Abstract:
An embodiment of regulation and shaping circuit includes a first input terminal for receiving a first input signal with a first frequency; a second input terminal for receiving a second input signal with a second frequency higher than the first frequency; a first circuital branch coupled to the first input terminal and, through first coupling means active at the first frequency, to an output terminal for providing an output signal; a second circuital branch coupled to the second input terminal and to the output terminal, wherein said second circuital branch comprises a negative feedback circuital loop adapted to control the output signal according to the second input signal.

Description:
PRIORITY CLAIM 
       [0001]    The present application is a national phase application filed pursuant to 35 USC §371 of International Patent Application Serial No. PCT/EP2007/062246, filed Nov. 13, 2007; which further claims the benefit of Italian Patent Application MI2006A002294, filed Nov. 29, 2006; all of the foregoing applications are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    An embodiment of the present invention relates to the field of circuits for the generation of signals to be used in pulse-width modulation communication protocols, particularly of the type used in satellite communication systems and in particular in apparatus for the reception of signals from artificial satellites. 
       BACKGROUND 
       [0003]    Satellite communication systems have several applications. For example they are used in the field of television, mobile telephony, and terrestrial and sea navigation. In such systems, the exchange of information is based on the propagation of a signal via satellite connections, which cover long distances without the use of particular infrastructures along the covered distances. 
         [0004]    For example, satellite television is a system consisting of an artificial satellite able to receive a television signal from one or more transmitter stations and directly retransmit it to different users. Each user is provided with one or more receiving antennas (such as parabolic antennas) which are directed towards one or more satellites, which antennas receive the television signal, perform a first conversation/selection of the high-frequency satellite signal and send it to a decoder (Set-Top Box) to which they are connected by a coaxial cable. The decoder is able to process the received television signal so that it can be exploited by the user through normal television apparatuses. The decoder forms what is referred to as the Internal Decoding Unit (“IDU”), whereas the receiving antennas form the Out Decoding Unit (“ODU”). 
         [0005]    In digital satellite television, the television signal processed by the decoder is in particular a digital signal. 
         [0006]    In some application, the decoder interacts with each receiving antenna using a communication protocol which requires the superimposing of a low frequency signal (for example, at 22 KHz) with a pulse width keying on a DC signal forming the supply voltage, which is provided to each receiving antenna by the decoder itself. 
         [0007]    In order to guarantee the correct exchange of information between each receiving unit and the decoder, it is necessary that the low frequency signal complies with particular requirements. For example, in the case in which the low frequency signal is a square wave, it is required that the amplitude thereof, the rising and falling time, the duty-cycle thereof (defined as the ratio between the time in which the signal is at the high voltage level and the period of the signal) take predetermined values. 
         [0008]    For this purpose, the decoders have regulation and shaping circuits adapted to provide the low frequency signal having the desired features. 
         [0009]    Examples of regulation and shaping circuits of the low frequency signal are known in the art. 
         [0010]    In  FIG. 1  an example of a conventional regulation and shaping circuit  100  of low frequency signal of the above-mentioned type is shown. 
         [0011]    The shaping and regulation circuit  100  receives in input an input voltage signal Vin and provides in output an output voltage signal Vout adapted for being used as a low frequency signal. 
         [0012]    For this purpose, the shaping and regulation circuit  100  comprises an input terminal  105  which receives the input voltage signal Vin, and an output terminal  110  which sends to a coaxial cable  115  to which it is connected the output voltage signal Vout. In the example at issue, the input voltage signal Vin consists of a square wave oscillating between a first predetermined voltage Vin′* and a second predetermined voltage Vin″*, with a frequency f 1  (for example, 22 kHz). Typically, the first predetermined voltage Vin′* and the second predetermined voltage Vin″* take positive values so that also the output voltage signal takes positive values (indeed, the communication protocol between the decoder and each receiving antenna provides for the use of voltage signals which take values higher than zero). 
