters are not
particularly suitable for mass production. In addition, the
dielectric resonator filters will support the TEM mode, as
well as other modes, but require post production tuning in
order to obtain optimum filter performance. This adds
further to the cost. Another type of filter that has been
used in cellular telephones is a tri-plate stripline
interdigital filter which consists of low dielectric
substrate sandwiched between a very high dielectric constant
substrate and a superstrate. The planar resonator pattern
is deposited or etched on the low dielectric constant
substrate and a number of shorting pins are used.
Unfortunately, this tri-plate stripline filter also requires
complex and therefore expensive fabrication techniques.
Further, this filter is not particularly suitable for mass
production. Cellular telephones are becoming increasingly
popular. It is desirable to construct the cellular
telephones as small as possible but the size of the
telephones is presently limited by the size of the filters
used in the telephones~ Cellular telephones that are
portable are ad~antageous. While portable cellular
telephones are known, a reduction in size would enhance
their popularity and make them easier to transport. It is
therefore extremely important to produce a filter that is
suitable for use in cellular telephones where the filter has
a size that is much smaller than conventional filters.
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It is an object of the present invention to
provide a filter that can be used in cellular telephones,
said filter being suitable for mass production and not
requiring any post production tuning, said filter being
noticeably smaller than previous filters and being cheaper
to manufacture without any sacrifice in performance.
A microstripline planar filter has a housing
containing a first substrate medium and a second substrate
medium. Said substrate mediums being inhomogeneous from one
another. The first substrate medium has a high dielectric
constant and a low loss. The first medium has a plurality
of resonators formed by a hairline metal pattern located
thereon. The pattern is located between the first medium
and the second medium. The second medium is selected from
the group of air and ceramic of high dielectric constant and
low loss. The first and second medium together produce a
very high average dielectric constant of the medium. The
filter has an input and output with wide impedance lines at
input and output tapping points, the filter having a high Q.
2Q In the drawings:
Figure 1 is a top view of part of a prior art
hairpinline filter;
Figure 2 is a top view of part of a microstripline
planar filter of the present invention;
Figure 3 is an exploded perspective view of the
microstripline filter of the present invention; and
Figure 4 is a graph showing the isolation and
return loss response of the filter shown in Figure 3.
Referring to the drawings in greater detail, the
filter shown in Figure 1 is a prior art tri-plate stripline
interdigital filter 2. The filter 2 is drawn to scale
relative to its actual size. It can be seen that the filter
2 of Figure 1 has a microstripline 4 printed on a substrate
6 with input 7 and output 8. The substrate 6 has a low
dielectric constant and the filter has a high Q. The filter
2 has a relatively large size and is unsuitable for use in
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2 ~ 3 2 ~ 2 ~
cellular telephones. When "high Qll or "very high Q" is
mentioned in this application, it shall be interpreted as Q
being greater than ~000.
Referring to Figure 2 in greater detail, a filter
10 of the present invention has a microstripline 12 printed
on a first substrate medium 14. The first substrate medium
is preferably hard ceramic. It can be seen that the filter
10 has an input 16 and an output 18 with wide impedance
lines that are printed inductors. The filter 10 has five
resonators 20, 22, 24, 26, 28 and is drawn to scale relative
to its actual size on the same basis as the filter 2. The
filter 10 is noticeably and substantially smaller than the
filter 2.
In Figure 3, a perspective view of the entire
filter 10 is shown. The microstripline 12 is printed on a
first substrate 14 and sandwiched between the first
substrate 14 and a second substrate medium 34. The
substrates are contained within a housing 36. Preferably,
the housing is made of aluminum that has been solar plated.
The filter 10 is a 5-pole Chebyshev filter having five
coupled microstripline resonators 20, 22, 24, ~6, 28. The
resonators resonate at the same quasi TEM mode
simultaneously. Microwave energy is coupled from the first
resonator 20 to the second resonator 22, from the second
resonator 22 to the third resonator 24, from the third
resonator 24 to the fourth resonator 26 and from the fourth
resonator 26 to the fifth resonator 28. The coupling
between adjacent resonators within the filter 10 occurs
through capacitative coupling and the amount of coupling is
determined by the size of a gap 38 between immediately
adjacent resonators. The size of the gap can vary
throughout the filter.
Energy is coupled into the filter through the
input 16 and out of the filter through the output 18.
The first substrate medium 14 is of a high
dielectric constant (i.e. ~r greater than eighty) and a very
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2~32727
high Q (Qo greater than eight thousand)O One suitable
material is hard ceramic. The second substrate medium 34 is
selected from the group of air or ceramic of high dielectric
constant and low loss. The two substrate media 14, 34
produce a filter having a very high average dielectric
constant of the medium. This results in a filter having an
ultrashort resonator length and an overall small filter
size.
The input 16 and output 18 have wide impedance
lines that are printed inductors and are preferably each
fourteen ohm lines. Without these wider lines, the filter
would suffer from severe tolerance problems and would not
operate satisfactorily.
Preferably, the microstripline is printed on the
hard ceramic first substrate with a strip thickness chosen
to provide a very high unloaded Q. The input 16 and output
18 are located on the two outermost resonators 20, 28
respectively.
The filter is housed in an aluminum housing which
2Q is solar plated.
In Figure 4, it can be seen that the filter 10 has
high performance characteristics. The filter 10 is twenty
percent smaller in size than conve~tional filters used in
cellular telephones.

Representative Drawing