GB2512032A - Filter - Google Patents

Abstract

An in-line bandstop filter for rejecting 4G mobile phone signals in a TV receiver comprises earthed quarter-wave ceramic resonator stubs 22 and series coil inductors 18 coupled as delay elements between the resonator stubs 22. The interfering 4G signal band may be just above the TV frequency band, and so the filter must provide a sharp transition at its lower frequency edge. The required sharp transition may be achieved by selecting inductor values that provide transmission poles near to the edge of the stopband (TP, figure 2).

Description

FILTER

The present invention relates to electronic filters and more specifically to UHF band-stop filters.

While the invention has broader potential applications, It has been created In response to a specific technical challenge. In the United Kingdom several licences for provision of so-called fourth generation (43*) mobile communication services are, at the time of filing of this application, expected shortly to be auctioned to service providers. One of the main frequency bands for 40 transmissions Is to be around 800 MHz. This band was previously used for analogue television transmissions, which have recently been discontinued In the UK. However the frequency range allocated to 40 mobile services Is immediately adjacent a band still being used for transmission of free-to-view digital television.

It is eqected that in some geographical areas 40 mobile transmissions may consequently cause interference for viewers of digital television.

it is also expected that 40 service providers will, as a condition of their licences, be required to participate In schemes for mitigating the interference problem for television viewers. In many cases this will involve provIding IndIvIdual viewers or households with a frequency selective filter, to be connected between an aerial and a television receiver, to suitably attenuate the unwanted signals and so remove the interference. The band to be removed, in this particular application, is from 791 to 821 MHz, which is the band allocated to the services.

Hence there exists a pressing commercial need for a band-stop filter, with a stop band covering the aforementioned range, which can be straightforwardly and economically mass produced and compactly packaged. It is desirable for the filter to have a narrow transition band since the 40 signal which is to be removed l.

by the filter and th.e television channel which, is to be transmitted are separated only by a narrow guard band.

GB 2347805B, in the name of the present inventor David CHve Baty, thaciosed a bandstop fUter with a stopband at 153 MHz that used muftipe d&ay circuits in a signal ilne. with ceramic resonators leading from the signal line to ground. Each delay circuit comprised a parallel combination of (i) a first inductor, (ii) a first capacitor and (iii) second and third capacitors in series with one another. A second inductor was connected from a point between the second and third conductors to ground. In this way the delay circuit, de&gned according to lumped element principles, served to provide a quarter wave delay between one resonator and the next.

While this filter is effective, it is desfrable to provide a fiiter which is improved in terms of (a) simpUcity of construction and/or (b) filter characteristics, particularly in that there should be a narrow transition band that is, there should be a sharp transition from frequencies that the filter transmits to frequencies that it strongly attenuates.

According to the present invention there is a UHF band stop filter comprising a delay element connected on an input side to a fllter input and on an output side to a filter output, a first ceramic resonator which is coupled to the input side of the delay element an.d is led to ground and a second ceramic resonator which is coupled to the output side of the delay element and is led to ground, characterised in that the d&ay element is formed by an inductor.

Compared with the filter circuit of GB 2347805B, the circuit according to the present invention not only provides a simplification in construction, and manufacture, but also for reasons which will be explained below provides a marked improvement in the filters characteristics.

Specific embodiments of the present invention will now be deathbed, by way of example only, with reference to the accompanying drawings, in which:-Figure 1 Is a circuit diagram of a band-stop filter embodying the present invention; Figure 2 is a graph showing theoretical (calculated) insertion loss and return loss of a band-stop fitter embodying the present invention; Figure 3 Is a graph showing measured insertion loss and return loss of a prototype band-stop filter embodying the present invention; FIgure 4 Is a graph of measured insertion loss of the same prototype filter, over a frequency range much narrower than that of Figure 3; Figures 5a and 5b are respectively side and end views of a ceramic resonator suitable for use In the circuit of Figure 1, shown to an enlarged scale; and Figure 6 illustrates the physical construction of a filter embodying the present Invention.

