The present invention relates to improvements m the design of filters of the type which are used in apparatus for the receipt of data signals such as those which are broadcast via satellite transmission systems. The data signals are typically received via apparatus which includes a feed horn connected to a waveguide which allows the data signals to be received from the antenna in relation to which the horn is mounted. The waveguide is then provided to allow the selective direction and processing of the received data signals in relation to the frequency bandwidth of the data which is desired to be received and processed and/ or the particular polarity of the data signals which is required to be processed and so different outlets and paths may be provided along the waveguide. Filters can be provided to allow the selection of the frequency of the data signals which is to be allowed to pass along and/ or from the waveguide. In particular, but not necessarily exclusively, the invention is directed towards filters which can be provided as or as part of the waveguide and which act as band pass filters which allow data signals within a selected frequency bandwidth to pass through the same low pass cut off filters of the type designed to allow radio frequency signals propagating just above the cutoff frequency of the waveguide to pass along the waveguide, and to reject higher frequencies due to the nature of the waveguide topology. Yet further, the filters are of particular use in relation to apparatus provided for the reception of Digital Television Signals transmitted via satellite broadcast systems so as to allow radio and/or television programmes to be generated from the data signals which are allowed to pass through the filter.

A problem with the conventional filters of this type is that the same tend to be relatively large m si2e m order to achieve the required characteristics for the required range of frequencies. This physical srze problem can make it difficult to include band pass filters as part of apparatus where there are srze constraints such as with apparatus provided at domestic premises for the reception of satellite television services. This apparatus has significant srze constraints and the filters are typically desired to be provided as part of Low Noise Blocks (LNB) which are provided as part of apparatus typically including an antenna designed to receive signals which are broadcast from one or more satellites and allow the receipt, and subsequent processing and transfer of the same, to receiving apparatus which can, for example, allow the generation of video and/ or audio data therefrom.

Conventionally the required dimensions of the band pass filters has meant that the apparatus in which the same are installed, such as the LNB, has had to be of a suitable srze to accommodate the band pass filters which can make the LNB bulky and cause other problems in the apparatus design. Typically, those experienced in the art could alternatively use a microstrip structure, however this inherently has higher losses can cause service error, and so is a less desirable solution than this invention.

The aim of the present invention is therefore to provide a filter of a design and dimensions which allows the same to be more easily included in apparatus in terms of the size requirements while, at the same time, ensuring that the performance of the filter still meets the required performance targets.

In a first aspect of the invention there is provided a filter for received data signals, said filter including a body having a cavity formed therein, said cavity defining a channel connected to an inlet and an outlet at spaced locations and wherein a plurality of recesses are formed as part of the cavity and spaced along the channel which is located intermediate the said inlet and outlet.

Typically the filter includes an input m the form of a microstrip transition.

Typically the base portion includes a bend formed therein prior to the output and typically after the last of the recess portions from the inlet. Typically the bend is a 90 degree bend. In one embodiment the provision of the bend allows microstnps provided at the inlet and outlet of the filter to be provided on the same side of the filter to thereby increase the compactness of srze of the filter.

In one embodiment the filter is a low pass cutoff filter.

In one embodiment the filter is a 2 ^nd order filter in which case two spaced recesses are provided. In an alternative embodiment the filter is a 3 ^rd order filter in which case there are 3 spaced recesses. Thus an 11 ^th order filter would typically have n recesses.

In one embodiment the portion of the channel on which the recesses are formed is straight. In another embodiment the said channel portion may be bent to accommodate one or more further recesses whilst continuing to attempt to minimise the si2e of the filter.

In one embodiment the number of recesses determines the "order" of rolloff, which determines how quickly the undesired frequencies are attenuated or the steepness of the transition between the band of frequency allowed to pass through the filter and that which is prevented. Typically the waveguide width of the base portion is selected to determine the frequency at which roll off or rejection occurs at the lower frequency value of the allowed frequency band and the number of recesses is used to control the roll off or rejection which occurs at the higher frequency value of the allowed frequency band.

