TECHNICAL FIELD

The present invention relates to the technical field of systems for radio frequency broadcasting ("Broadcasting") of signals, for example of radio or television signals.

In particular, the invention relates to a compact radio frequency LC filter with high power and very low return, particularly suitable for use in power equipment for the transmission of aforesaid radio or television signals.

BACKGROUND ART

In the technical field of radiocommunication electronics, and in particular in the field of broadcasting of radio and television signals (Broadcasting), high power radio frequency transmitters are commonly used, capable of working with antenna emission power ranging from hundreds of Watts to tens of Kilowatts. Such transmitters generally comprise feed sections, signal amplification sections and one or more LC filters, aimed at limiting the antenna emission spectrum to the required frequency band and to keep out-of-band frequencies within acceptable limits.

TECHNICAL PROBLEM

In the aforementioned transmitters, it is extremely important to keep under control the production of heat that inevitably takes place during the transfer of power during normal operation of the equipment, despite optimal circuit configurations and particularly efficient components and construction technologies. Low power dissipation is necessary to limit consumption, and thus the operating costs of the equipment, and to avoid thermal stress of the components due to the high temperatures produced by the power dissipation. This is true both for the strictly power-related components of the transmitter, such as the i MOSFET amplifier stages, and for the passive components, such as the LC filters, and especially the output low-pass filter, which carries the power signal to the antenna.

Regarding the LC filters, a solution generally adopted to limit power losses consists in their design with large sized components. Since the filters are often made in planar mode, that is with most of the inductive and capacitive components obtained directly in the printed circuit board, it is normal to envisage very large printed circuit boards, in which a low heat build-up required for the various components is easily obtained by increasing the surface area of the current flow. It also allows to reduce the production costs of filters, as it is possible to use materials that are not particularly sophisticated in terms of characteristics, such as dielectric constant and thermal conductivity.

LC radio frequency filters, even those manufactured using planar technology, also require customised calibration operations after being constructed and before being installed in the power transmitter. This is mainly caused by the fact that the geometry of planar inductors, which are realised with square-shaped spiral turns on the printed circuit board, is very critical, both with regard to the correct electrical dimensioning of the components and to the production of parasitic effects and out-of- band signal return.

A further necessity of the fabrication of LC filters for power equipment arises from the development of modular broadcast transmission systems, in which the overall transmission power is obtained from one or more standardised transmitter modules, which are integrated in a support structure (cabinet with a 'bus', or common connection system) situated closely side by side. Each module includes all the components necessary for substantially independent operation, as well as the logic for interfacing with the support structure and the other modules in the system. Transmitters of this type require components that are as compact as possible, capable of being housed in an independent transmitter module, yet without sacrificing the performance, efficiency and reliability characteristics typical of high-quality broadcasting systems.

OBJECTS OF THE INVENTION

It is an object of the present invention to propose a power LC filter for radio frequency equipment which is capable of assuring low current losses, and therefore limited heat generation, high cleanliness of the filtered signal due to the absence of parasitic effects and out-of-band signal return through the inductors, while having a compact construction suitable for use in high power transmitter modules.

Another object of the invention is to propose a radio frequency power LC filter which does not need calibration, but whose electrical characteristics are obtained by simple design dimensioning of the components.

A further object of the invention is to limit the construction costs of the power LC filter low, while maintaining its performance and reliability characteristics.

SUMMARY OF THE INVENTION

The above-mentioned and other objects are entirely achieved, in accordance with the content of the claims, by means of a compact LC filter for radio frequency power transmitters comprising an input terminal, an inductor and at least one capacitor, which form a resonant circuit, electrically connected to the input terminal to receive a radio frequency signal to be filtered, and an output terminal, electrically connected to the resonant circuit to carry a filtered radio frequency signal to the output. Two resonant circuits are connected in series. The input and output terminals and the resonant circuits are supported by a printed circuit board. Each capacitor is of the planar type, made by photoetching in a conductive layer of the printed circuit board, and each inductor comprises a substantially circular evolving spiral consisting of a flat strip of metal, attached at its ends to the printed circuit board, substantially parallel thereto and at a predetermined distance from the conductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will be apparent from the following description of a preferred, although not exclusive embodiment, in accordance with claims and with the help of the enclosed drawings, in which:

- Figure 1 is a preferred circuit diagram of a low-pass LC filter made according to the invention;

- Figure 2 is a plan view of a preferred embodiment of the filter of Figure

1 ;

- Figure 3 is a perspective view of the filter of Figure 2.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to Figures 1 and 2, a compact LC filter as a whole, according to the invention, is indicated with 100 and is particularly suitable for use as a low-pass output filter in a radio frequency power transmitter.

Since the latter is not the only use of the filter 100, although it is the use of choice, by way of example and not limitation, a preferred configuration according to the invention of a resonant two-stage low- pass LC filter will be described below. Obviously, it is understood that different configurations of filter circuits and other uses are considered to fall within the scope of the invention.

