MXPA96005049A - Apparatus to control the gain of conversion of a reduc converter - Google Patents

Abstract

The present invention relates to a reducing converter, such as a low sound block converter (LNB), of the external unit (100) of a terrestrial microwave or satellite receiver system, includes an apparatus for automatically controlling the gain of converter conversion reducer. In one embodiment, the automatic gain control apparatus includes a gain controllable RF stage (106) and a gain control signal generator (146) to generate a gain control signal in response to the output signal. of the reducing converter, both are included inside the external unit (100). In another embodiment, the gain controllable RF stage (106) is included in the external unit (200), but the gain control signal generator (220) is included in the internal unit (202) of the receiver system, and the signal Gain control is coupled to the external unit (200) in a direct or coded manner. In the latter case, the coded gain control signal may be coupled to the reduction converter via the coaxial cable (222) connected between the external (200) and internal (202) units. In a receiver system which processes digitally coded information, the error rate of the information, which is encoded, in the internal unit (202), can be used to generate the gain control signal for the controllable gain stage (106). ) of the external unit (20

Description

APPARATUS TO CONTROL THE GAIN OF CONVERSION OF A REDUCING CONVERTER DESCRIPTION OF THE INVENTION The invention relates to a reducing converter of a communication receiver and, more particularly, to an apparatus for controlling the conversion gain of a reducing converter. In a microwave distribution system for digital television signals, television signals are transmitted on radio frequency (RF) carrier signals in the L-band microwave frequency scale (for example 2150 to 2686 MHz). A highly directed dish-like antenna of a receiver system receives the microwave signals transmitted by the television signal transmitter. A low sound block converter (LN B) converts the entire scale ("block") of relatively high frequency microwave signals transmitted by the transmitter to a lower, more manageable scale of frequencies (eg, 128 to 408 MHz). This procedure is known as "low conversion". Typically, the LN B converter is part of an external unit, which includes the parabolic type receiving antenna and the N LB converter. The receiving system also comprises an internal unit. The RF signals of the external unit are coupled through a coaxial cable to the internal unit. The internal unit includes a selector for choosing the RF signals corresponding to a desired channel of the RF signals received from the LNB converter and for converting the chosen RF signal to a low intermediate frequency (IF) signal. The internal unit also includes a signal processing section for demodulating and decoding the IF signal. The RF signals produced by the LNB converter are sometimes referred to as "first IF signals", and therefore the IF signal produced by the selector is sometimes referred to as a "secondary IF" signal. Typically, there is only one transmission site for all microwave signals, which are received by the external unit. As such, the distance of that transmission site to the receiving antenna generally determines the signal strength of all RF signals received by the external unit. In some cases, where the receiver is relatively close to the transmitting site, the received RF signals may overload the LNB converter and the resulting distortion products may degrade the performance of the indoor unit. In a digital television transmission system, where the television information is encoded in digital form, said overload can decode errors within the signal processing section of the internal unit. These decoding errors are usually catastrophic since they can cause a total loss of television and audio-information image. To solve this problem, reception installation technicians are trained, when the system is installed, to use an antenna with a relatively low gain (for example a gain of 20 dB), when the reception site is near a transmitter site and the use of an antenna with a relatively high gain (for example a gain of 24 dB) when the receiving site is far from the transmitting site (for example 30 miles or more). However, even with an antenna with a low gain, an overload of the LNB converter may occur. In this way, technicians are also trained to use an LNB converter with a relatively low conversion gain for facilities that are close to the transmitting site. Generally, an installation technician can select one of the two LN B converters having a conversion gain of either 20 dB or 30 dB. Said equipment selection procedure has been found to be prone