         [0013]    In detail, the regulation and shaping circuit  100  includes a differential amplifier  120  having a relatively high gain (for example, ranging between 60 dB and 80 dB). The differential amplifier  120  has an inverting input terminal (denoted in  FIG. 1  with the symbol “−”) and a non-inverting input terminal (denoted in  FIG. 1  with the symbol “+”). The differential amplifier  120  receives as power supply a reference voltage, for example ground, and a power supply voltage Vdd (for example, 20V). Typically, the supply voltage Vdd is higher than the maximum voltage value which can be reached by the output voltage signal Vout. The non-inverting terminal of the differential amplifier  120  is connected to the input terminal  105 , whereas the inverting terminal is connected to a first terminal of a resistor R 1 , which has a second terminal kept to ground. The resistor R 1  has the first terminal connected to a first terminal of a resistor R 2 , which has a second terminal connected to the output terminal  110  of the regulation and shaping circuit  100 . An npn bipolar transistor T 1  is connected between an output terminal  125  of the differential amplifier  120  and the output terminal  110  of the regulation and shaping circuit  100 . In particular, the transistor T 1  has an emitter terminal connected to the output terminal  110 , a base terminal connected to the output terminal  125  of the differential amplifier  120  and a collector terminal, which receives the supply voltage Vdd. 
         [0014]    The regulation and shaping circuit  100  provides for the use of a negative feedback control loop (comprising the differential amplifier  120 , the transistor T 1 , the resistor R 1  and the resistor R 2 ) for controlling and setting the output voltage signal Vout. In particular, the differential amplifier  120  is connected to the network formed by the resistors R 1  and R 2  and by the transistor T 1  according to a conventional non-inverting configuration. 
         [0015]    In response to the input voltage signal Vin (applied to the input terminal  105 ) the regulation and shaping circuit  100  provides at the output terminal  110  the output voltage signal Vout having a square shape wave, whose amplitude is a function of the resistance of the resistors R 1  and R 2 . In particular, the output voltage signal Vout has an average value substantially equal to the supply voltage to be provided by the decoder to each receiving antenna and it is used as a low frequency signal (with a dynamic range of the order of some thousands of mV) during the exchange of information between the decoder and each receiving antenna. 
         [0016]    A drawback of such solution is that for the output voltage signal Vout not to be distorted (thereby keeping the amplitude needed for the correct transmission of the signal), it may be necessary to provide an output circuit (for example, of push-pull type) connected to the output terminal  110 . This causes an increase of the area occupied by the regulation and shaping circuit  100  in a semiconductor chip in which it is integrated. On the other hand, to guarantee the integrity of the wave shape of the output voltage signal Vout it may be needed to increase the voltage drop at the transistor T 1 , thus increasing the circuit consumption. 
         [0017]    Moreover, the frequency of the output voltage signal Vout depends on the pass band of the regulation and shaping circuit (with negative feedback)  100 . In particular, the maximum obtainable frequency of the output voltage signal Vout is limited by the pass band of the regulation and shaping circuit  100 . This can cause an undesired shift of the frequency of the output voltage signal Vout from the provided one. 
         [0018]      FIG. 2  shows another conventional regulation and shaping circuit  200 . The regulation and shaping circuit  200  receives in input a first reference voltage Vref and an input voltage signal Vin 1 , and provides in output an output voltage signal Vout 1 , consisting of a low frequency signal, superimposed on a DC voltage. 
         [0019]    For this purpose, the regulation and shaping circuit  200  includes a first input terminal  205 , through which it receives the first input reference voltage Vref, a second input terminal  210  through which it receives the input voltage signal Vin 1 , and an output terminal  215 . In the example at issue, the first reference voltage Vref takes a constant value for example, 1.25V; the input voltage signal Vin 1  consists of a square wave ranging from a first voltage value V 1  (for example, the ground) to a second voltage value V 2  (for example, 1V) with the frequency f 1  (for example, 22 kHz). 
         [0020]    The regulation and shaping circuit  200  includes a regulation circuital structure  100 ′ similar to that of the regulation and shaping circuit  100  (for this reason similar elements are denoted with the same reference numerals with the addition of an apex), which is connected to the input terminal  205 . In this case, the emitter terminal of the transistor T 1 ′ is coupled to the output terminal  215  by means of an impedance Z 1 . In particular, the impedance Z 1  has a first terminal connected to the emitter terminal of the transistor T 1 ′ and a second terminal coupled to the coaxial cable  115 ; moreover, the impedance Z 1  consists of an inductor L 1  which is connected in parallel to a resistor R 3 . 
