IE67876B1 - A System for the distribution of very high frequency signals - Google Patents

Abstract

The invention relates to a distribution system for very high frequency signals. This system comprises the use of a dual frequency conversion for the purposes of making it possible in particular to eliminate parasitic signals. It applies most particularly to the distribution of audio and video signals coming from satellites.

Description

The subject of the invention is a system for distributing very high frequency signals, and in particular audio-visual signals originating from satellites.

In a general way, the signals sent out by satel5 lites at very high frequency, for example between 10 and 15 GHz, are picked up by an antenna, for example a parabolic antenna, converted into signals of lower frequency (of the order of 1 to 2 GHz) then led, usually by cable, to a distribution system, in the case of communal distribution (see Nachrichtentechnische Zeitschrift - Vol. 40, 1987, pp. 646-52).

The distribution system essentially comprises a group of receiver-modulator subassemblies corresponding to the various television channels, the various radio frequency signals sent out by the modulators in accordance with the standards in force (for example AM audio and video, or AM video/FM audio) being sent to the U.H.F. or V.H.F. television receivers by means of a coaxial cable, via a coupler.

Each subassembly is most often locked in the factory on to the frequency to be received. The active elements making it possible to set the frequency of each subassembly may be frequency synthesizers which receive commands from a central unit, commands converted into datum values in order to set the local oscillators. These synthesizers may themselves consist of a phase-locked loop (or P.L.L.).

According to a technique usually used, the audio and video signals received by the modulator modulate an intermediate frequency, of the order of 30 MHz, the intermediate-frequency signals are filtered for the purposes of reducing the bandwidth for the signals sent in amplitude modulation, transposed (frequency multiplication) , then amplified and finally filtered at the frequency of the chosen channel, that is to say between 470 and 860 MHz with a bandwidth not exceeding 8 MHz. Put another way, in these systems, a frequency multiplication is performed from the intermediate frequency to the value of the frequency of the chosen channel . One of the major problems of these systems lies in the filtering of the signals resulting from this frequency multiplication exactly at the value chosen, for example 500 MHz, both in that the signals at 500 MHz are accompanied by parasitic signals with a slight frequency difference, for example between 470 and 530 MHz, and in that it is necessary to set up one filter per channel (or frequency) .

The invention therefore proposes a system for distributing signals sent out at very high frequency, and especially audio and video signals originating from satellites, this system making it possible both to obtain the desired frequency (F) exactly without parasitic signals with a slight frequency difference, and to obtain a plurality of frequencies without having recourse to one filter per channel or frequency.

This system for distributing signals sent out at very high frequency, which comprises a receiving antenna, a receiver, a modulator and means configured to provide the link between the antenna and the receiver and to link the output of the system to an output device (13), the signals received by the modulator being delivered at intermediate frequency (IF), the modulated signals thus received being transposed, amplified, filtered, on the chosen frequency F and sent to the output device, is characterized in that the modulator comprises: - a first local oscillator (6), - a first frequency converter (5) receiving the signal supplied by the said first oscillator and intended to transpose the modulated signals received at inter30 mediate frequency IF to a fixed frequency Fx lying above the frequency band F of the output channel of the system, the IF frequency being lower than the fixed frequency Fx; - a first filter (7) for receiving the output signal from the first converter and eliminating the frequency of the first local oscillator (6) and the frequencies lower than the fixed frequency Fx, which are contained in the said output signal of the first converter; - a second frequency converter (8) receiving the output of the first filter (7) at the frequency Fx, associated with a second local oscillator (9) controlled by a synthesizer (10) in order to transpose it by a frequency F2, F2 being the frequency of the output signal of the said second local oscillator, equal to Fx + F; - a second filter (11) receiving the output signal of the second converter in order to transmit only the frequency F sent to the output device (13).

For simplicity, the set of devices lying between the modulator (included) and the final filter will be called modulator.

Unlike the previously described system, the first conversion to a frequency higher than the desired frequency for the output signals makes it possible to widen the frequency bands of the signals to the maximum, with a view to facilitating the filtering thereof. In fact, as has been stated, transposition from 30 MHz to 500 MHz leads to the presence of signals at 470 and of signals at 530 MHz (in addition to signals at 500 MHz) , and the filtering of the correct frequency is a delicate matter. Moreover, it is appropriate to make use of a filter for each desired frequency. The use of a frequency higher than the desired frequency, preferably by at least 100 MHz, for example of a frequency of 1 GHz for the same desired frequency of 500 MHz, leads to signals at 500, 1500 and 2500 MHz, the separation of which by filtering is considerably easier.

Moreover, after the filtering eliminating the frequencies lower than the fixed frequency (Fx) chosen (1 GHz in this example), the use of a synthesizer controlled by a central unit, which can be linked and controlled by a single keyboard, makes it possible to select the output frequency directly, the frequency Fx itself being eliminated by the bandpass filter.

