Feeding method and device, particularly for a Doppler VOR system, modulator suitable for the same and Doppler VOR system

Abstract: The blending function, an approximation of the sinX/X function limitedly to the three central lobes, is obtained by amplitude overmodulation of a single signal according to a periodic function. The blending function can be controlled through the control of the periodic function.

Description: The present invention relates to a feeding device for a Doppler VOR system having an antenna array, including: switching means (SW) for switching a modulated message signal between antennas of said antenna array; filter means for performing a blending function in order that said switching of the modulated message signal between the antennas (E-1,...E-N) of the antenna array is gradual, said filter means including at least one amplitude modulator (MOD-A, MOD-B) capable of modulating a message signal with a periodic signal having a modulation index greater than 100% (per cent) to generate the modulated message signal; and a control logic unit for controlling said switching means (SW) and said filter means. Many systems, among which civil and military navaid systems (TACAN, VOR, ---), radar systems and so on require radio signal transmission with a moving radiation pattern (generally in a periodic fashion): very often rotating around its axis. Up to some decades ago, such motion effect was obtained by moving physically the transmission antenna; this was a problem for the reliability and accuracy of such systems. As soon as the electronic technology made it possible, such systems were little by little replaced with more modern systems in which the motion of the radiation pattern was simulated through an antenna array fed with electrical signals having phase and/or amplitude variable with time in a suitable relation with each other; in such systems it is therefore necessary to provide antennas with feeding devices capable of generating electrical signals having the suitable characteristics. This often involves the need of switching the same electrical signal from an antenna to another as it occurs e .g. in Doppler VOR systems; it is well known that, in order to obtain a perfect simulation of the motion, such switching must be gradual. In the course of the description reference will only be made to Doppler VOR systems for simplicity but, as already clarified, the same problems can be encountered in other systems. A VOR (Very high frequency OmniRange) system allows an aircraft equipped with a suitable receiver to be provided with a bearing information relative to a ground beacon whose geographical position is known. The VOR radiates a metric band (108-118 MHz) signal whose quality largely depends upon the place in which the VOR is installed; in fact, the omnidirectional radiation is subject to distortion due to undesired reflections by objects. For this reason, a conventional VOR is advantageously replaced with a Doppler VOR. In a Doppler VOR system, a "reference" signal, corresponding to a VHF carrier amplitude modulated by a sinewave having a frequency of 30 Hz, is transmitted according to an omnidirectional pattern, and a "variable" signal, corresponding to two side-bands, at +/- 9960 Hz of the VHF carrier frequency, modulated by a sinewave having a frequency of 30 Hz, according to a directional pattern, carrying azimuth information. These two sidebands are emitted independently from each other through separated circuits. The omnidirectional pattern is radiated through a central antenna while the other pattern is obtained by rotating on a circle two antennas diametrally opposite and each fed by one of the two sidebands at +/- 9960 Hz of the carrier. The revolution frequency of these antennas is 30 Hz. For practical reasons, as already mentioned, the physical rotation of the antennas which emit the two sidebands is simulated through the gradual switching of the D-VOR signal between the fixed antennas of a circular array in such a way as to simulate the progressive motion of the emitting point from one antenna to the next one; more specifically one carries out, in general, the switching of a radiofrequency signal amplitude modulated in accordance with a suitable time function, called blending function, which is able to attenuate the spurious modulations that are generated because of the discretization of the antennas themselves. Several solutions for the emission of the two sidebands of the amplitude modulation of the VHF carrier exist at present; in particular the emission of only one sideband, of two sidebands alternately, and of two sidebands simultaneously over two antennas or two diametrically opposed antenna groups; the present invention can be applied to each of these solutions. More details on operation of D-VOR systems and problems connected with their design, in particular of the blending function, of the distribution device and of the antenna array, are described by B.R. Johnson and J.G.N. Lee in "A Double-Sideband Doppler VHF Omnirange Beacon" - AWA Technical Review, Vol. 15, No. 1, 1973 and by F.L. van der Berg in "The Philips Doppler VOR Beacon RN200" - Philips Telecommunication review, vol. 34, No. 1, April 1976. It is clear that the design of a new feeding device for Doppler VOR system will have to face well known problems such as the choice of a suitable blending function, the generation of the same, the complexity of the distribution device, and try to solve them. The ideal blending function is notoriously sin X/X (provided that there is no interaction between the antennas of the array) but, given the physical impossibility of realizing it, approximations thereof are used: in the above-mentioned articles the use of the function sin x, limited to a half period is used. From the US patent 3,896,444 it is known the use, as blending function in a Doppler VOR system, of an approximation of the function sin X/X, restricted to the three center lobes, obtained through combination of two