         [0021]    An n-channel MOSFET T 2  has a drain terminal, which is connected to the output terminal  215 , a control terminal, which is connected to the second input terminal  210 , and a source terminal, which is connected to a first terminal of a resistor R 4 . The resistor R 4  has a second terminal kept to ground. 
         [0022]    During the operation of the regulation and shaping circuit  200 , the output voltage signal Vout 1  is obtained by superimposing on a DC output voltage signal due uniquely to the first reference voltage Vref (when the second input terminal  210  is kept to ground), a low frequency voltage signal due uniquely to the input voltage signal Vin 1  (that is the signal which it should be observed at the output terminal  215  when the first input terminal  205  is at ground). 
         [0023]    When the regulation and shaping circuit  200  receives only the first reference voltage Vref and the input voltage signal Vin 1  is kept to ground, the transistor T 2  is turned off so that no current flows in the circuital branch formed by the transistor T 2  and by the resistor R 4 . The output terminal  215  reaches a constant value which, similarly to the case of  FIG. 1 , depends on the value of the first reference voltage Vref and on the ratio between the resistances of the two resistors R 1 ′ and R 2 ′ (for very low frequencies, the inductor L 1  has a very low resistance, ideally it is a short-circuit, so that the voltage reached by the emitter terminal of the transistor T 1 ′ is directly transmitted to the output terminal  215 ). 
         [0024]    When the regulation and shaping circuit  200  receives only the input voltage signal Vin 1  and the first reference voltage Vref is kept to ground, the regulation circuit  100 ′ is inhibited, whereas the circuital branch formed by the transistor T 2  and by the resistor R 4  is conductive. In particular, in response to the input voltage signal Vin 1 , the output terminal  215  provides an output voltage signal having a square wave shape with a frequency equal to the frequency of the input voltage signal Vin 1 . 
         [0025]    In particular, the input voltage signal Vin 1  generates in output a signal having a small amplitude with respect to the DC value. Therefore, the output voltage signal Vout 1  is formed by a DC voltage value—for example, ranging from 13V to 18V—effect of the first reference voltage Vref− to which a small signal of 22 KHz is superimposed having a peak-to-peak amplitude significantly lower than the DC voltage value (for example, approximately 700 mV). 
         [0026]    In other words, the output voltage signal Vout 1  obtained by the superposition of the effects of the first reference voltage Vref and the input voltage signal Vin 1 , consists of a square wave having a frequency depending on the frequency of the input voltage signal Vin 1  and average value function of the first reference voltage Vref. In particular, such average value corresponds to the DC supply voltage to be provided to each receiving antenna, whereas the output voltage signal due to the input voltage signal Vin 1  is adapted to be used as a low frequency signal for communicating with the receiving antennas. 
         [0027]    It should be noted that the use of the transistor T 2  implies a lower control of the wave shape of the output voltage signal Vout 1 . Indeed, the lack of a negative feedback may prevent stabilizing the output voltage signal Vout 1  and thus the low frequency signal. 
         [0028]    Moreover, the amplitude of the output voltage signal Vout 1  depends not only on the values of the resistance of the resistors R 1 ′ and R 2 ′, but also on the impedance Z 1  and on the impedance of the coaxial cable  115 ′, so that it may not be possible to obtain an output voltage signal Vout 1  having an amplitude equal to the desired one. 
       SUMMARY 
       [0029]    An embodiment of the present invention provides a solution, which is based on the idea of controlling the output voltage signal by a negative feedback and a de-coupling element. 
         [0030]    In particular, an embodiment of the present invention proposes a regulation and shaping circuit comprising: a first input terminal for receiving a first input signal with a first frequency; a second input terminal for receiving a second input signal with a second frequency higher than the first frequency; a first circuital branch coupled to the first input terminal and, through first coupling means active at the first frequency, with an output terminal, for providing an output signal; a second circuital branch coupled to the second input terminal and to the output terminal, wherein said second circuital branch comprises a negative feedback circuital loop adapted to control the output signal according to the second input signal. 
