EP2731192A1

EP2731192A1 - Bandstop filter for interferring signals

Info

Publication number: EP2731192A1
Application number: EP12382438.5A
Authority: EP
Inventors: José Damboriena Jesús María San; Luciano Lavado Garcia
Original assignee: Angel Iglesias SA
Current assignee: Angel Iglesias SA
Priority date: 2012-11-08
Filing date: 2012-11-08
Publication date: 2014-05-14

Abstract

A bandstop filter comprising: a signal input (10, 20); a signal output (11, 21); connecting means (12, 22) for connecting said signal input (10, 20) and said signal output (11, 21); at least one resonator (13₁, 23₁) connected to said connecting means (12, 22), said at least one resonator (13₁, 23₁) having a resonant frequency with a corresponding wavelength, said at least one resonator (13₁, 23₁) comprising a helical transmission line (17₁, 27₁). The helical transmission line (17₁, 27₁) of said at least one resonator (13₁, 23₁) has an electrical length of n times one-quarter wavelength of the resonant frequency, n being an odd number different from 1.

Description

TECHNICAL FIELD

The present invention relates to filtering of unwanted electrical signals and, in particular, to filters based on helical elements configured as bandstop filters.

STATE OF THE ART

Compression systems in current digital television systems are designed to support several normal digital television channels (usually up to six, depending on the used coding and modulating techniques) of acceptable quality, in the spectrum of radioelectric frequencies previously used for a single analog channel. There are usually four or five terrestrial analog services per region and, as a consequence, the use of a single digital television channel for them all will considerably reduce the use of the spectrum. The difference between the quantity of spectrum used in the analog systems and the spectrum used in digital systems for allocating already existing analog services, and which has thus been released when analog television has migrated to digital (after a transitional period in which both technologies have co-existed), is called Digital Dividend (originally Spanish term "Dividendo Digital").
New coding, compression and modulating techniques for broadcasting of terrestrial digital television (such as the ones proposed by several recommendations of the International Telecommunication Union, ITU) have indirectly contributed to the process of creating the mentioned digital dividend. For example, the precursor Recommendation UIT-R BT.798 says that "terrestrial broadcasting for digital television be accommodated in the channels (with bandwidths of 6, 7 and 8 MHz) dedicated to the broadcasting of analog television in the bands of metric and decimetric waves. This Recommendation, in which it is forbidden that the bandwidth used for digital programs be greater than the bandwidth used for analog channels, has open a way to the development of advanced techniques of digital compression. That is to say, the digital dividend is due to the fact that digital compression systems allow to multiplex the transmission of several television programs in the spectrum previously used for a single analog television channel. This means that the possibilities of accessing to the spectrum of the digital dividend are still growing, as more advanced rules for terrestrial television (for infrastructure and compression) are progressively developed and introduced (for example, the second generation of systems for broadcasting transmission of digital terrestrial television, TDT), which offer greater binary capacity per Hertz than existing systems.
The amount of liberated spectrum due to the change from analog to digital transmission depends mainly on the national peculiarities, such as the geography and topography of the country, grade of penetration of the digital transmission services, needs of the regional or minority television services, and use of the spectrum by neighbor countries. This amount of liberated spectrum also depends on the digital television technology chosen for migrating the analog services. As a consequence, the size of the digital dividend also changes from region to region and from country to country. However, the specific location of the digital dividend also varies from one country (or region) to another one, because it depends on the frequency assignation in each country/region, normally between 200 MHz and 1 GHz. In particular in Europe, this liberated band (dividend) is placed in the range between 790 and 862 MHz.
The spectrum created with this digital dividend can be used for any type of services, such as additional terrestrial broadcasting services (which might even include the delivery of new interactive television programs and of high quality), mobile multimedia applications, mobile communications, wireless broad-band access systems (for example, it could be used to offer broad-band access to Internet, ubiquitous in areas not yet reached by terrestrial lines, which would help reducing the digital breach) and so on. In other words, it can be said that this spectrum is the opportunity to respond to the growing demand for new wireless communication services.
But it is the mobile telephony sector the most interested one in this digital dividend, given the number of new mobile services which are being offered (mobile television, access to Internet through mobile terminals, massive data transmission...). Besides, the liberated frequencies in the digital dividend (which, as already explained, are usually in the band between 200 MHz and 1 GHz), have better characteristics of the signals propagation than the characteristics of, for example, 2.4 GHz. The mobile sector has shown its interest in using these lower frequencies in order to improve capacity and thus to achieve an optimal balance between transmission capacity and operational scope. This way, less infrastructure would be required for obtaining wider mobile coverage, with the resulting reduction in costs of communication services, especially in rural areas. For all this, in many of the countries in which television transmission has changed from analog to digital, the liberated spectrum has been mainly assigned to mobile telephony communications and, in particular, to new generation mobile telephony, also known as LTE (Long Term Evolution), and also to other types of technologies, such as 4G mobile telephony or WiMAX.