to errors. For example, if the technician has incorrectly selected an antenna and a LNB converter combination for a particular site, an optimum signal level can not be provided, avoiding the too weak ends of a signal and the signal overload. Consequently, the internal unit can produce decryption errors that result in defects in the video and in the audio. Therefore, the applicant has recognized that there is a need in the art for an LN B converter that contains an automatic gain control circuit (AGC) system, which adjusts the conversion gain of the LN B converter. to compensate for the variation in the intensity of the received signal. Accordingly, the present invention relates to an apparatus for dynamically controlling the conversion gain control of a reducing converter, such as a LN B converter included in an external unit. More particularly, a low sound block converter (LN B) incorporating the invention comprises a gain controlled by the RF amplifier, a conversion stage including a mixer and a local oscillator, and an output amplifier, and a generator of gain control signal, which responds to an output signal of the output amplifier to produce a gain control signal. The gain control signal is applied to the gain controlled by the RF amplifier, so that the gain of the amplifier is substantially constant for input signal levels above a predetermined threshold. In one embodiment, the entire automatic gain control apparatus is fully included within the external unit. In another embodiment of the invention, portions of the gain control apparatus are partially included in the external unit and partially included in the internal unit. More specifically, the gain control signal generator is included in the internal unit of the receiver system. The gain control signal generated by the gain control signal generator is coupled through a transmission path to the external unit of the receiver system. For example, the transmission path may comprise the central conductor of the coaxial cable, which connects the external unit to the internal unit. To facilitate the transmission of control signal over a single conductor, such as the center conductor of a coaxial cable, an encoder apparatus is included in the internal unit to modulate a signal carried by the individual driver with the gain control information. For example, the gain control signal can be converted to a modulated low frequency tone frequency, which is coupled to the center conductor of the transmission line. The center conductor is coupled to a tone decoder inside the external unit, which regenerates the gain control signal to control the amplifier gain of R F. Alternatively, the magnitude of the supply voltage for the LNB converter, the which is typically carried to the LN B through the center conductor of the coaxial cable, can be modulated by the gain control signal. The teachings of the present invention can be easily understood by considering the following detailed description together with the accompanying drawings, in which: Figure 1 depicts a block diagram of a low sound block converter (LN B) containing a mode of the present invention; and Figures 2A and 2B together represent a block diagram of a communication system containing an alternative embodiment of the present invention. In the Figures, identical reference numbers have been used, where it is possible to designate the same or similar elements that are common in the Figures. The invention will be described with reference to a terrestrial microwave digital television distribution system, in which, the television information is transmitted in a coded and compressed form, in accordance with a predetermined digital compression standard such as the standard of the Group of Motion Picture Experts (commonly known as the MPEG standard). Those skilled in the art will understand that other embodiments of the invention may be used in other communication systems. For example, one embodiment of the invention may be used in a digital satellite television system, such as the DirecTV satellite television system operated by Hughes Corporation of California. Furthermore, since the invention is particularly useful when used in a digital communication system, where processing errors due to a small signal degradation for sound performance can be catastrophic, the invention is also useful in a system analog communication to improve the signal for sound performance. Figure 1 depicts a block diagram in a low sound block converter 100 (LNB), incorporating one embodiment of the present invention, such as may be employed in a terrestrial microwave digital television receiver system. The LNB converter 100 is included in the external unit of the receiver system, together with a parabolic receiver antenna (not shown). The LNB converter 