     
    
     
         [0031]    These and other features and advantages of one or more embodiments of the present invention will be made apparent by the following detailed description of an embodiment thereof, provided merely by way of non-limitative example, description that will be conducted making reference to the attached drawings. In particular: 
           [0032]      FIG. 1  shows a circuit schematic of a known regulation and shaping circuit. 
           [0033]      FIG. 2  shows a circuit schematic of a further known regulation and shaping circuit. 
           [0034]      FIG. 3  shows a schematic block diagram of a satellite communication system wherein the solution according to an embodiment of the present invention is usable; and 
           [0035]      FIG. 4  shows a circuit schematic of a regulation and shaping circuit according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    With reference to  FIG. 3 , a communication satellite system  300  for television applications is shown. An artificial satellite  310  is placed in orbit around the earth. For example, the orbit can be of geostationary type, in such a way that, for an observer on the earth, the artificial satellite  310  seems motionless in the sky. The artificial satellite  310  receives a television signal by a transmitter station (not shown in figure) and retransmits it to receiving antennas  315  (in the example at issue there are shown eight) placed on the earth. For this purpose, the artificial satellite  310  is provided with systems which serve the operation thereof, like one or more transponder units or electronic processors able to support the processing of received signals directly on-board of the satellite. 
         [0037]    Each receiving antenna  315  includes a parabolic mirror  320 , and an illuminator  325 . A circuit of first conversion/selection of the satellite signal, known as “Low Noise Block” or LNB, is associated with the illuminator. The parabolic mirror has a paraboloid shape (that is of a rotation solid obtained from the rotation of a parabola around an axis) and it is turned to the artificial satellite  310 , from which it receives the satellite signal with significantly high frequencies (for example, from 10.7 GHz to 12.7 GHz). The received signal is then reflected to the illuminator  325 , which is placed at the focus of the parabolic mirror  320 , in such a way to receive all the power of the carrier signal received by the parabolic mirror  320 . In such a way, the power of the received signal is sufficiently high for being adequately sent to and processed by a decoder  330  to which each LNB is connected by a coaxial cable  335 . 
         [0038]    In particular, the decoder  330 , for exchanging information with the LNBs placed on the receiving antennas  315 , uses a type of protocol thanks to which it can control several receiving antennas  315  connected to the same coaxial cable  335 . For example, the communication between the receiving antennas  315  and the decoder  330  can be based on the DISEqC (Digital Satellite Equipment Control) protocol, which allows managing up to eight input signals on the single coaxial cable  335 . 
         [0039]    The decoder  330  has, among the main functions, that of processing the received signal, for making it usable by the user through a television apparatus. For this purpose, the decoder  330  includes all the circuits needed for performing the demodulation and the decoding of the received signal, which has been previously coded by the emitter at the transmission time. For this purpose, a tuner  340  is provided, which allows selecting among the received signals the ones to be sent to a television set  345 . 
         [0040]    For controlling the LNBs on the antennas, the decoder  330  includes an LNB controller  350 , which sends the control signals to the LNBs  325  and moreover provides thereto the supply voltage. A micro-controller  355  is able to control the operations performed by the LNB controller  350 . 
         [0041]    In order to implement the communication protocol for the correct exchange of information between the decoder  330  and each receiving antenna  315 , a regulation and shaping circuit  360  is interposed between the LNB controller  350  and the coaxial cable  335 . In this way, a signal from the LNB controller  350  is converted into a low frequency signal, typically 22 KHz, adapted to be used as a low frequency signal to be superimposed onto the supply voltage for the exchange of information between the decoder  330  and the LNB on each receiving antenna  315 . 
         [0042]    With reference to  FIG. 4 , the circuit schematic of the regulation and shaping circuit  360  according to an embodiment of the present invention is shown. 
         [0043]    The regulation and shaping circuit  360  receives in input the first reference voltage Vref and the input voltage signal Vin 1  and provides in output the output voltage signal Vout 1  adapted to be used as supply voltage for the LNBs and as a low frequency signal during the exchange of information between the decoder and each receiving antenna. 
