CA2032727A1

CA2032727A1 - Cellular telephone filter

Info

Publication number: CA2032727A1
Application number: CA 2032727
Authority: CA
Inventors: Protap Pramanick
Original assignee: Com Dev Ltd
Current assignee: Com Dev Ltd
Priority date: 1990-12-19
Filing date: 1990-12-19
Publication date: 1992-06-20

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.

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.

~3327~r~

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 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 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.

Claims

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.