Abstract:
A method and apparatus for enhancing the effective dynamic range of an RF-to-optical-to-RF link. The transmission through an optical link of RF digital signals having magnitudes outside of the dynamic range of the link is performed by changing the amplitude of (e.g., attenuating) such signals prior to transmission, and restoring such signals to their original amplitude after transmission. Signal-to-Noise Ratio (e.g., Noise Power Ratio (NPR)) of such digital signals is thereby maintained above a predetermined minimum level. The method and apparatus have advantageous applications in bi-directional commercial CATV systems.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to transmission of RF digital signals through an optical link having a limited physical Dynamic Range, particularly a method and apparatus for enhancing the effective Dynamic Range of the link by sacrificing unneeded Signal to Noise Ratio, (e.g., Noise Power Ratio (NPR)) of such digital signals, the method and apparatus having advantageous applications in bi-directional commercial CATV systems.  
           [0003]    2. Related Art  
           [0004]    Commercial terrestrial Cable Television (CATV) systems now typically utilize optical links (e.g., fiber-optic cable, with RF transmitter-receiver pairs) to carry radio frequency (RF) digital signals over long distances.  
           [0005]    [0005]FIG. 1A depicts a typical RF digital signal transmission system  100  of the related art. The related art system  100  includes an electro-optical transmitter (Tx,  120 ) that transmits the digital information signals (e.g., carried in as RF digital electrical signals RFin) through an Optical-Out port ( 121 ) into an optical LINK ( 130 ) as light signals to the Optical-In port ( 141 ) of an opto-electric Receiver (Rx, 140 ) which outputs the received light signals as radio frequency (RF) digital signals (i.e., RF digital electrical signals RFout). The light signals passed through the optical link  130  (e.g., fiber optic cable) may be Wavelength-Division Multiplexed (WDM) signals. The RF digital electrical signals RFout output from the receiver  140  are generally reproductions of the original electrical signals RFin input to the transmitter  120  except for any gain/attenuation, noise and/or distortion introduced during passage through the system  100 . A conductor  110  carries the radio frequency (RF) electronic signals RFin into the transmitter  120 . The transmitter  120  may include a laser for transmitting the RF signals as light signals. A conductor  111  carries the radio frequency (RF) electronic signals RFout out of the receiver  140 . The transmitter  120  and the  140  are generally electrically isolated, and separated by significant distances (e.g., several miles). A separate local power supply (not shown in the diagrams) supplies electrical power to each of the transmitter  120  and the receiver  140 .  
           [0006]    The extent to which a digital signal within RFin has been distorted or combined with noise is characterized by persons skilled in the art by known measurements (or calculations) referred to as Signal-To-Noise Ratio (SNR) and/or Noise Power Ratio (NPR). For purposes of discussion and illustration herein, NPR will be considered to be the same and/or analogous to SNR, except that distinct values of NPR (as measured between RFin and RFout) will be associated with “amplitudes” (e.g., RF power levels) of digital signals input to the system, rather than associated with particular continuous wave (CW) signals. Digital signals (RFin) transmitted out at amplitudes having an NPR equal to or greater than a predetermined minimum NPR (e.g., the current Industry Standard minimum NPR is 40) are considered to have acceptable fidelity, while signals (RFin) transmitted out at amplitudes having an NPR less than the predetermined minimum NPR (e.g., 40) are deemed unacceptably distorted and/or noisy.  
           [0007]    In a typical digital signal transmission system of the related art, such as system  100  depicted in FIG. 1, the Noise Power Ratio (NPR) associated with digital signals (RFout) passing out of the system  100  may be characterized as a function of the amplitude (i.e., signal strength measured in dBmV) of the inputted signals (RFin). Digital signals RFin that have insufficient amplitude (i.e., having amplitude smaller than the smallest amplitude that will emerge from the system  100  with an NPR equal to or greater than the predetermined minimum NPR) will emerge too noisy or distorted (i.e., with a SNR that is too small and/or with a NPR less than 40). Digital signals RFin that have a large amplitude can be clipped (i.e., by the Tx, Link, or Rx) to an extent roughly proportional to their amplitude, thus introducing noise and/or distortion. Thus, digital signals RFin that have an amplitude greater than a maximum amplitude (that depends on system device characteristics), can emerge too noisy or distorted (i.e., a SNR and/or NPR that is less than 40). The range of signal amplitudes that includes amplitudes that are not too small, nor too large, and that will emerge from the system with an NPR equal to or higher than the predetermined minimum NPR, is referred to as the “dynamic range.” 
