EP0959519A1

EP0959519A1 - Low loss duplexer without setting

Info

Publication number: EP0959519A1
Application number: EP99303288A
Authority: EP
Inventors: Alain Nguyen
Original assignee: TRT Lucent Technologies SA
Current assignee: TRT Lucent Technologies SA
Priority date: 1998-05-18
Filing date: 1999-04-27
Publication date: 1999-11-24

Abstract

A duplexer for microwave signals that requires no system of setting by screws comprises two tunnels, each comprising a longitudinal passage and compartments demarcated by transversal partition walls. Said compartments, said longitudinal passages and said common part are hollowed out in the plane upper surface of a monolithic block, and said tunnels are closed on the top by a lid that adheres uniformly to said plane surface.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a duplexer for microwave signals.
Duplexers are usually devices found at the end of a system in items of radio equipment, i.e. at the antenna. They are designed to separate the signals transmitted from the signals received by the antenna. A duplexer conventionally comprises two passband filters, one responsible for filtering the transmitted signals and the other the received signals.

2. Description of the Prior Art

Conventionally, a passband filter for microwave signals consists of a tunnel comprising a succession of compartments communicating with one another through a longitudinal passage, the dimensions and number of compartments being a function of the size and the center frequency of the passband of the filter. A duplexer for microwave signals therefore generally comprises two tunnels of this type that are respectively connected by one end to the transmission part and to the reception part of the radio equipment and that open jointly into the other end on the antenna side.
These passband filters are designed to meet the following conditions:

high resistance under temperature throughout the range;
low loss in transmission and high return loss in the passband; and
high rejection in the near band.

With respect to the rejection of the highest frequencies, especially for the elimination of the harmonics of the filtered signal, the equipment is generally provided with a low pass filter interposed between the duplexer and the antenna.
To meet the first condition, existing duplexers are generally made of a material that is highly stable under temperature, for example invar which is an alloy of iron and nickel with a coefficient of thermal expansion that is practically zero. However, this type of material proves to be very costly and very difficult to machine. Thus, the method generally used to manufacture duplexers is to make tunnels out of invar plates and solder transversal partition walls thereto so as to obtain compartments in these tunnels.
These duplexers are then methodically provided with a system of setting by screws to obtain the desired signal filtering and transmission characteristics. Tapped holes are made in the upper wall of the tunnels to receive setting screws. In general, one setting screw is provided per compartment with another screw being provided in the partitition walls of each compartment in the longitudinal passage. The setting operation then consists in adjusting the part of the screw that projects into the compartment or into the longitudinal passage. This operation proves to be very complicated and very lengthy.
Consequently, the present invention seeks to overcome the prior art drawbacks by proposing a duplexer that does not require a system of setting by screws for the usual frequencies.

SUMMARY OF THE INVENTION

Indeed, an object of the present invention is a duplexer for microwave signals comprising two passband filters designed to process respectively incoming signals and outgoing signals and to process these signals simultaneously, these filters consisting of two tunnels that open jointly at one end by a common part into a first hole and open independently at the other end into a second hole and a third hole, each of these tunnels comprising a longitudinal passage and compartments demarcated by transversal partition walls,
wherein said compartments, said longitudinal passages and said common parts are hollowed out in the upper plane face of a monolithic block, said tunnels being closed at the top from the first hole up to the second and third holes by a lid that adheres uniformly to said plane surface,
the functional characteristics of the two filters being determined by dimensional parameters within said tunnels, these parameters including the thickness of each wall, the longitudinal and transversal dimensions of each compartment and the width of each longitudinal passage.
In order that the compartments, the longitudinal passages and the common part of the duplexer may be hollowed out with precision in the monolithic block, the block and the lid are preferably made of aluminum. Aluminum is indeed easier to machine than invar.
Since this material is less stable under temperature than invar, it is enough to provide for a slightly wider passband to compensate for the drifts in temperature of the material and increase the slope on the flanks of the passband of the filters to obtain the desired near band rejection.
Advantageously, the joining surface of the lid is covered with a uniform layer of a brazing alloy to obtain uniform adhesion on all the surfaces in contact with said monolithic block and with said lid after soldering.
Furthermore, the surfaces within said tunnels of said monolithic block are preferably subjected to a surface treatment to ensure efficient transmission of the signals in said tunnels. Said surface treatment may consist for example in adding a surface layer of silver.
Finally, in a preferred embodiment, the common part through which said tunnels open into the first hole has a Y-shape and the external sides of its arms are concave.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention shall appear from the following detailed description made with reference to the appended drawings, of which:

Figure 1 shows a view in perspective of a monolithic block of a duplexer according to the invention;
Figure 2 shows a view in perspective of the monolithic block of Figure 1 partially covered by a lid; and
Figures 3 and 4 are curves of results of the duplexer of Figures 1 and 2.