100 contains an input gate 102 coupled to the antenna, a first RF input bandpass filter 104, a gain controlled RF amplifier 106, a second RF input bandpass filter 108. , a conversion stage 109 including a mixer 110 and a local oscillator (LO) included in a phase locked loop (PLL) 112 included in an arrangement, an RF output filter 114, and an RF output amplifier 116 and a gain control generator 144. The illustrative microwave digital television distribution system operates on a scale of 2100 to 2700 MHz, including two different frequency bands from 2150 to 2162 MHz and from 2500 to 2686 MHz. Consequently, the first RF filter 104 comprises a pair of RF passband filters 118 and 120 connected in parallel between the input port 102 and the RF amplifier 106. The first RF filter 118 passes RF signals in the 2150 to 2162 MHz, and the second RF filter 120 passes RF signals in the 2500 to 2686 MHz band. RF amplifier 106 is used in this embodiment of the invention as an illustrative gain control device. As will be discussed later, other gain control devices may be used. The RF amplifier 106 is typically a two-stage amplifier, which provides a maximum gain of approximately 15 dB. Either one or both stages of the RF amplifier is manufactured using a GaAs FET or double gate MOSFET transistor, where, for each FET, the first gate forms an RF input terminal for the amplifier 106 and the second gate is a Gain control terminal to receive a gain control voltage. When the gain control voltage applied to the second gate is at its maximum value, the amplifier 106 has a maximum gain. As the gain control voltage is applied to the second gate, it is reduced, the gain of the amplifier 106 is reduced. Either one or both of the FETs can be used for gain control. Said RF AGC circuit using double gate MOSFET is described by Trout, "Small Signal RF Design with Double Gate MOSFET" (Small-Signal RF Design With Dual-Gate MOSFETs), Motorola Semiconductor Product Application Note, AN-078. As controlled gain amplifiers, other amplifier circuits can also be used as bipolar transistor amplifiers. The output signal from the second RF bandpass filter 108 is coupled to the conversion stage 109, which includes the mixer 1 10 and the local oscillator included in the PLL 1 12. The mixer 1 10 may comprise a conventional diode mixer. , which has a loss of approximately 6 dB. In this embodiment the frequency of the local oscillator is 2278 MHz. Consequently, the mixer 106 produces lower side components having RF signals in the frequency scale of between 116 MHz (ie 2278-2162 MHz) and 128 MHz (en say, 2278-2150 MHz), and an upper lateral component that has RF signals on the frequency scale between 222 MHz (ie, 2500-2278 MHz) and 408 MHz (ie, 2286-2278 MHz). The filter 114 has a pass band, which passes RF signals on a frequency scale of 116 to 402 MHz. The output signals of the filter 114, although they are on the RF frequency scale, can be determined as IF signals. (Intermediate Frequency), and in this way, the filter 114 is labeled an "IF Filter", in Figure 1. The output signals of the IF filter 114 are coupled to an output amplifier 116. The output amplifier 116 may comprise a two-stage amplifier that provides a total gain of approximately 20 dB. the output amplifier 116 is coupled through a capacitor 124 to an RF output 126, which in turn is connected to one end of the center conductor of a coaxial cable (not shown in Figure 1). The other end of the center conductor of the coaxial cable is connected to the RF input of the input unit of the receiver system (as indicated in Figure 2B). Typically, the DC power for the LNB converter 110 is provided via the center conductor of the coaxial cable from the input unit For this purpose, as indicated in Figure 1, an inductor 128 is connected via an output port 126 between the central conductor of the coaxial cable to a voltage regulating section CD 130, which generates several supply voltages for the active circuits of the LN B 100 converters. An automatic gain control signal generator (AGC) 146 includes an AGC detector 132 coupled in cascade with an AGC signal processor 134. In its simplest form, the AGC sensor 132 comprises a conventional diode detector 136, for example, which comprises a Schottky barrier diode in combination with a resistor and capacitor load. Specifically, the diode is connected, via its anode connection, to the output of the output amplifier 1 16. The cathode connection of the diode is connected to the resistor and capacitor load and to the AGC signal processor 134. detector 136 rectifies the output RF signal and applies the rectified signal to the AGC signal processor 134. If desired, the pole of the diode can be inverted. The AGC signal processor 134 in its simplest form, comprises a CD amplifier, such