           [0008]    [0008]FIG. 1B is a sketch depicting measured NPR (measured between RFin and RFout) graphed as a function of the amplitude of input signals (RFin), through the typical digital signal transmission system  100  of FIG. 1A. The dynamic range of the system  100  is characterized by the range of amplitudes (i.e., 10 dBmV to 40 dBmV) of input signals RFin that emerge from the system at NPRs equal to or greater than the predetermined minimum NPR (i.e., 40). As shown in FIG. 1A, it is typical in a system  100  of the related art that some signals having amplitudes within the dynamic range of the system  100 , will be transmitted through the system  100  associated with NPR values that substantially exceed the predetermined minimum NPR.  
           [0009]    A digital signal transmission system having maximally wide dynamic range, and particularly a system that does not introduce significant gain nor attenuation to power level of the outputted RF signal RFout relative to the input RFin, is desirable. However, conventional techniques for increasing the dynamic range of such a system  100  generally entail providing more-expensive system components (e.g., a higher fidelity transmitter Tx and/or receiver Rx) and/or higher quality (i.e, more expensive) optical LINK ( 130 ) media etc. The expense of such physical upgrades is often economically prohibitive. But, the limited dynamic range of an economical installed terrestrial system  100  can limit the usefulness, expandability, and performance of such a system.  
           [0010]    There is thus a need for a method for transmitting RF signals through an optical link, and an economical RF digital signal transmission system, that provide an enhanced (i.e., wider) effective dynamic range.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention overcomes physical and economic limitations on the dynamic range of a RF digital signal transmission system that includes an optical link. The disclosed invention is applicable to the entire cable television (CATV) frequency range, and may be especially useful in lower frequency CATV applications such as transmission of end-user supplied information, such as programming and/or website section information, from the set-top box back to the commercial CATV provider. RF digital signals that may be transmitted through the inventive RF digital signal transmission system may include QAM (e.g., M-ary Quadrature Amplitude Modulated), Baseband, QPSK and other types.  
           [0012]    The present invention exploits the knowledge that in a typical RF digital signal transmission system, such as system  100  shown in FIG. 1A, it is typical that some range of amplitudes (of RFin signals) within the dynamic range of the system  100 , will be transmitted with NPR values that substantially exceed the predetermined minimum NPR. In other words, some amplitudes (of RFin signals) will produce output signals (RFout) that have more SNR and/or NPR than is needed for faithful transmission of encoded information. The present invention further exploits a discovery that such “excess performance” (i.e., higher than necessary NPR) of a typical system  100  can be purposefully sacrificed with the result of effectively enhancing (widening) the dynamic range of such a system (i.e., without improving the quality of the optical LINK ( 130 ) nor improving the fidelity of either the transmitter (Tx) or the Receiver (Rx)).  
           [0013]    In a first general aspect, the present invention provides an apparatus for communicating radio frequency (RF) informational signals having a RF power level, through an optical link medium, said apparatus comprising: a first conductor adapted to carry said informational signals as electrical signals into the apparatus; a RF level sensor operatively coupled to the first conductor, adapted to measure the RF power level and to output a control signal according to said RF power level; a first RF attenuator adapted to be operatively controlled by the control signal, and adapted to attenuate the electrical signals from the first conductor prior to being communicated through said optical link medium; a transmitter adapted to transmit the electrical signals as optical signals into the optical link medium; a receiver adapted to receive the optical signals from the optical link medium, said receiver being operatively coupled to a second conductor adapted to carry said informational signals as electrical signals out of the apparatus.  