Typically the recess dimensions of the filter base portion are selected so as to define the frequency bandwidth allowed to pass through the filter and hence allow the filter to have a bandpass filter characteristic in which the frequency band of the received signals which are allowed to pass through the filter are defined. Typically, the length of the base portion can be selected at the time of design to allow the filter to have the selected cutoff value. Typically the filter utilises the cut off frequency of the channel or waveguide to determine the low side rejection value in terms of frequency.

In one embodiment the filter includes a transition plate which is located as part of, or is, a printed circuit board (PCB) and preferably the ground plane of the PCB is used to form one of the walls of the filter. Typically the components of the PCB or PCB's are mounted on the opposing side of the PCB surface to that which acts as a waveguide wall for the filter.

The ability to provide the waveguide filter in a compact srze means that the filter can be accommodated more easily in the LNB structure and hence, in turn allows the LNB to be provided of a smaller size and the design and shape of the same to be more flexible, which would provide an alternative to a microstnp filter.

In one embodiment the filter is formed as an integral part of the body of the LNB.

There is therefore provided a filter m accordance with the invention which is found to be less sensitive to manufacturing processes such that the manufacturing tolerances need not necessarily be so tight while, at the same time, providing a filter which is compact in size and the shape of which can be designed to suit specific manufacturing requirements.

Typically the filter is a low pass cut off filter for use in apparatus to receive data signals for use in the generation of radio and/ or television programmes. Typically the data signals are transmitted via satellite broadcast system and the filter acts as a waveguide connected to a feed horn from which the data signals are received via an antenna with respect to which the feed horn and hence filter is mounted. In a further aspect of the invention there is provided apparatus for the receipt of transmitted data signals, said apparatus including one or more filters as herein described in order to allow data signals received in a particular frequency band to pass therethrough and to prevent data signals of other frequencies from passing therethrough.

Specific embodiments of the invention are now described with reference to the accompanying drawings; wherein

Figures la-h illustrate views of a second order filter formed in accordance with the invention m one embodiment;

Figures 2a-d illustrate the components of the filter of Figures la-h;

Figure 3 illustrates a model of the cavity of a third order filter formed in accordance with the invention;

Figure 4 illustrates a model of the cavity of a fourth order filter in a further embodiment of the invention;

Figures 5 and 6 illustrate the performance of 2 ^nd and 3 ^rd order filters in accordance with 2 ^nd and 3rd order filters in accordance with the invention.

Referring now to Figures la-h and 2a-d the filter in accordance with the invention is formed, in this embodiment, as an assembly comprising a body portion 9 and lid portion 15 of a suitable metal or metal alloy and a microstnp transition plate 17 which are joined together by securing means 19. The filter may also be formed integrally, typically by casting, as part of another data receiving component such as an LNB.

The filter itself is formed by a cavity 11 in the body portion with the cavity including a channel 13 along which the received data signals within a permitted frequency band can pass from an inlet 6 at one end to an outlet 8 at the other. The channel includes a base portion 4 and which, at one end, is connected to the input 6 via the microstnp transition 17 and, at the other end at output 8 by the microstnp transition 17. The location of one or both of the inlet and/or outlet can be orientated with respect to the filter body in order to allow specific design and/or operation requirements to be taken into account such that, in this case, the microstnp transition and hence inlet 6 and outlet 8 lie in the same plane to the same side of the assembly.

In between the input and output locations are formed a plurality of recesses 10, which can also be referred to as ribs or fins. In the design shown, two spaced recesses are provided, the said recesses equally spaced apart along a portion of the channel. The number of recesses defines the order of the particular filter design such that, in the design shown in Figures 1 and 2 the filter can be referred to as a 2 ^nd order filter, having two recesses and, for example, in a further embodiment as shown m Figure 3, when three recesses are provided the filter is a third order filter.