amplitude modulated signals according TO rectified cos x function. The blending function generator uses modulators constructed with varactor diode bridges. The object of the present invention is to provide a feeding device for Doppler VOR systems including an antenna array which implements a blending function similar to sin X/X, through a circuitry which is not too complex and critical and that does not involve a high waste of energy of the signal entering the feeding device. This object is achieved through the feeding device as set forth in claim 1; further advantageous aspects of the present invention are set forth in the dependent claims. In the present invention the blending function, an approximation of the sin X/X function restricted to the three central lobes, is obtained by amplitude overmodulation of a single signal by a periodic function. The blending can be controlled through the control of the periodic function. The present invention will become more apparent from the following description of a not limiting embodiment thereof, suitable for a Doppler VOR system, taken in conjunction with the attached drawings, in which: Fig. 1 illustrates a block diagram of a feeding device in accordance with the present invention, Fig. 2 illustrates a switch/divider which can be used in the switching means of the device of Fig. 1, Fig. 3 shows a time diagram of signals outgoing from and coming into the switching means of the device of Fig. 1, Fig. 4 is a block diagram of a modulator according to the present invention which can be used in the device of Fig. 1, and Fig. 5 illustrates a possible detailed block diagram of the switching means of the device of Fig. 1. Prior to further describing the present invention it is better to clarify what does overmodulation mean. An amplitude modulator has, generally, two inputs and one output: one input is provided with the carrier signal and the other input with the modulating signal, generally at much lower frequency; at the output a modulated signal is emitted; the so-called modulation index depends on the amplitudes of the two input signals: in particular in case of sinusoids, if the amplitude of the carrier is exactly twice the amplitude of the modulating signal, the modulation index is 100% (per cent); if it is more than twice, the index is lower than 100% (per cent); while if it is less than twice, the index is greater than 100% (per cent); in the latter case one speaks of overmodulation but only in the event in which, when the envelope of the modulated signal passes through zero, an inversion in phase of the carrier sinusoid occurs and not its cancellation. For a better and exhaustive treatment of the amplitude modulation one may refer to H. Taub, D.L. Schilling, "Principles of Communication Systems", McGraw-Hill, 1971 and particularly to chap. 3 "Amplitude Modulation Systems". Normally telecommunication apparatuses are not adapted to handle overmodulated signals and conventional amplitude modulators do not output any signal when the modulating signal exceeds a certain limit. The method of the present invention is now described by reference to Fig. 3. Such method allows the antennas of the circular array of a Doppler VOR system to be fed with the D-VOR signal, or more precisely with its sidebands, through the use of filter means that realize a blending function, for switching gradually such signal between the antennas, by modulating the amplitude of the D-VOR signal with a periodic signal with a modulation index greater than 100% (per cent) and providing each antenna with a corresponding time fraction of the modulated D-VOR signal at the output of the filter means. The curve labelled A in Fig. 3 is the envelope of the modulated D-VOR signal at the output of the filter means; as can be seen better if the three consecutive time intervals labelled D1, D2, D3 are considered, the resulting function looks very close to sinX/X, restricted to the three central lobes. The curves labelled E-1, E-2, E-3, E-4 in Fig. 3 are the envelopes of the signals provided respectively to antennas E-1, E-2, E-3, E-4 of a Doppler VOR System, which are physically adjacent in the circular array. The modulated D-VOR signal can be distributed to the various antennas by reserving a different time fraction thereof to each antenna: e.g. intervals D1 and D2 to antenna E-1, intervals D3 and D4 to antenna E-4, and so on. There is also the possibility that such time fractions overlap partially each other: e.g. intervals D1, D2, D3 to antenna E-1, intervals D3, D4, D5 to antenna E-4, thus approximating still better the function sinX/X. There is also the possibility that such time fractions overlap totally each other. In Fig. 3 a sinusoid and a modulation index of 200% have been chosen as periodic signal, very simple to synthesize, but the person skilled in the art will be able each time to make the most appropriate choice. If the partial overlapping is chosen it is to advantage that, during a first positive alternation (D2) of the amplitude modulation, the modulated D-VOR signal is fed to a first antenna (E-1), during a first negative alternation (D3) subsequent the first positive alternation (D2), the modulated D-VOR signal is power-divided and fed simultaneously to the first antenna (E-1) and to a second antenna (E-4) logically adjacent to the first one, and, during the following positive and negative alternations, the modulated D-VOR signal is fed gradually to logically adjacent antennas until all the antennas of the array are covered. The concept of logical adjacency will be explained through an example. In order to better simulate the progressive motion of the emitting point from the position of an antenna, for example E-1, to that of another one, for example E-4, it is possible to place further antennas, for example antennas E-2 and E-3, between antenna E-1 and antenna E-4. The modulated signals to be provided to these further antennas are shown in Fig. 3 and correspond to the signal for antenna E-1 