           [0014]    In a second general aspect, the present invention provides an apparatus for enhancing the dynamic range of an optical transmission system, the optical transmission system including a RF transmitter for transmitting digital signals, an RF receiver for receiving the digital signals, and an optical link operatively connecting the RF transmitter to the RF receiver, the apparatus comprising: RF sensor adapted to measure the power level of RF digital signals to be transmitted by the RF transmitter, the RF sensor having a sensor output corresponding to said power level; and a first RF attenuator operatively coupled to the RF sensor and adapted to attenuate the RF digital signals prior to being transmitted by the RF transmitter, wherein the attenuation performed by the first RF attenuator corresponds to the sensor output.  
           [0015]    In a third general aspect, the present invention provides an optical transmission system comprising: an optical signal transmitter section; an optical signal receiver section; an optical link medium being operatively connected between the optical signal transmitter section and the optical signal receiver section to form an included transmission system having a dynamic range; an RF stabilization system operationally connected to said transmitter section and to a first conductor carrying in an RF signal having a first RF power level; an RF stabilization system operationally connected to said receiver section and to a second conductor carrying out the RF signal at a second RF power level; wherein the RF stabilization systems operate to make the effective dynamic range of the apparatus substantially wider than the dynamic range of the included transmission system.  
           [0016]    In a fourth general aspect, the present invention provides an apparatus for enhancing the dynamic range of an optical transmission system, the optical transmission system including an RF transmitter for transmitting digital signals, an RF receiver for receiving the digital signals, and an optical link operatively connecting the RF transmitter to the RF receiver, the apparatus comprising: an RF sensor adapted to measure the power level of RF digital signals to be transmitted by the RF transmitter, the RF sensor having a sensor output corresponding to said power level, wherein the sensor output is adapted to be transmitted to the RF receiver; RF attenuator operatively coupled to the RF sensor and adapted to attenuate the RF digital signals prior to being transmitted by the RF transmitter, wherein an attenuation performed by the RF attenuator is greater when the measured power level is higher than the dynamic range than when the measured power level is within the dynamic range; and a RF amplifier operatively coupled to the RF receiver and adapted to amplify the digital signals, wherein during operation of the apparatus the magnitude of the amplification performed by the RF amplifier is approximately the same as the magnitude of the attenuation performed by the RF attenuator.  
           [0017]    In a fifth general aspect, the present invention provides a method for enhancing an effective dynamic range of an optical transmission system including a transmitter, an optical link, and a receiver, and for transmitting RF electronic signals as light signals through the optical link to the receiver that outputs the light signals as transmitted RF electronic signals, the method comprising: measuring a first RF power level of the RF electronic signals to be transmitted; transforming the RF electronic signals to a transformed RF power level before the RF electronic signals are transmitted as light signals by the transmitter, wherein the noise power ratio (NPR) of the transmitted RF electronic signals is greater than it would be if such transforming had not been performed; and outputting the transmitted RF electronic signals at within ±0.5 dB of the first RF power level.  
           [0018]    In a sixth general aspect, the present invention provides a method for communicating radio frequency (RF) informational signals having a RF power level, through an optical link medium, said method comprising: providing a first conductor adapted to carry said informational signals as electrical signals into the apparatus; providing a RF level sensor operatively coupled to the first conductor, adapted to measure the RF power level and to output a control signal according to said RF power level; providing a first RF attenuator adapted to be operatively controlled by the control signal, and adapted to attenuate the electrical signals from the first conductor prior to being communicated through said optical link medium; providing a transmitter adapted to transmit the electrical signals as optical signals into the optical link medium; providing a receiver adapted to receive the optical signals from the optical link medium, said receiver being operatively coupled to a second conductor adapted to output said informational signals as electrical signals; and outputting said electrical signals at said second conductor at ±0.5 dB of the RF power level.  
           [0019]    The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description.  
