TECHNICAL FIELD

The present invention relates to the technical field of high frequency electronic circuits, for example of the type used in systems for radio frequency broadcasting ("Broadcasting") of signals, such as radio or television signals.

In particular, the invention relates to a planar inductor with high power and very low return, particularly suitable for use in the above mentioned high frequency power circuits, for example aimed at being used as components for the transmission of the 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 power supply sections, signal amplification sections and one or more LC filters, aimed at limiting the antenna emission spectrum to the required frequency band and at keeping 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 occurs while transferring 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 i produced by the power dissipation.

This is true both for the strictly power-related components of the transmitter, such as the MOSFET amplifier stages, and for the passive components, such as the LC filters, and especially the output low-pass filter, which transfers the power signal to the antenna.

Regarding the LC filters, a solution generally adopted to limit power losses consists in their design with generously 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 low heat build-up values required for the various components are easily obtained by increasing the surface area of the current flow. This also allows a reduction of 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.

As an example of a conventional high frequency circuit, figure 1 illustrates a radio frequency LC filter 100, comprising a printed circuit board 101 in which a plurality of inductors 102 are made using planar technology, directly etched on the printed circuit of the board 101 .

Such inductor manufacturing technology has a simple construction and low cost. However, it points out a problem of insufficient thermal energy disposal, especially when high powers, typically above 1 kW flow through the RF circuits in which the inductors are inserted. Mainly, but not exclusively, this weakness is caused by the substantial heat generation produced by losses in the dielectric. In addition, return phenomena occur out-of-band due to the imperfect inductive behaviour of the turns. Finally, the circuits made with inductors of the above described type 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 made with square coils 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.

The above mentioned problems relating to parasitic effects and out-of-band signal returns could be prevented by using the type of inductor 120 illustrated in figure 3, made with an air-core wound wire 121 with circular turns 122 forming an approximately cylindrical coil, which is the most classic configuration and is closest to the concept of perfect inductance, in that it is not affected by parasitic effects and return of spurious out-of-band signal.

However, even this configuration has certain drawbacks, mainly due to the fact that, in order to obtain the desired inductance value, it is always necessary to carry out precise measurements and subsequent calibrations of the installed inductor, due to the significant variations in the electrical parameters of the component as a function of even small differences in geometry, and the difficulty of obtaining the exact geometry of the component. A further drawback is the bulkiness of the three-dimensional inductor configuration described above once installed.

The above described drawback is most evident in LC filters aimed at being installed in broadcast transmission systems of the modular type, 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) 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 an inductor which can be used in power circuits for radio frequency equipment and which is capable of ensuring characteristics of low current losses, and therefore limited heat generation, high signal cleanliness due to the absence of parasitic effects and out-of-band signal return, while having a compact construction suitable for use in high power transmitter modules.

Another object of the invention is to propose an inductor for radio frequency power circuits in which calibration operations are not necessary, 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 inductor, and consequently of the radio frequency circuits in which it is installed, while maintaining unchanged its performance and reliability characteristics.

SUMMARY OF THE INVENTION

The aforementioned and other objects, are entirely achieved, in accordance with the content of the claims, by means of a planar inductor, in particular for radio frequency power circuits, aimed at being installed as an air-core component of a printed circuit board of the above mentioned radio frequency power circuit. The planar inductor comprises a spiral consisting of a flat strip of electrically conductive material and provided at its ends with corresponding terminals, aimed at attaching the inductor to the printed circuit board, in a substantially parallel configuration and at a predetermined height therefrom. BRIEF DESCRIPTION OF THE DRAWINGS

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

- Figure 1 (prior art) is a circuit diagram of a low-pass LC filter made using planar printed inductors of conventional type;

- Figure 2 (prior art) is a perspective view of a conventional wire inductor wound in circular turns;

- Figure 3 is a plan view of a planar inductor made according to a preferred embodiment of the present invention;

- Figure 4 is a perspective view of the inductor of Figure 3;

- Figure 5 is a perspective view of a power circuit operating at high frequency, on which a pair of inductors made according to the invention is installed;

- Figure 6 is a perspective view of a planar inductor operating in air (set to operate at a distance from the circuit) in a different embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to Figures 3 and 4, and to a preferred embodiment of the invention, the reference number 1 indicates a planar inductor for high frequency power circuits, as a whole, with the high frequency power circuits comprising for example a compact LC filter 200 (see also Figure 5), particularly suitable for use as a low-pass output filter in a radio frequency power transmitter.