as an operational amplifier 138. The rectified signal generated by the AGC detector 132 is applied to a first input of the operational amplifier 138. A threshold voltage provided by a source 140 is applied to a second input of the operational amplifier 138. The operational amplifier 138 provides a maximum output signal until the rectified signal RF output signal exceeds the threshold voltage. At that point, the output signal of the operational amplification will be reduced as the rectified signal increases. The output signal of the operational amplifier 138 is the gain control signal provided to the RF amplifier 106. In this way, after the rectified signal has exceeded the threshold voltage, as the rectified signal increases in magnitude, the gain control signal is reduced in magnitude. As described above, in the described embodiment, the gain control signal is applied to a gate of a GaAsFET or double gate MOSFET to control the entire conversion gain of the LN B 100 converter. As a gain control device Alternatively, a PI diode attenuator N 144 can be placed between the first RF filter 104 and the RF amplifier 106. The RF amplifier 10, in this case, could have a fixed gain and the attenuation of the diode attenuator PI N it could be controlled by the gain control signal. As such, a reduction in the magnitude of the gain control signal could increase the attenuation of the input signal to the RF amplifier 106. Of course, the relationship between the magnitude of the gain control signal and the attenuation value they may vary depending on the specific circuit arrangement, which is used to implement the invention. For example, the operational amplifier can produce a control signal that increases with increasing output signal strength and, as such, the attenuation could increase with the increase of the gain control signal. The gain control can also be provided in discrete steps of gain control. For example, an attenuator may be "connected" or "disconnected" from the path between the first RF filter 104 and the RF amplifier 106, in response to the gain control signal. When the detected output signal is greater than the voltage threshold, the gain control signal could cause the attenuator to "disconnect". When the output signal is below the voltage threshold, the attenuator could not be "connected". More than one gain control step can be provided with the addition of more attenuators. In the embodiment of the invention described with respect to Figure 1, the provisions of the automatic gain control are completely contained within the external unit. In the embodiment of the invention shown in Figures 2A and 2B, part of the gain control circuit system is included in the external units and part is included in the internal unit. More specifically, the controllable gain stage remains in the reducing converter of the external unit, but the gain control signal generator is included in the internal unit and is coupled to the reducing converter of the external unit. Figures 2A and 2 B together represent the embodiment of the invention, wherein the portions of the automatic gain control arrangement are both incorporated in the external unit 200, shown in Figure 2A and in the indoor unit 202, shown in Figure 2B. The external unit depicted in Figure 2A is similar to that in Figure 1; therefore, only the differences will be discussed below.

The internal unit 202, shown in Figure 2B, comprises a selector including a first RF filter 204, an RF amplifier 206, a second RF filter 208 and a conversion stage 209 including a mixer 210 and a local oscillator included in a phase locked loop (PLL) 212. The RF output signals, which are received from the external unit 200 via the coaxial cable at the RF input 223, are coupled through a capacitor 201 to the selector 203. The two RF filters 204 and 208, and the RF amplifier 206 filter and amplify the received RF signals. The local oscillator signal received by the mixer 210 of the phase locked loop 212 has a frequency, which is controlled by a microprocessor (not shown) to select the particular RF signal, which corresponds to the desired channel of the signal "block" of RF received from the external unit 200. The internal unit 202 also includes an IF amplifier. 214, a first IF filter 216 and a second IF amplifier 218. The first IF signal produced by the mixer 210 is amplified and filtered and then amplified again by the IF amplifier 214, the IF filter 216 and the amplifier of IF 218. The IF signal processed by the IF amplifier 214, the IF filter 216 and the IF amplifier 218 can be referred to as a "second IF signal", since the output RF signals of the unit external 200 can be referred to as "first IF signals". By way of example, the second signal of I F can have a frequency of the order of 44 MHz.