The microstnp transition 17 is provided with a ground face 12 as illustrated in Figure lh which shows (in hatching) the face 12 of the microstnp transition, with the body portion 9 removed and shows how the respective microstrips 6'8' for inlet 6 and outlet 8 respectively are formed m the transition plate, most typically a PCB, and are positioned to overlie rebated portions 21 in the lid portion 15 and which are m communication with the channel 13 and hence allow the passage of data signals of the permitted frequency band from the inlet 6 through the first rebated portion 21, along the channel 13 to the second rebated portion 21 and hence to the outlet 8. It will also be seen that the ground side 12 of the transition plate is located along and forms part of the wall of the channel 13, which can also be referred to as a waveguide, and includes the input 6 and output 8 formed therein as shown. The base portion also includes a 90 degree bend 16, the outline of which is shown and which leads to the output 8 as shown. Figure 3 illustrates a further embodiment of the invention m the form of a third order filter. In this case only the outline of the filter cavity is shown m a model form and so there is indicated the external walls and shapes of the cavity which is formed. Once again there is provided the outlet 6 and 8 and part of the transition plate 17 with the same having in internally facing surface 12 as described above and an external face 14 and three recesses 10.

The filters shown m Figures 1-3 have the recesses formed along a straight portion of the channel 13. However m certain circumstances it may be necessary to alter the shape of the filter to make the same more compact while, m this example, increasing the order of the filter so that m the filter of Figure 4, which again shows in model form the outline of the shape and form of the walls of the cavity of the filter formed in the body, a 4 ^th order filter is provided with four recesses 10 spaced along a U shaped channel 13 which leads from the inlet 6 to the outlet 8 via microstrip transition 17. As before the cavity 11 and channel is formed in the body 9 and the outlet and inlet are formed as rebates

Thus, in order to determine the operating characteristics of the filter m accordance with the invention, the number of recesses provided, as described above, is used to determine the particular order of the filter and, typically the greater the order of the filter then the greater the roll off of the frequency above a certain frequency value. Figure 5 illustrates graphically the performance of a filter m accordance with the invention as shown m Figures 1 and 2, which is of a 2 ^nd order, having two recesses 10, while Figure 6 illustrates the performance of the filter m accordance with Figure 3 having three recesses, and therefore being of a third order. Typically, the length of the 3 ^rd order filter will be greater than that of the 2 ^nd order filter due to the need to accommodate the extra recess but the overall length of the filter m accordance with the invention is still significantly less than that of conventional filter designs and can be further reduced by suitably shaping the channel 13 as illustrated in Figure 4. Typically the waveguide width of the filter is selected to control the cut off on the lower side of the frequency band which is to be allowed through the filter and which therefore allows the filter to act as a bandpass filter in which a specific frequency band of the signals received by the apparatus in which the filter is installed, is allowed to pass through the filter.

The number of recesses 10 provided therefore determines the "order" of rolloff, which determines how quickly the undesired frequencies are attenuated or the steepness of the transition between the band of frequency allowed to pass through the filter and that which is prevented. Typically the length X of the channel 13 as shown m Figure 2a is selected to determine the rejection value or roll off rate at the lower frequency of the allowed frequency band of the data signals which are received and which can pass through the filter. The recess dimensions of the channel portion 13 are selected so as to define the frequency bandwidth allowed to pass through the filter and hence allow the filter to have a bandpass filter characteristic in which the frequency band of the received signals which are allowed to pass through the filter are defined. Typically, the waveguide width of the channel can be selected at the time of design so the filter utilises the cut off frequency of the waveguide transmission line to determine the low side rejection value m terms of frequency.

There is therefore provided a filter design which allows significant reductions m srze to be achieved whilst still allowing the required performance to be achieved and therefore allows the same to be installed in smaller apparatus designs and therefore increases the design options whilst allowing potential costs savings to be achieved.