         [0021]    [0021]FIG. 1A is a block diagram depicts a typical RF digital signal transmission system of the related art;  
         [0022]    [0022]FIG. 1B is a sketch depicting measured NPR graphed as a function of the amplitude of input signals, through the digital signal transmission system of FIG. 1A;  
         [0023]    [0023]FIG. 2 is a block diagram depicting a RF digital signal transmission system supporting an enhanced dynamic range, in accordance with embodiments of the present invention;  
         [0024]    [0024]FIG. 3 is a sketch depicting enhanced NPR graphed as a function of the amplitude of input signals, through the digital signal transmission system of FIG. 2;  
         [0025]    [0025]FIG. 4A is a block diagram depicting the internal components of a first embodiment of the first RF Level Transforming circuit  250  in system  200  as shown in FIG. 2;  
         [0026]    [0026]FIG. 4B is a block diagram depicting the internal components of a first embodiment of the first RF Level Transforming circuit  250  in system  200  as shown in FIG. 2;  
         [0027]    [0027]FIG. 5 is a sketch depicting enhanced NPR graphed as a function of the amplitude of input signals, through the digital signal transmission system of FIG. 2;  
         [0028]    [0028]FIG. 6 is a block diagram depicting the internal components of a first embodiment of the optional second RF Level Transforming circuit  260  in system  200  as shown in FIG. 2. 
     
    
       [0029]    It should be noted that the same element numbers are assigned to components having the same, or approximately the same functions and structural features. Thus, elements in different figures and labeled with the same element number may be identical, or substantially similar in composition, structure and/or function, and where the function of such element has already been explained, there is no necessity for repeated explanation thereof in the detailed description.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    [0030]FIG. 2 is a block diagram depicting a RF digital signal transmission system for transmission of informational signals (i.e. informational signals carried into the system  200  as RF electronic signals RFin, and out of the system as RF electronic signals RFout) through an optical link  130 , and supporting an enhanced dynamic range, in accordance with embodiments of the present invention. The system  200  includes an external conductor  210  for carrying in radio frequency (RF) digital information signals as electronic signals RFin. The system  200  also includes an external conductor  211  for carrying out the radio frequency (RF) digital informational signals as electronic signals RFout. Embodiments of the system  200  may further include an intact RF transmitter-LINK-receiver system  100  of the related art. Alternatively, the active components (e.g.,  120 , and  140 ) of the related art system  100  may be modified so as to be physically and electrically integrated with additional components (e.g.,  250  and  260  respectively) that are unique to the inventive system  200 .  
         [0031]    As shown in FIG. 2, the system further comprises a first RF Level Transforming circuit  250 , adapted to sense (i.e., measure, integrate, or compute) the average power level (i.e., amplitude) of the electronic signals (RFin) entering the system during a predetermined (or during a dynamically variable) period of time (e.g., ranging from one second to several minutes), and to accordingly transform (i.e, amplify or attenuate) the level of such inputted electronic signals (before they are transmitted by the transmitter  120  through the link  130 ), in such a manner as to enhance the effective dynamic range of the included system  200  so that it is wider than the physical dynamic range of the included transmitter-link-receiver system  100 .  
         [0032]    The active components (e.g.,  250  and  260 ) unique to the system  200 , (which will be more particularly described below) are adapted to perform a method for enhancing the effective dynamic range of a system  100  including a transmitter  120 , an optical link  130 , and a receiver  140 . The method performed by the system  200  comprises: measuring during a period of time the original signal power level (i.e., amplitude) of RF electronic signals (RFin), wherein the RF electronic signals are to be transmitted as light signals by a transmitter  120  through an optical link  130  to a receiver that outputs the light signals as RF electronic signals; transforming (e.g., attenuating) the amplitude of such RF electronic signals to a transformed (e.g, attenuated amplitude) level before the RF electronic signals are transmitted as light signals by the transmitter  120 , whereby the noise power ratio (PR) of the transmitted RF electronic signals is greater than it would be if such attenuation had not been performed. The will generally also include a second transformation of the signals after the passing through the receiver  140 , such that the RF signals RFout output from the system  200  shall have approximately the original signal power level (i.e., amplitude). Thus, the second transformation at the output end (e.g., at  260 ) of the system  200  will perform the inverse transformation (mirror) as was performed at the input end (e.g., at  250 ).  