The above mentioned compact LC filter 200 is made on a printed circuit board 10, provided with a layer of conductive material 201 on which some planar components of the filter 200 are made, which do not pertain to the invention and therefore will not be further detailed. A pair of planar inductors 1 made according to the invention are installed in the filter 200.

The inductor 1 is advantageously obtained by photoetching and is mounted in air (set at a distance from the circuit borad), which is permitted by the mechanical characteristics obtained by the particular construction thereof. In this way, the inductor 1 is not affected by the properties of the dielectric of the printed circuit board 10 housing the LC circuit. More precisely, the inductor 1 is fixed in a suitable position to the conductive layer 11 , parallel thereto and at a predefined height therefrom, which is preferably between 2 and 5 mm.

According to a preferred embodiment of the invention, the inductor 1 comprises a substantially circularly evolving spiral 1a comprising a flat strip of metal (preferably, but not necessarily, silver-plated copper). The length of the spiral 1a, the distance between consecutive turns, the width of the flat strip and its thickness define very precisely the inductance value of the inductor 1 , 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 1a greater than 1 .5 mm.

In the dimensioning of the inductor 1 , its thickness is particularly important since, in addition to contributing to the definition of the power transmission and to the component resistance to thermal stress, it must be capable of giving to the component mechanical characteristics than maintain its stable shape once mounted in air in the RF circuit. Essentially, the minimum acceptable thickness for the use in power devices is considered to be no less than 0.5 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 envisaged thickness of the spiral is 1 mm.

Figure 6 illustrates an example of an inductor 110 for high power circuits operating in radio frequency according to a different embodiment of the invention.

The inductor 110 is also made, for example, of silver-plated copper using the photoetching technique and is provided with terminals 111 ,112 aimed at fixing it to the conductive surface of the device on which it is installed, for example, again an LC radio frequency filter. The fixing is obtained by interposing spacers 113,114 aimed at keeping the inductor 110 at a distance from the surface of the device. In this way, the inductor 110 is not affected by the properties of the dielectric of the printed circuit board housing the LC circuit.

The inductor 110 has a square spiral track 110a, very similar to the one conventionally used for the conformation of the above described planar printed inductors, with comers blunted by linear segments.

With the configuration and type of installation described above, by appropriately dimensioning the width and thickness of the track 110a, for example with the dimensional parameters already specified for the preferred form of embodiment of the invention, there is a significant improvement in the thermal dissipation performance of the inductor 110, which is similar to the performance of the above described inductor 1 , obtained according to the preferred embodiment.

In comparison to conventional moulded inductors, however, there are still some problems related to the presence of parasitic effects and out-of-band signal return, which lead to a non optimal efficiency of the component and the circuits in which it is installed. A good circuit compactness is maintained due to the flatness of the inductor 110.

Essentially, the inductor 1 , 110 made in the manner described above is extremely stable, and its electrical and thermal characteristics can be perfectly reproduced, respecting the dimensional ratios described above.

Unlike the planar inductors printed in the conducting layer 201 of the circuit 200, it has also been experimentally verified that the above described inductor 1 , made according to the preferred embodiment of the invention, is substantially free from unwanted out-of-band returns.

Unlike conventional air-core inductors with cylindrical coils, which have optimal characteristics regarding out-of-band return, but require calibration to define their exact inductance value, this inductance value for the inductor 1 ,110 of the invention is precisely defined by its geometry and by the dimensional specifications of the spiral, and therefore does not require any calibration.

However, 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.