The above-mentioned compact LC filter 100 is made on a printed circuit board 10, provided with a layer of conductive material 11 on which the planar components of the filter are made, as will be described in further detail below. According to the invention, in order to obtain the compactness of the device which is a necessary requirement, the printed circuit board is made of material with a high dielectric constant and high thermal conductivity. This makes it possible to make planar capacitor armatures on the conductive layer 11 of small dimensions, with the same required capacitance value, and to have a good transfer of the heat dissipated by the inevitable power losses caused by strong currents necessary to transfer the signal power generated by the preceding RF amplifier stage to the antenna.

In particular, the material selected to make the printed circuit board 10 is alumina, a ceramic oxide of aluminium, characterised by a very high dielectric constant (greater than 9.5 - the materials normally used for printed circuit boards have a dielectric constant no greater than 2) and by an excellent thermal conductivity, generally greater than 12 W/m°K.

Another usable material, less valuable than alumina but much less expensive, which still allows good performance and sufficient compactness and limits production costs in cases where this is a priority over optimising dimensions, is a composite PTFE based material reinforced with glass fibre and a ceramic component, for example known by the technical-commercial name TC600. This material still has a high dielectric constant, greater than 6, and a not very high thermal conductivity, which can be compensated for by using more efficient heat sinks with a larger surface area.

As for the circuit configuration of the filter 100, with reference to figure 1 , an input terminal 1 is provided, which is used to introduce the power RF signal into the filter. A first resonant circuit 2, connected to the input terminal 1 , comprises an inductor 21 and a capacitor 22, likewise connected to said input terminal 1 and to a common circuit node. A further capacitor 24 is connected in parallel to the inductor 21 . In the illustrative filter configuration, a second resonant circuit 2 is provided, arranged in series with the above described first circuit. An additional capacitor 23 is provided between the two resonant circuits, connected to the common node between the two resonant circuits 2 and to the common circuit node.

The second resonant circuit 2 is in turn connected to an output terminal 3, aimed at transferring the filtered power RF signal to the antenna.

The circuit configurations and operation modes of LC filters are universally known, and will not be described in further detail, since their functional characteristics are irrelevant with respect to the objects of the present invention. By way of information only, for a configuration of the low-pass filter 100 which requires a cut-off frequency of 125 MHz with 3 dB attenuation, the following values of the above-described components are provided: 63 nH for the inductors 21 ; 19 pF for the capacitors 22; 36 pF for the additional capacitor 23; and 10 pf for the additional capacitors 24.

Referring now to Figures 2 and 3, according to the invention, the capacitors 21 and the additional capacitor 23 are of the planar type, an armature of which is made in the conductive layer 11 of the printed circuit board 10. For all the aforementioned capacitors, the second armature is made in a further conductive layer present in the opposite surface of the board 10, which forms the aforementioned common circuit node. By contrast, the further capacitors 24 are of the discrete type and are soldered onto the conductive layer 11 .

Each of the inductors 21 is advantageously air-core inductor and is fixed in a suitable position to the conductive layer 11 , parallel thereto and at a predetermined distance with respect thereto, preferably between 2 and 5 mm.

The inductor 21 comprises a substantially circularly evolving spiral 21a consisting of a flat strip of metal (preferably, but not necessarily, silver-plated copper). The length of the coil 21a, the distance between consecutive turns, the width and thickness of the flat strip very precisely define the inductance value of the inductor 21 , its power transmission characteristics and its resistance to thermal stress, according to empirically defined results derived from research and field experiments.

Applications on RF transmitters of rated power greater than a few hundred watts require flat strip wider than 1.5 mm and distances between two consecutive turns of the coil 21a greater than 1 .5 mm.

By way of example, a spiral length of 123 mm, a flat strip width of 2 mm and a distance between consecutive turns of 1.6 mm are provided to obtain an inductor 1 with an inductance value of 63 nH, capable of operating up to a rated power of 1 .5 kW. The thickness of the spiral is 1 mm.

Essentially, the inductor 21 made in the above described manner is extremely stable, and its electrical and thermal characteristics can be perfectly reproduced, respecting the dimensional ratios described above.

Unlike the planar inductors made by photoetching in the conductive layer 11 or the planar air-core inductors with the conventional square spiral turns, the above described inductor 21 is substantially free of unwanted out-of-band return. Unlike conventional air-core inductors with cylindrical turns, which have optimal characteristics with respect to out-of-band return, but require calibration to define their exact inductance value, the inductance value for the inductor 21 of the invention is precisely defined by its geometry and by the dimensional specifications of the spiral 21a, and therefore does not require any calibration.

It is understood that what above is an example and not a limitation, therefore possible modifications of details are considered from now on to be within the protective scope defined by the claims below.