         [0033]    As an example, in the case of the exemplary NPR function (shown in FIG. 1B) for the transmitter-link-receiver system  100  included within system  200 , it may be observed that a signal having amplitude 50 dBmV will pass through the system  100  with an associated NPR of less than 40 (i.e., inadequate), while a signal having the smaller amplitude 30 dBmV will pass through the system  100  with an associated NPR of approximately 50. Thus, if the 50 dBmV signal were transformed (e.g., attenuated) down to 30 dBmV before passing through the system  100 , the emerging attenuated signal would theoretically be associated with an NPR of nearly 50 (e.g., the peak NPR value, at 30 bBmV).  
         [0034]    [0034]FIG. 3 is a sketch depicting exemplary NPR graphed as a function of the power level (i.e., amplitude) of input signals (RFin), through the digital signal transmission system  200  of FIG. 2. FIG. 3 further depicts how the ENHANCED DYNAMIC RANGE of system  200  may be compared with the “original dynamic range” of the included system  100  (which was first depicted in FIG. 1B). In practice, the process of attenuating the 50 dBmV of signals might to some extent introduce distortion and/or noise into the RF electronic signal, resulting in some reduction of the final NPR for the outputted signals. However, the 10 NPR-unit margin of “excess performance” (i.e., 50−40=10) (i.e., computed at the attenuated-to amplitude of 30 bBmV by subtracting the actual NMR value 50 from the predetermined minimum value 40) can be exploited to output RF signals from the system  200  with an NPR of at least 40 (e.g., about 45 as shown in the sketch of FIG. 3). Thus, by sacrificing the “excess” NPR, RF signals of amplitudes that would ordinarily not pass through the system  100  with sufficient NPR (i.e., signals of amplitudes beyond the physical dynamic range of the system  100 ) can be transformed (e.g., attenuated) and then passed though the system  100  with the requisite NPR (i.e., equal to or greater than 40).  
         [0035]    A pre-transmission attenuation scheme could be advantageously practiced (within limits imposed by the fidelity of real attenuating circuits) with signals of any amplitude greater than the top of the dynamic range (e.g., greater than 40 dBmV) of the system  100 . Accordingly, NPR values equal to or greater than 40 could be accorded to signals of amplitudes beyond (i.e., greater than) the dynamic range (e.g., greater than 40 dBmV) of the system  100 , thus effectively extending the upper limits of the effective dynamic range of the system  200  beyond the original dynamic range of system  100  (as illustrated in FIG. 3).  
         [0036]    Attenuation of electronic RF signals (RFin) of amplitudes beyond the dynamic range of the included system  100  could be carried out by many schemes and many circuits known to persons skilled in the art. For example the first RF Level Transforming circuit  250  might be designed such that all signals having amplitudes greater than a certain predetermined number of dBmV (e.g., greater than 35 dBmV) shall be selectively attenuated. Selective attenuation can be facilitated by some type of RF power level sensor, within the system  200  (e.g., within the first RF Level Transforming circuit  250 ) to detect the average RF power level of the RF signals passing through the system at that point in time and to generate a control signal to continuously control or intermittently trigger the attenuation.  
         [0037]    [0037]FIG. 4A is a block diagram depicting the internal components of a first embodiment of the first RF Level Transforming circuit of system  200  as shown in FIG. 2. The first RF Level Transforming circuit  250  includes at least an RF attenuator  255  adapted to attenuate RF power levels (amplitudes) of electronic signals (RFin) passed to the system  100 . The first RF Level Transforming circuit  250  may further include RF level sensor  251 . RF level sensor  251  could be an explicit component of the system  200  extrinsic to the RF attenuator  255  (as shown in FIG. 4A) or the RF power level sensing function could be performed as an implicit function of embodiments of an RF attenuator  255  itself.  