The band-stop filter circuit 10 Illustrated in Figure 1 comprises an input 12 connected to an output 14 by a signal tine 16 incorporatIng a number of Inductors 18. That is, the inductors 18 are connected in series between the input 12 and the output 14. Three Inductors 18 are shown in Figure 1 but other embodiments may use one, two or more than three Inductors, according to design requirements. The most simple circuit embodying the invention would comprise only the components seen in dotted box A in Figure 1 and have only a single inductor iSa. The input side of Inductor 18a Is coupled through a capacitance 20a to a ceramic resonator 22a which in turn is connected to ground. The output sIde of inductor 18a is connected to ground via a series combination of a capacitance 20A and a ceramic resonator 22k :3 The basic circuit of box A can be expanded by adding one, two or more repeating units as seen in boxes B and C of Figure 1. Each repeating unit comprises an additional inductor 18b, 18c incorporated In the signal line 16 (that is, connected in series with the first inductor 18a between it and the output 14) and, on the output side of the inductor 18b, I Sc, a further series combination of a capacitance 2Db, 2Cc and a ceramic resonator 22b, 22c. An increased number of these repeating units can provide improved filter characteristics, at the expense of increased complexity and cost.

The ceramic resonators 22 are quarter wave resonators. Ceramic resonators are well known devices which come in various forms and it is not the intention of the applicant to limit the scope of the invention to any particular form for this device, but an example of a suitable coaxial ceramic resonator 22 is illustrated in Figure 5 and comprises an approximately cuboidal block 50 of ceramic material which is silver plated on ail of its outer faces except for end face 52.

A through-going bore 54 leads from the end face 52 to the opposite end face 58, and the input to the resonator is formed by a wire (not shown) conductively secured to the bore's inner surface. The device's other terminal is formed by the outer plating, which can for example be soldered to a ground plane in embodiments of the invention. Cerwic materials used in suitable devices typically have a relative permittivity in the range 10-100. In the present invention, the ceramic resonators serve as quarter wave stubs at the filter's stop frequency.

That is, each resonator is effectively one quarter of a wavelength long at this frequency.

The inductors 18 are in the present embodiment formed as miniature wire coils and form quarter wave couplings between neighbouring ceramic resonators 22. The values of the inductors 18 are calculated to (a) give the required wavelength separation between the resonant sections and (b) to introduce near-passband transmission poles, helping to give a narrow transition band (as to which, see below), In operation, the effect of the quarter wave coaxial ceramic resoriators 22 which are electrically one quarter of a wavelength long at a wavelength in the stop band, separated by the Inductors 18, whIch are also electñcaily one quarter wavelength long at similar frequencies, is to create a complex standing wave pattern that will either transmit the signal to the output 14 or, In the frequency range corresponding to the stop band will short It to ground and so prevent signal transmission.

To this extent the present filter circuit is similar to the earlier version disclosed in GB 23478058. However the present circuIt is both simpler than that earlier circuit and superior in its characteristics.

Looking now at Figure 2, line X represents calculated insertion loss (that Is, the attenuation of the signal from input to output) on a logarithmic, decibel (dB) scale. Line V represents return loss (reflected power dMded by input power) on the same scale. Note that the graph clearly shows the favourable skewing of selectivity (being the small transition frequency bandwidth n MHZ) between the lower frequencies where the signal is transmitted relatively unimpeded and the stop-band frequencies where the signal Is blocked by the filter, compared to the transition bandwidth between the stop-band frequencies and the upper passband frequencies. This is desirable, in relation to the specific application discussed above, because the digital signal that is to be transmitted and used by a television receiver is on the lower frequency side of the stop-band. What is needed is a filter that transmits that lower frequency range and blocks the unwanted mobile telephone signals in the stop-band, with a sharp transition between those two ranges. The Illustrated filter meets these requirements.