Put another way, this system makes it possible not only to arrive easily at the desired frequency, for example by simple control of the 2nd converterby means of a digital keyboard, but also to be able to control the desired frequency at will without being obliged to interpose the filters corresponding to each of the said desired frequencies into the circuit. That being so, and unlike traditional systems, the setting-up of the distribution system, that is to say the choice of the various output frequencies (corresponding to the various television channels for example) may be made no longer by the manufacturer, that is to say in the factory, but directly on the operating site.

In the system described above, use is made of synthesizers, filters, amplifiers, receivers, modulators, frequency converters, local oscillators which are known individually and which do not separately constitute subjects of the invention.

The invention lies in fact in this combination of devices and more precisely in the use of converters in the previously cited conditions.

Purely by way of illustration, the plate shows an installation diagram for these various elements.

On this plate, the symbols and markings have the following meanings: 1. Video-frequency access 2. Audio-frequency access 3. Intermediate frequency modulator 4. 1st intermediate frequency filter . 1st frequency converter 6. Oscillator associated with the 1st frequency converter 7. 2nd intermediate frequency filter 8. 2nd frequency converter 9. Oscillator associated with the 2nd frequency converter . Frequency synthesizer 11. Output filter 12. Amplifier 13. Final receiver (for example television set) 14. Central unit . Programming keyboard The information given previously relates to the modulator (in the sense given above). This means that, on either side of this modulator, the distribution system may comprise conventional hardware. In other words, upstream of the modulator, a conventional receiving antenna may be used, in particular a parabolic antenna, a converter which is itself conventional as well as a conventional receiver. Likewise, beyond the modulator, the signals may be sent to a traditional coupler before being led to the final receiver, jointly with signals picked up by conventional antennae as appropriate.

It is possible however, and this constitutes a variant of the distribution system in accordance with the invention, to associate with the modulator a receiver which is also agile, that is to say comprising frequency synthesizers consisting of a phase-locked loop (P.L.L.) and receiving their commands from a central unit. In such a system, the signals originating from the (parabolic) antenna at very high frequency (for example from 10 to 15 GHz) pass into a frequency converter, an amplifier, then are transposed to intermediate frequency set by the synthesizer controlled by the central unit to the desired frequency, then are filtered and demodulated so as to extract the video signals and audio signals. As has been said, the modulator in accordance with the invention offers a means of generating signals at a desired frequency (output frequency) simultaneously in a way which is very effective as far as the elimination of parasitic signals is concerned, and simple, as much as a result of dispensing with the filters corresponding to each of the output frequencies as by the fact of the possibility offered for setting the said frequencies directly on the operating site, whereas traditional systems involve pre-setting in the factory, frequency by frequency, and, in the event of breakdown, require preset modulators to be held, corresponding to the faulty modulator.

Put another way, in a system for distributing associated described, model of by simple several tens of channels, the novel modulator makes it possible to replace any-of the faulty modulators, without it being necessary to keep as many spare modulators as there are modulators in service.

In the event that the agile modulator is with an agile receiver, as previously it is noted that the same receiver/modulator pair makes it possible, control for example by means of a keyboard, to distribute all the frequencies corresponding to the tens of channels mentioned above.

The modulator (and the receiver) have been described above more specifically in the application to television signals sent out by satellite. It goes without saying that the system itself can receive and distribute other types of signals, such as for example digital information, and the invention obviously extends to all other applications of this system.

Claims (5)

1. System for distributing signals sent out at veryhigh frequency, comprising a receiving antenna, a receiver, a modulator and means configured to provide the 5 link between the antenna and the receiver and to link the output of the system to an output device (13), the signals received by the modulator being delivered at intermediate frequency (IF), the modulated signals thus received being transposed, amplified, filtered, on the 10 chosen frequency F and sent to the output device, characterized in that the modulator comprises: - a first local oscillator (6), - a first frequency converter (5) receiving the signal supplied by the said first oscillator and intended 15 to transpose the modulated signals received at intermediate frequency IF to a fixed frequency F x lying above the frequency band F of the output channel of the system, the IF frequency being lower than the fixed frequency F x ; - a first filter (7) for receiving the output 20 signal from the first converter and eliminating the frequency of the first local oscillator (6) and the frequencies lower than the fixed frequency F x , which are contained in the said output signal of the first converter; 25 - a second frequency converter (8) receiving the output of the first filter (7) at the frequency F x , associated with a second local oscillator (9) controlled by a synthesizer (10) in order to transpose it by a frequency F 2 , F 2 being the frequency of the output signal 30 of the said second local oscillator, equal to F x + F; - a second filter (11) receiving the output signal of the second converter in order to transmit only the frequency F sent to the output device (13).

2. System according to Claim 1, characterized in 35 that the fixed frequency F x is higher than the maximum desired output frequency by at least 100 MHz.

3. System according to either of Claims Ί and 2, characterized in that the receiver comprises frequency synthesizers consisting of a phase-locked loop (P.L.L.), which are controlled by a central unit.

4. System according to any one of Claims 1 to 3, characterized in that the output frequency of the modulator and possibly of the receiver is controlled by a 5. Numerical keyboard.

5. Use of a system according to any one of Claims 1 to 4 for distributing audio and video signals originating from a satellite.