         [0038]    A discrete RF level sensor  251  could be operated as a digital switch enabling the RF attenuator  255  when the RF power level (i.e., amplitude) of RFin exceeds (and/or falls below) a predetermined threshold amount. Accordingly, a selective attenuation scheme might be implemented as a uniform attenuation scheme, such as where a constant (e.g., 10 dBmV) attenuation is applied to all signals of amplitudes greater than the predetermined of dBmV. The effect of such a selective but uniform attenuation scheme would likely extend the dynamic range upward by an amount approximately equal to the magnitude of the uniform attenuation. Such a “uniform attenuation scheme” need not be perfectly uniform over a range of RF power levels of RFin, nor even predictable (i.e., the amount of transformation of amplitude of signals RFin coming into the system  200  at a particular RF power level need not be predictable nor deterministic at any point in time), as long as the transformation (i.e., amplification/attenuation) scheme can be mirrored and/or the transformation counteracted in real time by equipment provided at the receiver end of the system  200 .  
         [0039]    The attenuation of the RF power level may also be linear and/or deterministic, e.g., proportional to the RF power level at conductor  210 . The RF sensor could produce a control signal that is proportional or approximately proportional to the RF power level. The RF attenuator  255  may be or operate like potentiometer controlled by the control signal, so as to increase the attenuation approximately in proportion to an increase in RF power level. The RF attenuator  255  may be implemented with a PIN diode (e.g., a PIN diode forward biased by a current controlled by the RF sensor). Alternatively, various linear and nonlinear attenuation schemes could be implemented within the circuit  250 , whereby the dynamic range of the system  200  could be extended above the upper limits of the dynamic range of system  100 . Accordingly, the RF level sensor  251  could be operated as an analogue sensor that continuously controls the magnitude of the attenuation performed by RF attenuator  255  so as to (proportionally, linearly, or otherwise deterministicly, or non-deterministicly) increase the attenuation (i.e., reduce the amplitude of the high RF level signal passing to system  100 ) as the RF power level of RFin increases towards or above the top of the dynamic range of system  100 .  
         [0040]    [0040]FIG. 4B is a block diagram depicting the internal components of a second embodiment of the first RF Level Transforming circuit  250  in system  200  as shown in FIG. 2. In alternative embodiments of the invention, the first RF Level Transforming circuit  250  may further include an RF amplifier  254  adapted to amplify RF power levels (amplitudes) of electronic signals (RFin) passed to the system  100 . The RF amplifier  254  could be used to amplify the RFin signal at times when the RF power level (i.e., amplitude) of signals RFin is below the lower bounds of the dynamic range of the system  100 , so as to extend the dynamic range of the system  200  lower than the lower bound of the dynamic range of system  100  (as shown in FIG. 5). The RF level sensor  251  could be adapted to control the operation of the RF amplifier  254 , in an inverse manner as it is used to control the operation of the RF attenuator  255 . In typical embodiments, only one of the RF amplifier  254  and RF attenuator  255  will be substantially active at any given point in time (i.e. typically, the RF attenuator  255  will not significantly affect the amplitude of the signals while the RF amplifier  254  is active, and vice versa, and various schemes for avoiding contention between the RF attenuator  255  and the RF amplifier  254  are know to persons skilled in the art. In some embodiments of the invention, a single circuit component may perform the functions of both the RF attenuator  255  and the RF amplifier  254 , alternating between such functions depending upon the output of the RF level sensor  251 ). Providing an RF amplifier within the first RF Level Transforming circuit  250  facilitates the downward enhancement of the dynamic range of system  200  to below the lower limit of the dynamic range of system  100 .  
         [0041]    [0041]FIG. 5 is a sketch depicting enhanced dynamic range of embodiments of system  200  that include Amplifier  254  graphed as a function of the amplitude of RF input signals, through the digital signal transmission system of FIG. 2. The enhanced dynamic range of the system  200  is extended below the dynamic range of system  100  by amplifying RF signals (RFin) having amplitudes below the dynamic range of system  100 .  