         [0044]    For this purpose, the regulation and shaping circuit  360  comprises a first input terminal  405  through which it receives the first reference voltage Vref, a second input terminal  410 , through which it receives the input voltage signal Vin 1 , and the output terminal  415 . 
         [0045]    In the example herein considered, the first reference voltage Vref takes a constant value. Alternatively, the terminal  405  can receive an input voltage signal having a very low frequency. The input voltage signal Vin 1  includes a square wave, which is modulated by variable width pulses. 
         [0046]    The regulation and shaping circuit  360  includes a first and second circuital branches  420  and  425  placed respectively between the first input terminal  405  and the output terminal  415 , and between the second input terminal  410  and the output terminal  415 . 
         [0047]    The first circuital branch  420  has a structure similar to the regulation and shaping circuit  100  shown in  FIG. 1  (for this reason, similar elements are denoted with the same reference numerals, adding two apexes). The first circuital branch  420  is connected to the output terminal  415  by an impedance Z 2 , which comprises an inductor L 2  connected in parallel to a resistor R 5 . 
         [0048]    The second circuital branch  425  includes a differential amplifier  430  having a relatively high gain (for example, ranging from 60 dB to 80 dB). The differential amplifier  430  has an inverting input terminal (denoted in  FIG. 4  with the symbol “−”) and a non-inverting input terminal (denoted in  FIG. 4  with the symbol “+”). The differential amplifier  430  receives as a supply the ground and the supply voltage Vdd (for example, ranging from 12V to 18V). The non-inverting terminal of the differential amplifier  430  is connected to the second input terminal  410 , whereas the inverting terminal is connected to a first terminal of a capacitor C 1 , which has a second terminal, connected to the output terminal  415 . An output circuit or stage  435  is placed between an output terminal of the differential amplifier  430  and the first terminal of the capacitor C 1 . In the example at issue, the output circuit  435  comprises an n-channel MOS transistor N 3  and a p-channel MOS transistor P 3 . The transistors N 3  and P 3  are connected in a push-pull configuration and have the corresponding control terminals connected to the output terminal of the differential amplifier  430 , and the source terminals connected to the inverting terminal of the differential amplifier  430 ; moreover, the transistor N 3  has a drain terminal which receives the supply voltage Vdd, whereas the transistor P 3  has a drain terminal which is kept to ground. Alternatively, different types of transistors can be used, for example the output stage  435  can comprise bipolar transistors. Moreover, the output stage  435  may have a different structure, for example it may have an emitter-follower configuration. 
         [0049]    The regulation and shaping circuit  360  provides for using a negative feedback control loop, comprising the differential amplifier  430  and the output circuit  435 , in order to control and stabilize the output voltage signal Vout 1 . In particular, the differential amplifier  430  and the output circuit  435  are connected to each other according in a typical non-inverting configuration having a unit gain. 
         [0050]    During the operation of the regulation and shaping circuit  360 , the output voltage signal Vout 1  is obtained by the superposition of a DC output voltage due uniquely to the first reference voltage Vref (visible by keeping to ground the second output terminal  410 ) with an output voltage signal due uniquely to the input voltage signal Vin 1  (visible by keeping to ground the first input terminal  405 ). 
         [0051]    When the regulation and shaping circuit  360  receives only the reference voltage Vref and the input voltage signal Vin 1  is kept to ground, the circuital branch  425  is disabled, so that no current flows therethrough. In DC regime, the capacitor C 1  is an open circuit, so that possible fluctuations of the voltage reached by the source terminal of the transistors N 3  and P 3  do not interfere in any way on the voltage reached by the output terminal  415  of the circuit. 
         [0052]    Similarly to the case of  FIG. 1  and  FIG. 2 , the emitter terminal of the transistor T 1 ″ reaches a voltage depending on the value of the first reference voltage Vref and on the value of the resistances of the resistors R 1 ″ and R 2 ″. Such voltage value is reached also by the output terminal  415 , since the impedance Z 2 , for frequencies close to zero, forms a low impedance circuital path (ideally, a short circuit). 