Figures 3 and 4 show measured experimental data and serve to demonstrate that these desirable characteristics are In fact achieved by physical embodiments of the invention. In FIgure 3 transmission loss is once more represented by flne X and reflection toss by line V, and transmission loss is seen to be strati outside the narrow stop-band S. Figure 4 shows transmission loss X over a smaller frequency range. The abrupt change from transmission to very large signal attenuation in transition band Z is apparent.

This desirable characteristic is, at least in part, due to the introduction of low side near passband transmission poles, indicated at TP on FIgure 2 and lying at approxImately 780 MHZ in the illustrated embodiments.

As compared with the circuit of GB 2347805B, the illustrated circuit differs in that whereas the earlier circuit used a lumped element network designed according to low frequency delay line theory to provide the required phase separation between the resonators. this network is replaced, in the circuit illustrated herein, by a single Inductive element, This new circuit has a remarkable effect on the performance of the filter in that it profoundly alters the shape of the filter characteristic in the region of the stop-band, making the transition between the lower passband and the stop-band much narrower than the transition band between the stop-band and the upper passband.

The physical construction of the filter cIrcuit 10 requires consideration because, at the relevant frequencies, it is not merely the topology of the circuit that determines Its characteristics but also aspects of its design such as component separation, and the consequent lengths of conductors connecting the components. Figure 6 shows a suitable construction. Input 112 and output 114 are formed as conventional coaxial connectors whose outer female connectors are electrically connected to a metal fitter housing 140 which screens out external interference. The signal line 116 incorporates wire coIls 118 forming the required Inductors. Metal tabs lead from the signal line 116 to the shunt capacitors 120 and through them to the ceramic resonators 122, which are soldered to the filter housing 140, which thus serves as a ground plane.

Other physical embodiments are of course possible.

Claims (14)

1. CLAIMS1. UHF band-stop filter comprising a delay element connected on an input side to a filter input and on an output side to a filter output, a first ceramic resonator which Is coupled to the input side of the delay element and is led to ground and a second ceramic resonator which Is coupled to the output side of the delay element and is led to ground, characterised in that the delay element is formed by an inductor.
2. 2. A UHF band stop filter as claimed In claim 1 wherein the delay element comprises a single inductor and no additional components.
3. 3. A UHF band stop filter as claimed in claim I or claIm 2 wherein each ceramIc resonator serves as a quarter wave line at a frequency In or adjacent a filter stop-band.
4. 4. A UHF band stop filter as claimed In claim 3 wherein the inductor provides a quarter wave delay at or adjacent the filter stop-band,
5. 5. A UHF band stop filter as claimed in claim 3 or claim 4 wherein interaction of the Inductor with the ceramic resonators provides transmission poles on the low wavelength side of the stop-band.
6. 6. A UHF band stop filter as claimed in claim 5 wherein the transmission poles provide a skewed filter characteristic having a narrow transition band on the low wavelength side of the stop-band.
7. 7. A UHF band stop filter as claimed in any preceding claim having one or more additional inductors connected In series with the first rnenuoned inductor between It and the filter output, each additional inductor being coupled on its output side to a respective additional ceramic resonator which is led to ground.
8. 8. A UHF band stop filter as claimed in any preceding claim wherein each ceramic resonator is coupied to the respective deiay element through a respective capadlance.
9. 9, A UHF band stop flier as daimed hi any preceding daim having a stop-band at a frequency b&ow I GHz.
10. 10. A UHF band stop filter as daimed in any preceding claim having a stop-band at a frequency above 500 MHZ.
11. 11, A UHF band stop filter as claimed in any preceding claim having a stop band between 780 and 832 MHz.
12. 12. A UHF band stop filter as claimed in any preceding claim having a stop band between 780 and 862 MHz.
13. 13. A UHF band stop filter as claimed in any preceding claim having a stop band between 780 and 820 MHz,
14. 14. A UHF band-stop fUter substantiay as herein described with reference to, and as iUustrated in, the accompanying drawings.