phase shifted respectively of 120 and 240 degrees. Clearly these further modulated signals cannot be obtained from signal A through a simple switching and will need, for example, separate generation. According to the above, antenna E-4 is logically adjacent to antenna E-1 as far as the switching of signal A is concerned, and is physically adjacent to antenna E-3. Having described the method, there follows a description of the apparatuses which allow the realization of such a method. In Fig. 1 a block diagram is shown of a feeding device in accordance with the present invention suitable for Doppler VOR systems with simultaneous emission of the two sidebands. The feeding device receives at its inputs a D-VOR signal, labelled VS, which is composed of a carrier signal CS, that contains the "reference" information and passes directly to the central antenna of the Doppler VOR system, and of two sidebands SB1 and SB2, which together will provide the azimuth information and therefore can be considered as a message signal, and it provides at its outputs N radiofrequency signals for the N antennas E-1, ..., E-N of the circular array of the Doppler VOR system. The two sidebands SB1 and SB2 are respectively fed to the inputs of filter means formed essentially by two identical amplitude modulators MOD-A and MOD-B whose function is to time-filter the two sidebands with a blending function and which generate at their respective outputs two modulated message signals A and B; naturally if the Doppler VOR system were of the type emitting the two sidebands alternately or emitting one sideband only, the duplication of the devices and of the signals would be unnecessary. The modulated message signals A and B are fed at the inputs of switching means SW that switch them between the antennas E-1 ,..., E-N; such switching is gradual thanks to the filtering of the two modulators. A control logic unit, not shown in Fig. 1, is on the other hand necessary for controlling the switching means SW and the modulators MOD-A and MOD-B. In order to implement the method in accordance with the present invention, amplitude modulators MOD-A and MOD-B must be such as to modulate the two sidebands SB1 and SB2 with a periodic signal with modulation index greater than 100% (per cent); such periodic signal is fed to the two modulators in the form of a modulation control signal CNT, typically in the form of a sequence of digital patterns. In case of a single modulated message signal, the switching means SW are such as to subdivide it into time fractions, determined by the control logic unit, and feed in succession each time fraction to an antenna, e.g. the first time fraction to antenna E-1, the second one to antenna E-2 and so on; in case of two signals, on the contrary, the switching means SW are such to subdivide it into time fractions determined by the control logic unit, and feed in succession the time fractions to two different antennas (in case of Doppler VOR systems with two antennas that are located in opposite positions in the circular array), e.g. the first time fraction of the first signal to antenna E-1 and at the same time the first time fraction of the second signal to antenna E-25, the second time fraction of the first signal to antenna E-2 and at the same time the second fraction of the second signal to antenna E-26. More advantageously, both in case of a single modulated message signal and in case of two, the switching means SW are such as to allow particular time fractions to be fed to at least two different antennas, e.g. those logically adjacent, simultaneously according to predetermined power ratios. Moreover one can arrange switching means SW such that the two cases described previously may follow on alternately. Such simultaneous feeding to two different antennas may be obtained, e.g., through the device represented in Fig. 2 and called switch/divider. The switch/divider has an input I1 and two outputs O1 and O2 and three operating positions K1, K2, K3; a first position K1 corresponds to the direct connection of the input I1 with the outpt O1, a second position K3 corresponds to the direct connection of input I1 with output O2, and a third position K2 corresponds to the simultaneous connection of input I1 with the two outputs O1 and O2 and a power division to the two outputs through two elements C1 and C2. Such a structure can be advantageously realized through microstrip architectures and controlled PIN diodes and with suitable transmission lines. Naturally the switch/divider represented in Fig. 2 can be modified according to design requirements, providing a higher number of outputs or of intermediate position characterized by different power division ratios: e.g. in a first intermediate position it could provide 1/4 of the power at the output O1 and 3/4 at the output O2; in a second intermediate position it could provide 1/2 of the power at the output O1 and 1/2 at the output O2; in a third and last intermediate position it could provide 3/4 of the power at the output O1 and 1/4 at the output O2. Prior to describe in more detail how switching means SW can be realized, the structure of a modulator according to the present invention and particularly advantageous for use in the feeding device according to the present invention will be described by reference to Fig. 4. In Fig. 4 it can be seen the structure of amplitude modulator MOD-A which is identical to the one of MOD-B. Modulator MOD-A receives at its input the first sideband SB1 and outputs three modulated message signals A1, A2, A3 identical but 120 degrees out of phase with each other and that correspond to the modulated message signal A of Fig. 1; naturally the modulator MOD-B receives at its input the second sideband SB2 and outputs three modulated message signals B1, B2, B3, corresponding to the modulated message signal B of Fig. 1. The choice of generating three identical message signals is particularly advantageous, as will be seen, as a compromise between wasted power