The "second" processed IF signal is coupled to a signal processing section 220, which modulates, decodes and decompresses the digital television information to produce digital signals representing the audio and video information. The signal processing section 220 also converts the digital signals to a corresponding analog signal, suitable for displaying a television image and producing a corresponding audible response. A power supply unit 232 generates supply voltages for various sections of the input unit 202. In addition, the power supply 232 generates a supply voltage for the LNB converter of the external unit 200. Typically, as shown in FIG. Figure 2B, the supply voltage of LNB is coupled to the LNB converter of the external unit 200 via the center conductor of the coaxial cable 222 connected between the RF output port 126 of the LNB converter 200 and the input port 223 of the internal unit 202. The capacitor 201 prevents the supply voltage form of LNB from being coupled to the input of the selector 203, and the inductor 226 prevents the RF input signals from being coupled to the power supply 232. A generator of The gain control signal 221 of the signal processing section 220 responds to the magnitude of the demodulated signal to generate a gain control signal. More specifically, the demodulator portion of the signal processing section 220 produces pulse signals corresponding to the digitally encoded television information. The gain control signal generator 221 generates a signal representing the magnitude of the pulse signals. The representative magnitude of the signal is compared to a threshold and generates a gain control signal, which has a magnitude related to the magnitude of the pulse signals when the threshold is exceeded. This procedure can take place analogously or digitally. Alternatively, the gain control signal of the external unit can be generated in response to the magnitude of the second I F signal. Further, the gain control signal generator for the LN converter B of the external unit 200 can share portions of the signal processing section 220 used to generate a gain control signal for the RF amplifier 204 of the external unit 202, as indicated in Figure 2B. The gain control signal may also be derived in response to errors produced by the digital signal decoder within the signal processor 220. More specifically, the decoder includes error correction provisions, such as a previous error correction section. (FEC) that operates, for example, according to the well-known algorithm of Reed-Solomon. The gain control signal generator 221 can verify the bit error register detected by the FEC section of the decoder and can adjust the gain control signal to change the conversion gain of the reducing converter to reduce the error rate . For example, when an error rate becomes unacceptable, the gain control signal may be changed to increase the gain of the RF amplifier 106 in the LN converter B 200 by a predetermined amount. If the error rate is improved, the amplifier gain of R F 106 causes it to have a gain increase by a certain amount. This procedure is repeated until the error rate is no longer improved. Conversely, if in the first gain increment or any subsequent increase, the bit error rate worsens, the gain of the RF amplifier 106 is caused to be repeatedly reduced in cases until the error rate is stabilized. The external gain control signal generated by the gain control signal generator 221 is coupled to a gain control signal encoder 224. The gain control signal decoder 224 encodes the gain control signal and couples the gain control signal. encoded signal to a transmission path that carries the encoded signal to the external unit 200. Advantageously, the transmission path is the central conductor of the coaxial cable 222 connected between the internal unit 202 and the external unit 200. For example, the decoder of Control signal 224 can convert the gain control signal to a relatively low frequency tone, such as an audio-frequency tone, having a frequency that depends on the magnitude of the gain control signal. For this purpose a controlled voltage oscillator can be used. The gain control signal for the external unit 200 may be encoded using other coding techniques, such as pulse width modulation, amplitude modulation, and the like. The coded gain control signal is coupled via a low pass filter, comprising the inductor 226 (which is already used to isolate the power supply 232 of the RF input signals), to the center conductor of the coaxial cable 222. As shown in Figure 2A, in the external unit 200, a decoder 230 is coupled through the inductor 128 and a capacitor 129 to the center conductor of the coaxial cable to decode the coded gain control signal. In the case where the tone signal has a frequency modulated according to the magnitude of the gain control, the decoder 230 may comprise a frequency discriminator. The output of the decoder 230 is a DC voltage corresponding to the gain control signal, which was generated inside the internal unit 202. The decoded gain control signal is applied to the controllable gain stage, such as the amplifier RF 106, of the external unit 200. As described with reference to Figure 1, the controllable gain stage may comprise a controllable gain amplifier, such as a GaAsFET, MOSFET, or bipolar transistor amplifier, or a controllable attenuator .

The gain control signal for the LN B 200 converter does not necessarily have to be coded. The gain control signal can be applied to the center conductor without coding, if the center conductor is not used to supply power to the external unit. In addition, depending on the power supply sound specification of the LN B 200 converter, the gain control signal can be superimposed on the power supply voltage of LN B to modulate its level. If the gain control signal modulation of the supply voltage of LN B is maintained at a relatively low level, the supply voltages produced by the regulator 130 in the LN B 200 converter will not change in response to the control signal of profit However, the modulation of the gain control signal can be detected by the decoder and converted to the gain control signal for the controllable gain control stage. Instead of coupling the gain control signal of the internal unit 202 to the LN converter B 200 via the coaxial cable 222, a separate path can be used, for example comprising a pair of twisted conductors. One or more of the following advantages result from the gain control provisions of the reductant converter arrangements, which have been described: The amplitudes of the RF output signals are maintained at a relatively low level. constant, although the amplitude of the amplitudes of the RF input signals, through the antenna, may vary significantly. The signal of the sound performance is improved. The high-level received RF signals are not distorted by the reducer conversion procedure. In a digital receiver system, this minimizes audio and video drops due to decoding errors. The amplitude variations due to the well-known phenomenon of microwave weakening that is reduced. The criticality of the installation procedure is reduced. Another use of the gain control signal from the external unit is to perfectly position the antenna during installation. The gain control signal may be used to indicate the signal strength to an installer either visually, such as by means of a meter, or aurally, such as by means of an audio-tone generator, which generates a response to audio that changes the frequency or amplitude as the signal strength changes. The embodiment shown in Figure 1, in which a gain control signal is generated within the LNB converter 100, is particularly useful in this respect, since the signal intensity indicating device can be coupled directly to the signal generator. of gain control 146 of the external unit, where the alignment of the antenna is presented. Although various embodiments and modifications, which incorporate the teachings of the present invention, have been described in detail, those skilled in the art can readily advise many other modalities that continue to incorporate these teachings and are within the scope of the invention defined by a more of the following claims.