         [0053]    When only the input voltage signal Vin 1  is applied to the regulation and shaping circuit  360 , whereas the reference voltage Vref is kept to ground, no current flows through the circuital branch comprising the transistor T 1 ″ and the resistors R 2 ″ and R 1 ″, and the emitter terminal of the transistor T 1 ″ reaches the ground. However, such value does not interfere in any way with the value reached by the output terminal  415 , since at the frequencies of the input signal Vin 1  the impedance Z 2  has a very high resistive component (equal to the resistance of the resistor R 5 ). For this purpose, the resistor R 5  is chosen in such a way to have a significantly high resistance (ideally infinite, so that the impedance Z 2  forms an open circuit). 
         [0054]    The circuital branch  425  transfers the input voltage signal Vin 1  to the non-inverting terminal of the differential amplifier  430  by means of the negative feedback loop comprising the differential amplifier  430  and the output circuit  435 . Moreover, the input voltage signal Vin 1  is directly transferred also to the output terminal  415  at the frequency of the input signal Vin 1 , since the capacitor C 1  has a low impedance (ideally a short-circuit). 
         [0055]    The output voltage signal Vout 1  obtained by superposing the effects due to the first reference voltage Vref and to the input voltage signal Vin 1 , consists of a square wave with a frequency equal to the frequency of the input voltage signal Vin 1  with an average value depending on the reference voltage Vref. 
         [0056]    However, it is possible to control the frequency of the output voltage signal Vout 1  provided by the regulation and shaping circuit  360  to the LNB by means of the input voltage signal Vin 1 . Thus, the circuit according to an embodiment of the present invention allows varying (for example, increasing) the frequency of the output voltage signal Vout 1  by acting on the input voltage signal Vin 1  exclusively. 
         [0057]    In particular, the use of the negative feedback loop allows controlling the frequency, the duty-cycle and the rising and falling time of the output voltage signal Vout 1  so as to avoid that possible departures of its wave shape from the desired one cause an error during the exchange of information between the decoder and each receiving antenna. 
         [0058]    Moreover, it is possible to control the average value of the output voltage signal Vout 1  by the resistances of the resistors R 1 ″ and R 2 ″. In particular, such values are chosen in such a way that the DC component of the output voltage signal Vout 1  has a value equal to the supply voltage (for example, ranging between 13V to 18V) to be supplied to each receiving antenna. 
         [0059]    It should be noted that, also in this case, the output voltage signal Vout 1  is formed by the DC voltage value to which a small signal of 22 KHz is superimposed having a peak-to-peak amplitude significantly lower than the DC voltage value (for example, approximately 700 mV). 
         [0060]    For high frequencies, the use of the impedance Z 2  having a high significantly resistive component (ideally an open circuit) allows decoupling the emitter terminal of the transistor T 1 ″ from the coaxial cable  315  to which it is connected. In such a way, for frequencies equal to at least the frequency of the input signal Vin, the voltage value reached by the output terminal  415  is not affected by the low output resistance of the emitter terminal of the transistor T 1 ″, so that the obtained low frequency signal may be exactly superimposed to the DC value. 
         [0061]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although one or more embodiments of the present invention have been described with a certain level of detail, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice. 
         [0062]    For example, the possibility of using an embodiment of the present invention in fields different from that of the satellite communication systems is not excluded. For example, the regulation and shaping circuit  360  may be used in any type of application where it is required that a signal having a predetermined frequency, duty-cycle, rising and falling time is superimposed on a continuous voltage. 
         [0063]    In any case, different waves shape of the input voltage signals are possible (for example, sinusoidal). 
         [0064]    Even if in the preceding description a reference has been made to a regulation and shaping circuit integrated in a semiconductor material chip, it is not excluded that the proposed regulation and shaping circuit can be realized on a physical support (such as a printed circuit board—PCB) using discrete components. 
         [0065]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the embodiments described above many modifications and alterations. Particularly, although one or more embodiments have been described with a certain degree of particularity, it should be understood that various omissions, substitutions, and changes in the form and details as well as other embodiments are possible. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment may be incorporated in any other embodiment as a general matter of design choice.