and circuit simplicity, but it is clear, for a person skilled in the art, that a different number of outputs would be possible: even one in the boundary case. The amplitude modulator MOD-A comprises: a power divider PD receiving at the input the first sideband SB1, and having a central output and two side outputs having predetermined power ratios with the input and such that the power of the two side outputs is equal, at least two phase shifters PS1 and PS2 controlled by phase-shift control signals CNT1 and CNT2 corresponding to the sole modulation control signal CNT in Fig. 1, having inputs electrically connected to two side outputs of the power divider PD, a variable attenuator VA controlled by an attenuation control signal CNTA, receiving at its input the central output of the power divider PD, and a Butler matrix BM having three inputs (-1, 0, +1), inputs -1 and +1 being connected to the outputs of the two phase shifters PS1 and PS2, and the input O being connected to the output of the variable attenuator VA. Such a modulator is based upon the known representation of the amplitude modulation through rotating vectors: input O corresponds to the vector of the carrier and inputs -1 and +1 correspond to the two vectors of the modulating signal that by rotating in the opposite direction and adding up to the first one, determine modulation. The rotation of vectors is obtained by varying the phase of the corresponding signals through phase-shifters PS1 and PS2, while the addition is obtained through the Butler matrix BM. The modulation index at the outputs of the Butler matrix BM depends upon reciprocal dimension of the vectors and therefore by the partition ratio of the power divider PD (very easily changeable): in a preferred realization the power divider shares the power into three equal parts and the resulting modulation index is equal to 200% (per cent). The variable attenuator VA may be used for a fine adjustment of the modulation index either during installation of the Doppler VOR system or during operation in consequence of monitoring information. In addition the variation of its attenuation, if quick enough, may influence the shape of the blending function; this possibility is useful if slightly different blending functions would be used for different antennas and/or a not perfectly simmetrical blending function would be necessary. The presence of the variable attenuator VA is not strictly necessary but is advantageous. In view of the generation of three identical, out-of-phase signals A1, A2, A3 and the use of a Butler matrix, the wasted power in the modulator is extremely limited as the sum of the powers of the signals is fairly shared between the three outputs A1, A2, A3 without being reflected or sent to internal loads. The index of the Butler matrix and consequently the number of outputs is chosen according to the preservation level of the spectral characteristics of the frequency modulation and to the cancellation level of the spurious amplitude modulations due to discretization of the antennas; such index is advantageously chosen equal to the number of the antennas divided by the frequency modulation index. Moreover, such a modulator is very versatile as far as the blending function to be applied is concerned; in fact, since the controlled phase shifters PS1 and PS2 are often realized through microstrip architectures and PIN diodes controlled by numeric signals, its shape will depend on the sequence of numbers fed to the phase shifters, element very easy to be changed, even in real time. From the above it is apparent that the feeding device parameters can be changed easily, and their determination can be done either analytically or tentatively or both. Referring now to Figs. 4 and 5 the structure of the switching means SW is described on the assumption that two modulators are used of the type described above and shown in Fig. 4. The switching means SW are organized on three hierarchic levels: the first level is formed by switches/dividers CD (Fig. 4) designed to carry out the switching of modulated message signals A1, A2, A3, B1, B2, B3 between logically adjacent antennas; the second level is formed by reversing switches DPDT (Fig. 5) designed to feed alternately the modulated message signals to the same antennas; the third level is formed by multiple switches SPFT (Fig. 5) to carry out the switching of each of the modulated message signals between all the antennas of the array. All the three types of switching devices (the switches/dividers CD, the reversing switches DPDT, the multiple switches SPFT) require control logic signals for their operation. The six signals A1, A2, A3, B1, B2, B3, at the outputs of the two Butler matrixes BM, are fed respectively to the inputs of six switches/dividers CD each generating respectively two signals A11, A12, A21, A22, A31, A32, B11, B12, B21, B22, B31, B32. Such latter signals are fed in pairs (A11 and B11, A12 and B12, etc.) respectively to the inputs P1 and P2 of six reversing switches (Double Pole Double Through) DPDT. The reversing switches DPDT have two outputs T1 and T2 and two operating conditions: input P1 connected to output T1 and, at the same time, input P2 connected to output T2, or input P1 connected to output T2 and, at the same time, input P2 connected to output T1. Each output of reversing switches DPDT is fed to a different multiple switch (Single Pole Fourth Through) SPFT and therefore the multiple switches are twelve. Multiple switches SPFT have one input and four outputs arid obviously four operating conditions corresponding to the possible connections of the input with the outputs. Finally, the outputs of the multiple switches are fed to the antennas, which in this instance are forty-eight; an example of the shape of the envelopes of signals fed to antennas E-1, E-2, E-3, E-4 (physically adjacent) is shown in Fig. 3. By feeding suitable control logic signals to all these devices, it is possible to obtain the desired rotation effect.