         [0042]    [0042]FIG. 6 is a block diagram depicting the internal components of the optional second RF Level Transforming circuit  260  in system  200  as shown in FIG. 2. In some embodiments of the invention, the system  200  may comprise an (optional) second RF Level Transforming circuit  260  adapted to reverse the transformation (i.e., to provide amplification to exactly counteract a signal attenuation performed by the first RF Level Transforming circuit  250 ; or to provide attenuation to counteract an amplification performed by the first RF Level Transforming circuit  250 ). The second RF Level Transforming circuit  260  can mirror the first RF Level Transforming circuit  250  and adjust its Amplification/Attenuation so as to maintain (at RFout) the same RF power level (i.e., amplitude) as was input to the system  200  (at RFin). The result of this is to enhance the dynamic range of the system  200  while also providing RFout at the original RF power level (i.e., amplitude) as was originally received at RFin. The second RF Level Transforming circuit  260  may be particularly useful in CATV systems wherein Automatic Gain Controllers (AGCs) are present, to avoid contention of the system  200  with such AGCs.  
         [0043]    In embodiments of the invention where the second RF Level Transforming circuit  260  is supplied for maintaining the RF power level at RFout, it is desirable that a continuous communication exists between the first RF Level Transforming circuit  250  and the second RF Level Transforming circuit  260 . The RF level sensor  251  may be adapted to output a signal that indicates the power level of RFin and which can be converted to an RF signal encoding that power level information which can be injected into the system  100  and then transmitted through the optical link ( 130 , see FIG. 2) to a RF decoder  261  in (or operatively coupled to) the second RF Level Transforming circuit  260 . The RF decoder  261  will decode the RF signal encoding that power level information, and supply that information to control the RF Amplifier  264  and/or RF Attenuator  265  within the second RF Level Transforming circuit  260 , so as to maintain the power level (i.e., amplitude) of RFout the same as the power level of RFin. The RFout signals will generally be faithful reproductions of the original digital signals (RFin) without significant changes in amplitude (i.e. no great net gain nor attenuation) except for any noise and/or distortion introduced during passage through the system  200 . Generally, the amplitude (i.e., RF power level) of the RF digital signals (RFout) emerging from the system  200  will be approximately the same as the RF power level of the same digital signals (RFin) when they entered the system  200 . Typically, the RF power level (i.e., amplitude) of the emergent RFout signals will be within plus or minus one-half decibels (±0.5 dB) of the RF power level of RFin. Embodiments of the system  200  may be also be designed so that the RF power level (i.e., amplitude) of the emergent RFout signals will be within plus or minus one-quarter decibels (±0.25 dB) of the RF power level of RFin, etc.  
         [0044]    Alternative embodiments of the invention may be implemented by adapting the circuits disclosed in commonly assigned co-pending US patent application No. ______, filed ______ 2001, (titled, RF LEVEL STABILIZATION OF OPTICAL LINK OVER TEMPERATURE) and incorporated herein by reference. For example, the “RF sensor” in FIGS. 4 and 5 of the co-pending application may be adapted to perform the functions of sensor  251  of FIG. 2 herein, and be operatively connected to the RF Amplifier and RF Attenuator circuits depicted in FIG. 4 and FIG. 5 of the co-pending application, to enable those circuits to perform the methods of the present invention (i.e., to enhance the dynamic range of the transmitter-optical link-receiver system  400  and  500 ) included in FIGS. 4 and 5 of the co-pending application. The RF power level measured by an RF power level sensor may be encoded (e.g., by modulator  520  of FIG. 5) and transmitted through the optical link to control the RF Amplifier and RF Attenuator at the RF output end of the system. The RF Level Stabilization circuits of the co-pending are well adapted to implement the method of the present invention, wherein the RF power level (i.e., amplitude) supplied to the transmitter (e.g., “laser”  142 ) is maintained at a constant RF level. Optimally, the constant RF level would be at or near the “peak” of the NPR curve of the transmitter-optical link-receiver system. The selection of the RF level at or near the “peak”of the NPR curve would tend to maximize the dynamic range of the constant-level RF digital signal transmission system disclosed in FIG. 4 and FIG. 5 of the co-pending application.  
         [0045]    Embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.