Abstract:
A method for controlling gain in a network is provided. The method includes receiving signals for transmission over a network and adjusting the level of the received signals. The method further includes inserting an additional signal indicative of the level adjustment and transmitting the signals and the additional signal over the network. The method also includes extracting the additional signal after transmission over the network and compensating for the level adjustment based on the extracted signal.

Description:
RELATED APPLICATIONS 
     This application is related to co-pending application Ser. No. 09/517,685 filed on Mar. 2, 2000, entitled “Automatic Gain Control for input to Analog to Digital Converter” (the “&#39;685 Application”). This application is also related to co-pending application Ser. No. 09/599,701, filed on Jun. 22, 2000, entitled “Automatic Level Control for Input to Analog to Digital Converter” (the “&#39;701 Application”). The &#39;685 and the &#39;701 Applications are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to the field of electronics and, in particular, to automatic level control for an input to an analog to digital converter. 
     BACKGROUND 
     Linear Amplifiers, analog to digital (A/D) converters, and analog laser transmitters are used in a wide variety of communication circuits. For example, A/D converters are used in high frequency digital transmission systems that receive analog input signals. The A/D converter is an electronic circuit that receives the analog input signal and produces a digital output signal. The digital output signal is produced based on samples of the analog input signal taken over time and processed through a plurality of output registers. The A/D converter samples the digital signal based on a clock signal. In high-speed applications for digital transmission systems, these clock signals typically operate at speeds in the tens to hundreds of megahertz range. 
     A/D converters and other circuits, such as amplifiers and laser transmitters, typically found in communication circuits have a limited dynamic range in that the circuits are designed to process analog signals over a specified range of values. In an A/D converter, when the input signal exceeds the specified peak input signal level, the output registers of the A/D converter overflow. This occurs even when the input voltage exceeds the peak input level by a value that would cause the A/D converter output to exceed its maximum level by a single least significant bit. Further increases beyond this point produce spurious output signals (distortions) that are proportional to the degree of overload of the A/D converter&#39;s output registers. Similar distortions are produced when an amplifier or a linear laser transmitter is driven beyond its optimum linear range (which is known as the compression point in linear systems). The spurious signals can be detrimental to the operation of the devices using the communication network. However, if this overload is kept at sufficiently low levels in terms of amplitude and frequency of occurrence, then the resultant distortion can be acceptable in many applications; but operating too far below the optimum drive point reduces the signal to noise ratio of the communication network and degrades the performance. Thus for best performance, it is necessary to operate near the optimum point while not exceeding it. Such an operation does not leave much room for overdrive condition necessitating expensive high linearity devices or automatic gain control mechanisms as described below. 
     To reduce the effect of overflow conditions, some electronic systems are adjusted to operate well below the peak input voltage range of the A/D converter and other electronic circuits with limited dynamic range. When initially setting up the system, the input voltage is increased gradually while the circuit is monitored, e.g., by an overflow register of the A/D converter. In the case of a typical AID converter, when an overflow condition is reached, the A/D converter provides an output of narrow pulses from the overflow register. The frequency of occurrence of the pulses is typically proportional to the severity of the overflow. Based on the monitored output of the overflow register, the input to the A/D converter is adjusted, e.g., by setting an attenuator, such that the expected maximum input signal will not exceed the peak input range of the A/D converter. In some systems, this adjustment is set at or near 50% of the peak input voltage level. Unfortunately, this reduces the effectiveness of the A/D converter and reduces the signal to noise ratio for the electronic device. 
     In some circuits, it is desirable to maintain the peak input voltage of the analog signal at or near the maximum value for the range of input signals accepted by the devices with a limited dynamic range, e.g., an analog to digital converter. Thus, automatic gain control circuits have been used in conjunction with circuits including analog to digital converters, amplifiers and laser transmitters. Typically, the automatic gain control circuit monitors the input to the device with the limited dynamic range. The automatic gain control circuit further generates a feedback signal based on the monitored input signal. The feedback signal is provided to an amplifier to control the level of the input signal to the analog to digital converter. This feedback signal attempts to keep the peak voltage level of the input signal at or near the full-scale value of the input for the analog to digital converter. 