MORE DETAILED DESCRIPTION

Figures 1 and 2 provide a more particular illustration of a duplexer covering the 12.875 GHz - 13 GHz frequency band for transmission and the 13.125 GHz - 13.25 GHz frequency band for reception. Naturally, this type of duplexer may be used for other frequency bands in the microwave domain. It would then be enough to modify the characteristics of the compartments and of the longitudinal passage of the tunnels as well as the number of compartments.
According to the invention, each duplexer comprises firstly a monolithic block in which two tunnels are hollowed out and a common block joining these two tunnels and secondly a lid to close said block on the top.
For reasons of clarity, the duplexer of Figure 1 is shown without its lid. Consequently, only one monolithic block 1 is shown in Figure 1. This block is a parallelepiped comprising six plane rectangular faces. The material used to manufacture the block is an aluminum-based alloy. This material is for example the alloy whose AFNOR designation is 2618 A. This alloy is especially easy to machine and has a relatively low thermal expansion coefficient.
Two parallel tunnels 2 and 3 are hollowed out into the plane upper surface of the monolithic block. These two tunnels open jointly at a first end into a hole 4 by means of a common Y-shaped part 5. The hole 4 is located on the antenna side.
The tunnels 2 and 3 comprise a succession of compartments 6 demarcated by transversal partition walls 7 which are in sets of two that face each other on either side of a longitudinal passage. This longitudinal passage is referenced 8 for the tunnel 2 and 9 for the tunnel 3. The dimensional parameters within the tunnels 2 and 3 determine the functional characteristics of the two filters of the duplexer, namely the transmission losses and the return loss of the filters in the passband and their rejection in near band. The thickness of the partition walls 7, the longitudinal and transversal dimensions of the compartments 6 and the width of the longitudinal passages 8 and 9 fix these characteristics with precision. The prior art system of setting by means of screws is then superfluous, at least in the range of usual frequencies.
For the compartments 6, the longitudinal passages 8 and 9 and the common part 5 can be made by the milling technique which provides a totally satisfactory degree of precision (of about +/- 15 microns) for the applications in view. A mill with a radius of 2 millimeters is then enough to obtain the desired precision. In the example of Figures 1 and 2, the longitudinal dimension of the compartments and the width of the longitudinal passages are defined with a precision of +/- 15 microns. The other dimensions are defined with a smaller precision of the order of +/- 20 microns.
The internal walls of the tunnels 2 and 3 are advantageously provided with surface treatment to ensure the efficient transmission of the signals. This treatment consists for example of the addition of a surface layer of silver. This layer will also be used to protect the block from possible oxidation. It is preferably extended through the block.
As shown in Figures 1 and 2, the common part 5 has a Y-shape. For reasons of gain in space in particular, the external sides of the arms of the Y have concavities. The concavities shown in Figures 1 and 2 are dihedrons. As a variant, it is possible to provide for a common T-shaped part. According to another embodiment, it is also possible to make the tunnels 2 and 3 converge on the common hole 4 by positioning them in the form of a V.
In the embodiment shown in Figures 1 and 2, the monolithic block 1 comprises, upline with respect to the tunnels 2 and 3, orthogonal elbows with steps 10 and 11 used to obtain a 90° change in direction.
Figure 2 shows a view in perspective of the monolithic block of Figure 1. This block is closed on top by a flat lid 12 which is shown partially. This aluminum lid is designed to adhere uniformly to the entire plane upper surface of the monolithic block 1. The joining of the block 1 and of the lid 12 is done by brazing. To do so, the joining surface of the lid 12 is covered with a uniform layer of a brazing alloy on a thickness of 20 micrometers. This brazing alloy is preferably formed by 60% tin and 40% lead. The adhesion between the surfaces in contact of the block 1 and the lid 12 is obtained by soldering by heating the entire unit. The layer of alloy covering the lid is used both as a filler metal for the brazing and as a protection layer for the lid. Advantageously, the monolithic block 1 has pre-positioning pins 13 as well as tapped holes 14 so as to facilitate the positioning of the lid 12 with respect to the block 1 and to place this lid and block flat against each other by means of screws.
Furthermore, windows 15 and 16 are hollowed out through the lid 12 to form the exit at 90° from the elbows 10 and 11.
Figures 3 and 4 show the results obtained in tests on a prototype corresponding to a duplexer as shown in Figures 1 and 2, namely a duplexer covering the 12.875 GHz - 13 GHz band of frequencies in transmission and the 13.125 GHz - 13.25 GHz band of frequencies in reception. The parameters S₂₁ and S₁₁ illustrated in Figures 3 and 4 respectively show the transmission losses and return losses of the duplexer of the invention. Figure 3 shows the value of the parameters S₂₁ and S₁₁ on the transmission band of the duplexer and Figure 4 shows the value of these parameters on the reception band.
Since the frequency drift of the passband of the filters of the duplexer throughout the range of temperatures does not exceed 15 MHz around the frequency response to a temperature of 25°C, the passband of the filters of the duplexer have been widened by 30 MHz to cope with this drift. That is, the frequency band covered by the duplexer at a temperature of 25°C is taken to be equal to 12.860 GHz - 13.015 GHz for transmission and 13.110 GHz - 13.265 GHz for reception.
The measurement curves show that:
a) the transmission losses (parameter S₂₁) are always below 1 decibel;
b) the return loss (parameter S₁₁) is always greater than 17 decibels;
c) by superimposing the curves, it is seen that the near band rejection is greater than 56 decibels.
According to an alternative embodiment, it is possible to consider covering the frequency band 12.875 GHz - 13 GHz (transmission) and 13.125 GHz - 13.25 GHz (reception) by using two duplexers and thus having, for each duplexer, a transmission band and a reception band that is half as wide. This would make it possible to reduce the constraints when designing the duplexer. In particular, the rejection of the near band would not have to be as great. However, this would have the drawback of doubling the number of pieces of equipment.
In conclusion, and given what has been described here above, the duplexer of the invention has the following advantages:

simplicity of manufacture leading to low cost price;
the elimination of lengthy and costly setting times;
a high degree of reproducibility to enable mass production on an industrial scale;
excellent electrical performance characteristics.

Claims

A duplexer for microwave signals comprising two passband filters designed to process respectively incoming signals and outgoing signals and to process these signals simultaneously, said filters consisting of two tunnels that open jointly at one end by a common part into a first hole and open independently at the other end into a second hole and third hole, each of these tunnels comprising a longitudinal passage and compartments demarcated by transversal partition walls,

wherein said compartments, said longitudinal passages and said common parts are hollowed out in the upper plane face of a monolithic block, said tunnels being closed at the top from the first hole up to the second and third holes by a lid that adheres uniformly to said plane surface,

the functional characteristics of the two filters being determined by dimensional parameters within said tunnels, these parameters including the thickness of each wall, the longitudinal and transversal dimensions of each compartment and the width of each longitudinal passage.
A duplexer for microwave signals according to claim 1, wherein said monolithic block and said lid are made of aluminum,
and wherein the joining surface of said lid is covered with a uniform layer of a brazing alloy to obtain uniform adhesion on all the surfaces in contact with said monolithic block and with said lid after soldering.
A duplexer for microwave signals according to claim 2, wherein the brazing alloy comprises 60% of tin and 40% of lead.
A duplexer for microwave signals according to claim 2 or 3, wherein at least the surfaces inside said tunnels of said monolithic block are subjected to surface treatment to ensure the efficient transmission of the signals in said tunnels.
A duplexer for microwave signals according to claim 4, wherein said surface treatment consists of the addition of a surface layer of silver.
A duplexer for microwave signals according to one of the claims 1 to 5, wherein the common part by which said tunnels open into the first hole has a Y shape.
A duplexer for microwave signals according to claim 6, wherein the external sides of the arms of the Y-shaped common part are concave.
A duplexer for microwave signals according to claim 7, wherein the concavities of the outer edges of the arms of the common part are dihedrons.