Claims: A feeding device for a doppler VOR system having an antenna array (E-1,..., E-N), including: switching means (SW) for switching a modulated message signal between antennas of said antenna array; filter means for performing a blending function in order that said switching of the modulated message signal between the antennas (E-1,..., E-N) of the antenna array is gradual, said filter means including at least one amplitude modulator (MOD-A, MOD-B) capable of modulating a message signal with a periodic signal having a modulation index greater than 100% (per cent) to generate the modulated message signal; and a control logic unit for controlling said switching means (SW) and said filter means, characterized in that said amplitude modulator (MA, MB) comprises: a power divider (PD) having an input for receiving the message signal (SB1, SB2) and having a central output and two side outputs with predetermined power ratios with said input such that the power at the central output and at the two side outputs are equal; at least two phase shifter (PS1, PS2) controlled by phase-shift control signals, having inputs electrically connected to said two side outputs of said power divider (PD); and a Butler matrix (BM) having three inputs, an input -1, an input + 1 and an input 0, with inputs - 1 and + 1 each connected to an output of one of the at least two phase shifters, and the input 0 connected to said central output of said power divider (PD), having outputs (A1, A2, A3; B1, B2, B3) corresponding to said modulated message signal, and having a Butler matrix index equal to the number of antennas divided by the frequency modulation index. Feeding device according to claim 1, characterized in that said control logic unit generates time fraction signals and that said switching means (SW), in response to said time fraction signals, switches the modulated message signal to at least two antennas at the same time according to predetermined power ratios. Feeding device according to claim 2, wherein said time fractions corresponding to different antennas are overlapped either partially or totally. Feeding device according to claim 1 further comprising means for receiving two incoming signals, corresponding to two sidebands (SB1, SB2) of said message signal characterized in that said filter means comprises two modulators (MOD-A,MOD-B) for modulating the two incoming signals, for outputting two modulated message signals (A, B) and in that said switching means receives said two modulated message signals (A, B) and outputs time fractions of said two modulated message signals respectively and successively to at least two different antennas of said antenna array. Feeding device according to claim 1, characterized in that a first hierarchic level of said switching means is comprised of switches/dividers (CD) designed to carry out the switching of said two modulated message signals (A1, A2,A3; B1, B2, B3) between logically adjacent antennas, each of said switches/dividers (CD) having an input (I1) two outputs (O1, O2) three operating positions (K1, K2, K3), a first position (K1) corresponding to the direct connection of said input (I1) with a first (O1) of said two outputs, a second position (K3) corresponding to the direct connection of said input (11) with a second (O2) of said two outputs, and a third position (K2) corresponding to the simultaneous connection of said input (I1) with said two outputs (O1, O2), and two elements (C1, C2) for dividing the power to said two outputs. Feeding device according to claim 5, characterized in that said switches/dividers (CD) are realized through a microstrip architecture and controlled PIN diodes, and with suitable transmission lines. Feeding device according to claim 1 characterized in that said amplitude modulator further comprises an attenuator (VA) controlled by an attenuation control signal (CNTA), and further responsive to the central output of said power divider (PD), for providing an output signal to said input 0 of said Butler matrix (BM). Feeding device according to claim 1, characterized in that said Butler matrix (BM) outputs three identical modulated message signals (A1, A2, A3; B1, B2, B3), 120° degrees out of phase each other.

Inventors: BERTOCCHI GIUSEPPE, BERTOCCHI, GIUSEPPE
Applicants: AIRSYS NAVIGATION SYSTEMS S P, AIRSYS NAVIGATION SYSTEMS S.P.A.
Application year: 1995
Main classification: H01Q21
IPC codes: G01S1, H01Q3, G01S7, H01Q21