     Conventionally, the feedback signal for A/D converters is generated using analog circuitry. For example, such feedback loops typically include one or more of the following analog control blocks: a log amplifier, a summing amplifier, an integrator, and a differentiator. Unfortunately, these analog feedback control loops typically suffer from the so-called “clip” effect. This means that when the input signal exceeds the full-scale input for the analog to digital converter, the control circuitry is unable to quickly reach steady-state operation. Some control loops attempt to use digital circuitry to overcome these problems. 
     Automatic gain or level control for circuits that have a limited dynamic range may introduce other problems into transmission systems. When automatic level control is used, total gain from system input to system output may vary over time as a function of input signal level. In some transmission systems, devices coupled to the transmission system expect a substantially constant or uniform gain from the system. For example, cable modems coupled to hybrid fiber/coax (HFC) networks include circuitry that is designed to monitor and adjust for changes in gain in the HFC network. Thus, as the automatic level control circuit attempts to compensate for an increasing input from a cable modem, the modem responds by further increasing its output provided to the network. Such a course of events can have disastrous consequences. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in automatic level or gain control that extend the dynamic range of circuits without compromising the operation of associated circuits and systems. 
     SUMMARY 
     The above-mentioned problems with automatic gain control in telecommunications systems and other problems are addressed by the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention provide information on gain adjustments in circuitry associated with a transmitter to circuitry associated with a receiver to produce offsetting gain adjustments at the receiver. Advantageously, the use of this “dual gain control” produces a channel with a substantially constant gain function while allowing the dynamic range of circuitry associated with the transmitter to be extended. 
     More particularly, in one embodiment a method for controlling gain in a network is provided. The method includes receiving signals for transmission over a network and adjusting the level of the received signals. The method further includes inserting an additional signal indicative of the level adjustment and transmitting the signals and the additional signal over the network. The method also includes extracting the additional signal after transmission over the network and compensating for the level adjustment based on the extracted signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an embodiment of a telecommunications system that uses a gain correction circuit at a receiver to compensate for adjustments of an automatic gain control circuit associated with a transmitter according to the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     FIG. 1 is a block diagram of an embodiment of a telecommunications system, indicated generally at  100 , including first and second gain control circuits  101  and  103  that implement a counter-acting dual gain control function to produce a substantially constant gain channel over network  114  between input  102  and output  126  according to the teachings of the present invention. This allows system  100  to advantageously improve the performance of circuits with limited dynamic range without adversely impacting or limiting the effectiveness of circuits providing input to system  100 . First and second gain control circuits  101  and  103  are described in detail below. 
     First gain control circuit  101  is coupled between input  102  and transmitter  110 . Transmitter  110  is coupled to network  114 . Transmitter  110  includes circuitry or devices with a limited dynamic range. For example, in some embodiments, transmitter  110  is a digital transmitter with an analog to digital (A/D) converter with a limited dynamic range. In other embodiments, transmitter  100  includes circuits such as A/D converters, amplifiers, laser transmitters and the like that have a limited dynamic range. 
     In one embodiment, network  114  comprises a hybrid fiber/coax network. In other embodiments, network  114  comprises other appropriate networks for transmitting data, e.g., voice, video, or high-speed data. Network  114  also provides an output to receiver  116 . Second gain control circuit  103  is coupled between receiver  116  and output  126 . 
     For simplicity in describing the functionality of telecommunications system  100 , FIG. 1 illustrates a unidirectional connection from input  102  to output  126 . However, it is understood, that telecommunications system  100  is not limited to a unidirectional network. Rather, bidirectional communication is accomplished over network  114  by providing complementary pairs of first and second gain control circuits  101  and  103  at each point connecting to network  114  thus allowing bidirectional communication with each direction including counter-acting dual gain control to produce substantially constant gain on both channels between endpoints. 
     First gain control circuit  101  is adapted to receive signals at input  102  for transmission over network  114 . First gain control circuit  101  includes the series combination of signal insertion circuit  104 , variable attenuator  106 , and signal sampling circuit  108 . First gain control circuit  101  further includes gain controller  112 . Gain controller  112  is coupled to receive a feedback signal from signal sampling circuit  108 . Gain controller  112  further provides input to variable attenuator  106  and signal insertion circuit  104 . 