This spectrum liberated due to the migration from analog to digital television (digital dividend) and which is being assigned to the above-mentioned services (wireless internet, mobile telephony...) is very close to the frequency band used for terrestrial digital television services (in some cases, the signals used by these new telephony services are in the band between 791 and 821 MHz, very close to some television signals which occupy the band of 790 MHz). An important problem then arises, since terrestrial digital television signals (TDT) are interfered by signals of those new services. This causes degradation in the quality of the TDT service. This is especially problematic in those television channels which use frequencies which are located at an end of the band dedicated to television (near 790 MHz), that is to say, in the limit area with the liberated spectrum band.
The interference caused by these new signals on the digital television signals is important, because the new signals are very different from the television ones and besides are very close by (in some cases, at a distance of 1 MHz), causing the following undesired effects: On the one hand, since they are high-power signals compared to the digital television signals, they normally cause saturation in the wide-band amplifiers and in the tuners of the television receiving system. On the other hand, these signals usually have a bandwidth which occupies frequencies out of the assigned channel, thus causing interference (co-channel interference) in the television channels, consequently deteriorating the signal-to-noise ratio of the received TV signal.
In order to improve the reception of TDT signals, the power of the received TV signal can be increased (for example, by means of repeaters), but this is an expensive solution. The reception of TDT signals can also be improved by using filters with adequate responses, in order to eliminate the interfering signals. Due to the limited separation in frequency between the useful signal and the interfering one, such filters are currently only achieved using cavities of great volume and high cost.
Conventional filters having discrete LC components (respectively coils and capacitors) do not offer a sufficiently abrupt transition between the pass-band and the stop-band so as to offer a valid solution to the posed problem.
Patent US5932522 describes a bandstop filter formed by three resonators, each of which is composed of a transmission line and two capacitors. The transmission lines can be helical. The electrical length of the transmission lines is chosen to be between one-quarter wavelength and one-half wavelength of the resonant frequency. However, this filter neither achieves rejection of the unwanted frequency band in a manner as sharp as required in the described circumstances. Besides, the maximum electrical length suggested in this document is one-half wavelength of the resonant frequency. Longer lengths are discouraged because of their inherent disadvantage: they occupy too much space. Tuning of the resonators is achieved by means of screws acting as capacitors.
In sum, a filtering system is needed, which enables to solve in an efficient way, the problem of worsening in the quality of reception of digital television signals, in the scenario described above.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a bandstop filter which achieves rejection of the unwanted band in a manner which is more abrupt than the rejection achieved with conventional filters
According to an aspect of the present invention, there is provided a bandstop filter comprising: a signal input; a signal output; connecting means for connecting the signal input and the signal output; at least one resonator connected to the connecting means, the at least one resonator having a resonant frequency with a corresponding wavelength, said at least one resonator comprising a helical transmission line; The helical transmission line of the at least one resonator has an electrical length of n times one-quarter wavelength of the resonant frequency, n being an odd number different from 1.
The bandstop filter is preferably placed in a housing. More particularly, the at least one resonator is located within a cavity of said housing.
The at least one resonator preferably comprising a capacitor, more preferably an adjustable capacitor in order to vary the resonant frequency of said at least one resonator.
In a preferred embodiment, the capacitor is implemented by means of a first conductive wire and a second conductive wire. The first conductive wire is preferably configured to achieve rough tuning of the resonant frequency of the at least one resonator, said first conductive wire being connected as a prolongation of the helical transmission line. The second conductive wire is preferably configured to achieve fine tuning of the resonant frequency of the at least one resonator, said second conductive wire being connected to ground.
In a particular embodiment, the helical transmission line of the at least one resonator has an electrical length of 3 times one-quarter wavelength of the resonant frequency.
In a particular embodiment, the bandstop filter comprises a plurality of resonators connected to the connecting means, each of the plurality of resonators comprising a helical transmission line.
In a particular embodiment, the bandstop filter is configured to reject the frequencies of the frequency band between 791 MHz and 821 MHz.
Additional advantages and features of the invention will become apparent from the detail description that follows and will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 shows the electric scheme of a filter according to an embodiment of the present invention.
Figure 2 shows the physical aspect of the filter whose electric scheme is shown in figure 1.
Figure 3 shows the response of the filter according to an embodiment of the invention, in comparison to the response of a conventional filter.

DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION

In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.
In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the terms "about" and "around" and "substantially".
The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Next embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing apparatuses and results according to the invention.
Referring to figure 1, a filter of the present invention includes an input 10 and an output 11 connected by connecting means 12. A plurality of resonators 13₁ 13₂ 13₃ 13₄ are connected to respective terminals 14₁ 14₂ 14₃ 14₄ at connecting means 12. It is remarked that connecting means 12 presents an electrically small connection. Each resonator 13₁ 13₂ 13₃ 13₄ has a helical transmission line 17₁ 17₂ 17₃ 17₄ coupled to one of the terminals 14₁ 14₂ 14₃ 14₄. The resonators have a resonant frequency with a corresponding wavelength.
The helical shape of the transmission line 17₁ 17₂ 17₃ 17₄ is selected because it permits the use of a relatively long transmission line in a relatively small amount of space.
The helical transmission lines 17₁ 17₂ 17₃ 17₄ are configured in a bandstop configuration and are tuned to work at n times λ/4 (n times one-quarter of wavelength), n being an odd number different from 1. In other words, the electrical length of each transmission line 17₁ 17₂ 17₃ 17₄ ("stretched") is nλ/4 (n=3, 5, 7...). In a preferred embodiment, they are tuned to work at 3λ/4.
Each resonator 13₁ 13₂ 13₃ 13₄ also has a capacitor 15₁ 15₂ 15₃ 15_4. Preferably, the capacitors 15₁ 15₂ 15₃ 15₄ are adjustable. They are used for fixing the tuning of the resonators. This is explained in detail later.
The filter of figure 1 is designed to work in frequencies up to 1 GHz. In a preferred embodiment, it is designed to have a bandstop which rejects the LTE downlink band (791-821 MHz).
Figure 2 shows the physical aspect of the filter of figure 1. Elements shown in figure 2 have been provided with equivalent numerals of the same elements shown in figure 1 (for instance, resonator 13₁ 23₁). The filter is placed in a housing 28.
The housing 28 is shown with an open top so that the filter can be seen. Outside the housing 28 are an input terminal 20 and an output terminal 21. Connecting means 22 is connected between the input and output terminals 20 21 from which resonators 23₁ 23₂ 23₃ 23₄ are connected at terminals 24₁ 24₂ 24₃ 24₄. The resonators 23₁ 23₂ 23₃ 23₄ are each located in a cavity. Each resonator 23₁ 23₂ 23₃ 23₄ has a helical transmission line 27₁ 27₂ 27₃ 27₄ connected to one of the terminals 24₁ 24₂ 24₃ 24₄. The helical transmission lines 27₁ 27₂ 27₃ 27₄ in the resonators 23₁ 23₂ 23₃ 23₄ are preferably manufactured from any material conventionally used in the manufacture of current helical transmission lines, such as copper wire (or silvered copper wire). The resonators have a resonant frequency with a corresponding wavelength. Although figure 2 shows a particular embodiment in which the filter is formed by four resonators, a different number of resonators can be implemented (also one single resonator).
As illustrated in figure 2, the upper end of each helical transmission line 27₁ 27₂ 27₃ 27₄ is open (to the air). Its connection to ground is achieved thanks to the capacitors provided by conductive wires.
In a particular embodiment, the helical transmission line 27₁ 27₂ 27₃ 27₄, conductive wires 25₁ 25₂ 25₃ 25₄ 26₁ 26₂ 26₃ 26₄ (explained in detail later) and metallic walls of each resonator cavity are mounted and soldered on a double layered printed circuit board (PCB). The PCB is located within a metallic housing (or chassis) which comprises the input and output terminals 20 21. The chassis is closed by two metallic covers. In alternative embodiments a single layered PCB can be used, or even no PCB can be used (other mechanical supports and electrical connections are possible). The metallic walls which delimit the cavities can either be an integral part of the chassis or be independent from the chassis.
Fixing the tuning of each helical element (helical transmission lines 27₁ 27₂ 27₃ 27₄) in each resonator is achieved by means of two conductive wires 25₁ 25₂ 25₃ 25₄ 26₁ 26₂ 26₃ 26₄ which act as variable capacitors (in figure 1, capacitors 15₁ 15₂ 15₃ 15₄). One of the conductive wires (or capacitors) in each resonator allows for quick approach to the tuning (rough tuning) and the other one allows for fine tuning.