Claims (1)

1 - . 1 - An apparatus comprising: a reducing converter of an external unit (100,200) of a receiver system for converting the frequency band of RF signals received from an antenna to a low frequency band and having an output (126) for coupling the converted RF signals to an internal unit (202) including a selector (203) and a demodulator (220) and coupled to the output (126) of said reduction converter via an axial cable (222), characterized by: a generator a gain control signal (146) that responds to said converted RF signals to produce a gain control signal; means (106), included within said reducing converter and coupled to the gain control signal generator (146), for controlling the conversion gain of the reducing converter in response to said gain control signal. 2. The apparatus of claim 1, characterized in that: said gain control signal generator (146) comprises a signal detector (132) that responds to said RF signals converted to a signal representing the magnitude of the signals of RF converted. 3 - The apparatus of claim 2, characterized in that: said gain control signal generator (146) further includes means (138) for amplifying said signal representative of magnitude. 4 - The apparatus of claim 3, characterized in that: said amplifying means (138) amplify said signal representative quantity, when the magnitude of said signal representative quantity exceeds a threshold. 5. The apparatus of claim 1, characterized in that: said gain control signal generator (146) is included within said external unit (100). 6. The apparatus of claim 1, characterized in that: said gain control signal generator (146) is included within said internal unit (202). 7. The apparatus of claim 6, characterized in that: said control signal generator (146) responds to an output signal of the demodulator (220) of said internal unit (202). 8. - The apparatus of claim 6, characterized in that: an encoder (224) is included with the internal unit (202) and responds to said gain control signal to generate an encoded gain control signal; a decoder (230) is included in the external unit for decoding said coded control signal to generate a decoded gain control signal to control the conversion gain of said reduction converter; a transmission path is coupled between the encoder (224) and said decoder (230) for coupling the encoded control signal of the encoder (224) to said decoder 9. - The apparatus of claim 8, characterized in that said transmission path is said coaxial cable (222) and the encoder and decoder are coupled to a central conductor of said coaxial cable (222). 10. The apparatus of claim 8, characterized in that: said encoder (224) is a tone generator for generating a tone having a frequency modulated according to the gain control signal; and said decoder (230) is a tone decoder for generating said decoded gain control signal in response to the frequency of the tone signal. 11. The apparatus of claim 10, characterized in that: said internal unit (202) includes a power supply (232) for generating various supply voltages for said internal unit (202) and a supply voltage for the external unit ( 200); said supply voltage for the external unit (200) is coupled via the central conductor of the coaxial cable (222) to said external unit; and said tone signal is superimposed on the supply voltage for said external unit (200). 12 * The apparatus of claim 8, characterized in that: said internal unit (202) includes a power supply (232) to generate several supply voltages for said internal unit (202) and a supply voltage for said external unit (200); said supply voltage for the external unit (200) is coupled via the center conductor of said coaxial cable (222) to the external unit (200); and said coded gain control signal is superimposed on the supply voltage for said external unit (200). 13. The apparatus of claim 1, characterized in that: said gain control means include a gain controllable RF amplifier (106). 14 - The apparatus of claim 1, characterized in that: said gain control means are a controllable attenuator.