     In one embodiment, gain controller  1   12  instructs signal insertion circuit  104  to insert a message with the signals receive at input  102  for transmission over network  114 . This message indicates the level of gain of gain controller  112 . This message is used by second gain control circuit  103  to compensate for the gain of first gain control circuit  101  as described in more detail below. In another embodiment, signal insertion circuit  104  inserts a reference signal with the signals received at input  102 . In this embodiment, the effect of variable attenuator  106  is communicated to second gain control circuit  103  by the inserted reference signal. Essentially, second gain control circuit  103  learns the setting of variable attenuator  106  based on the received reference signal from first gain control circuit  101 . 
     In one embodiment, first gain control circuit  101  implements an automatic level control procedure such as the procedure described in co-attending, commonly assigned application Ser. No. 09/599,701 incorporated by reference above. In one embodiment, variable attenuator  106  comprises a plurality of PIN diodes coupled in either a “T,” a “pi” or other appropriate configuration to produce an acceptable attenuation for signals provided to transmitter  110 . In other embodiments, variable attenuator  106  comprises a variable gain amplifier(current or voltage controlled), a digital scaling circuit or other appropriate circuit with an adjustable gain for adjusting the level of signals to be transmitted over network  114 . Thus, the terms “gain” and “attenuation” are used interchangeably to indicate a change in the magnitude of signals transmitted over network  114 . Further, it is understood that these terms also include a unity gain. 
     Second gain control circuit  103  is coupled between receiver  116  and output  126 . Second gain control circuit  103  includes the series combination of variable attenuator  118  and signal sampling circuit  120 . Second gain control circuit  103  further includes signal extraction circuit  122  and gain controller  124 . Signal extraction circuit  122  is coupled to signal sampling circuit  120 . Signal extraction circuit  122  provides an input to gain controller  124 . This input is based on the signal inserted into first gain control circuit  101  by signal insertion circuit  104 . Gain controller  124  provides a control input, based on the extracted signal, to variable attenuator  118  to adjust the gain of second gain control circuit  103  to counter-act the gain of gain control circuit  101 . In one embodiment gain controller  124  implements an inverse gain function to compensate for the gain of first gain control circuit  101 . 
     In one embodiment, variable attenuator  118  comprises a plurality of PIN diodes coupled in either a “T,” a “pi” or other appropriate configuration to produce an acceptable attenuation for signals from receiver  116 . Again, the term “attenuation” encompasses attenuation, gain, and unity gain. In other embodiments, variable attenuator  118  comprises a variable gain amplifier (current or voltage controlled), a digital scaling circuit or other appropriate circuit with an adjustable gain. 
     Advantageously, the use of first and second gain control circuits  101  and  103  allows system  100  to extend the dynamic range of various components of system  100 , e.g., transmitter  110 , while producing a channel in network  114  with substantially constant gain between input  102  and output  126 . 
     In operation, system  100  provides transport of data over network  114  with substantially constant gain between input  102  and output  126 . The signals are received at input  102  from, for example, a cable modem or other appropriate data source. First gain control circuit  101  adjusts the gain of the signals received at input  102  to compensate for the limited dynamic range of devices associated with transmitter  110 . For example, gain controller  112  adjusts the gain of variable attenuator  106  to set the peak level of input signals to an A/D converter for transmitter  110  at or substantially near the peak level acceptable by the A/D converter as described in the U.S. application Ser. No. 09/599,201 incorporated by reference above. 
     The amount of gain or attenuation implemented by variable attenuator  106  is communicated to second gain control circuit  103  via a signal inserted by signal insertion circuit  104 . In one embodiment, signal insertion during  104  inserts a reference signal for transmission to second gain control circuit  103 . This reference signal passes through variable attenuator  106  and thus carries information on the gain of variable attenuator  106 . In another embodiment, signal insertion circuit  104  inserts a message from gain controller  112  to indicate the level of gain or attenuation implemented by variable attenuator  106 . 
     At second gain control circuit  103 , signals received over network  114  are compensated for the gain or attenuation of first gain control circuit  101 . Gain controller  124  controls the gain of variable attenuator  118  based on a signal extracted by signal extraction circuit  122 . Essentially, gain controller  124  causes variable attenuator  118  to implement a counter-acting gain to compensate for the gain of variable attenuator  106 . Thus, system  100  provides a dual gain control function that provides a communication channel over network  114  between input  102  and output  126  that has a substantially constant gain. 
     CONCLUSION 
     Although specific embodiments have been illustrated and described in this specification, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. For example, in other embodiments, circuits other than PIN diodes are used to implement the variable attenuator function. Further, networks other than hybrid fiber/coax networks are used in other embodiments of system  100 .