The conductive wire 26₁ 26₂ 26₃ 26₄ which allows for rough tuning is connected as a prolongation of the helical transmission line 27₁ 27₂ 27₃ 27₄in its open end (represented by a little circle having reference 18₁ 18₂ 18₃ 18₄ in figure 1). The conductive wire 26₁ 26₂ 26₃ 26₄ is connected by a galvanic connection to the open end of the helical transmission line 27₁ 27₂ 27₃ 27₄ (in a particular embodiment, through the PCB). This wire permits to lengthen a bit the length of the helical transmission 27₁ 27₂ 27₃ 27₄ itself. This conductive wire 26₁ 26₂ 26₃ 26₄ becomes thus an integral part of the helical resonator and can besides be coupled with variable (tunable) capacitor to the main body of the helical transmission line and to the walls of the cavity within which the helical element is housed. Adjusting the tuning of the resonator is thus achieved. Exemplary ways of adjusting the tuning of the resonator is done moving the conductive wire 26₁ 26₂ 26₃ 26₄, for example bending it, or cutting it. In a preferred embodiment, it is done by moving it, either reducing or increasing the distance between the conductive wire and the helical transmission line.
The second conductive wire 25₁ 25₂ 25₃ 25₄ is connected to ground (represented by a little circle having reference 19₁ 19₂ 19₃ 19₄ in figure 1).and also acts as a variable (tunable) capacitor at the open end of the helical transmission line 27₁ 27₂ 27₃ 27₄.
The effect of this second conductive wire 25₁ 25₂ 25₃ 25₄ is weaker than the one of the first conductive wire. The fixing in the tuning is thus finer. Exemplary ways of adjusting the tuning of the resonator is done moving the conductive wire 25₁ 25₂ 25₃ 25₄, for example bending it, or cutting it. In a preferred embodiment, it is done by moving it, either reducing or increasing the distance between the conductive wire and the helical transmission line.
In other words, first and second conductive wires 25₁ 25₂ 25₃ 25₄ 26₁ 26₂ 26₃ 26₄ serve as adjustable capacitors. Conductive wires 26₁ 26₂ 26₃ 26₄ are electrically connected to the open end of the helical transmission line and conductive wires 25₁ 25₂ 25₃ 25₄ are electrically connected to ground. Both serve as each capacitor in figure 1.
In a preferred embodiment, first conductive wires 26₁ 26₂ 26₃ 26₄ and second conductive wires 25₁ 25₂ 25₃ 25₄ are disposed parallel to the helical transmission line 27₁ 27₂ 27₃ 27₄, as shown in figure 2. This position provides an optimal compromise between capacity and movement.
As already explained, each resonator 23₁ 23₂ 23₃ 23₄ may be tuned to its proper bandwidth and center frequency by adjusting respective first and second conductive wires.
In conventional bandstop filters, tuning is achieved by means of screws inserted in the cavity of the resonator, as explained for example in US5932522 . However, tuning the resonator with the wires of the present invention provides with several advantages, such as mechanical simplicity derived from several aspects, such as the absence of physical contact between different parts of the filter, absence of friction, and so on. Considerable reduction in manufacturing and maintenance costs is thus achieved.
The inventors have observed, when designing the helical transmission lines 27₁ 27₂ 27₃ 27₄ for resonation at nλ/4 (n = 3, 5, 7...), that the rejection of the unwanted band is considerably more abrupt than such rejection when the helical transmission lines resonate at λ/4. This is illustrated in figure 3, where the response of the inventive filter (of figures 1 or 2) is shown in comparison to the response of a filter resonating at λ/4. Figure 3 shows the response of the inventive filter when configured to reject the frequency bands of 791-821 MHz approximately, which is the band dedicated to LTE. The filter abruptly rejects the signals at these frequencies, while does not affect signals in the TDT band.
The inventive filter occupies a bit more space than conventional filters tuned at λ/4, but offers impressive advantages in terms of its response. Besides, the inventive filter occupies less space than conventional filters (having cavities of great volume) designed to correctly separate the useful signal from the interfering one at the mentioned frequencies.
Although in figure 1 the filter is formed by four resonators, the number of resonators can vary, including having one single resonator, depending on the requirements (mainly loss in the band-pass and rejection in the stop-band) of each application and on the available physical space.
On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

Claims

A bandstop filter comprising:
a signal input (10, 20);

a signal output (11, 21);

connecting means (12, 22) for connecting said signal input (10, 20) and said signal output (11, 21);

at least one resonator (13₁, 23₁) connected to said connecting means (12, 22), said at least one resonator (13₁, 23₁) having a resonant frequency with a corresponding wavelength, said at least one resonator (13₁, 23₁) comprising a helical transmission line (17₁, 27₁);

the bandstop filter being characterized in that said helical transmission line (17₁, 27₁) of said at least one resonator (13₁, 23₁) has an electrical length of n times one-quarter wavelength of the resonant frequency, n being an odd number different from 1.
The bandstop filter of claim 1, which is placed in a housing (28).
The bandstop filter of claim 2, wherein said at least one resonator (13₁, 23₁) is located within a cavity of said housing (28).
The bandstop filter of any preceding claims, wherein said at least one resonator (13₁, 23₁) further comprising a capacitor (15₁).
The bandstop filter of claim 4, wherein said capacitor (15₁) is adjustable in order to vary the resonant frequency of said at least one resonator (13₁, 23₁).
The bandstop filter of any claims 4 or 5, wherein said capacitor (15₁) is implemented by means of a first conductive wire (26₁) and a second conductive wire (25₁).
The bandstop filter of claim 6, wherein said first conductive wire (26₁) is configured to achieve rough tuning of the resonant frequency of said at least one resonator (13₁, 23₁), said first conductive wire (26₁) being connected as a prolongation of the helical transmission line (17₁, 27₁).
The bandstop filter of either claim 6 or 7, wherein said second conductive wire (25₁) is configured to achieve fine tuning of the resonant frequency of said at least one resonator (13₁, 23₁), said second conductive wire (25₁) being connected to ground.
The bandstop filter of any preceding claims, wherein said helical transmission line (17₁, 27₁) of said at least one resonator (13₁, 23₁) has an electrical length of 3 times one-quarter wavelength of the resonant frequency.
The bandstop filter of any preceding claims, comprising a plurality of resonators (13₁ 13₂ 13₃ 13₄, 23₁23₂23₃23₄) connected to said connecting means (12, 22), each of said plurality of resonators (13₁ 13₂ 13₃ 13₄, 23₁ 23₂ 23₃ 23₄) comprising a helical transmission line (17₁ 27₁ 17₂, 27₂ 17_3, 27₃ 17₄, 27₄).
The bandstop filter of any preceding claims, configured to reject the frequencies of the frequency band between 791 MHz and 821 MHz.