Patent Publication Number: US-8537941-B1

Title: Bit-synchronous feedback control of receiver sensitivity

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/361,692, filed Jul. 6, 2010 and herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to receivers used in digital systems and, more particularly, to an arrangement for providing bit-synchronous modulation of feedback within the receiver in order to improve the sensitivity of the receiver. 
     BACKGROUND OF THE INVENTION 
     Digital data is stored and transferred for many applications. Communications is becoming more and more digital. For example, telephony, which was originally analog throughout the world is now almost totally digital. Also, musical performances are now digitally recorded and distributed on CDs. Visual material that had previously been distributed via film and/or VHS tape is now contained on DVDs and Blue Ray disks; television broadcasting has now also entered the digital domain. 
     Analog “front ends” for digital receivers are thus needed to support the various digital technologies. When a CD, DVD or hard drive is accessed, a receiver is needed to boost the relatively weak signal into a stronger representation that may be properly interpreted in the digital domain. In some cases (such as, for example, with a CD or DVD), a laser is focused on a moving track that causes the light that reaches the receiver to vary as a function of time. The receiver includes a type of opto-electronic conversion device (usually, a photodiode) that converts the received optical signal into a relatively weak electrical signal. One task of the receiver is to amplify this weak signal to a level sufficient to discriminate between the transmission of digital “one&#39;s” and digital “zero&#39;s”. 
     Receivers are also a necessary component of communication systems. For example, in a digital fiber-optic link, an optical signal is conveyed over considerable distances by an optical fiber. In some configurations, an optical receiver converts this relatively weak optical signal into a (weak) electrical signal via, for example, a photodiode; electrical amplification is then used to increase the level of the electrical signal to a point where it can be reliably converted into a digital stream of one&#39;s and zero&#39;s. 
     All of the above examples require digital, baseband receiver circuits to convert a weak electrical signal into a more robust form that can be readily converted into a digital stream of one&#39;s and zero&#39;s. A baseband receiver, in contrast to a tuned broadcast receiver, operates with signals where the important frequency range extends from nearly DC (zero frequency) to a level comparable with the bit rate used for transmission. 
     For a given type of input signal (i.e., optical or electrical) and data encoding method, the main figure of merit for a receiver is its sensitivity. Receiver sensitivity in the digital domain is related to the Bit Error Rate (BER) of the system. When a weak digital signal is received, an occasional bit may be incorrectly labeled (namely, defined as a digital “one” when a digital “zero” was originally transmitted, or vice versa). The ratio of mis-labeled bits to the total number of bits received is thus defined as the BER. For example, in the Ethernet Standards (IEEE 802.3), the minimum level of performance is specified as a BER of 10 −12 . Modern digital data systems now require a BER on the order of 10 −6  to 10 −12 , depending upon the application. Stated differently, the receiver sensitivity is the minimum signal power needed to achieve the specified BER. If the receiver is presented with a signal below its sensitivity limits, it will have a much higher BER, since random fluctuations (generally attributed to “noise sources” in the receiver) will cause the apparent signal to be mis-interpreted upon occasion. 
     Receiver sensitivity is important inasmuch as greater sensitivity may indicate that a specific receiver will operate under a greater number of adverse conditions (e.g., poor media—in the case of digital data recordings; greater distances—in the case of transmission systems). For example, in the case of optical fiber transmission systems, a 2 dB improvement in receiver sensitivity may lead to an increase of 5-10 km in the transmission distance signal range. 
     Optimizing receiver sensitivity is not trivial, and receiver circuits are often designed as a compromise between competing elements, such as noise versus bandwidth. A need therefore remains for a receiver configuration that is not constrained by the competing interests of maintaining a low noise level and a relatively wide operating bandwidth. 
     SUMMARY OF THE INVENTION 
     The needs remaining in the prior art are addressed by the present invention, which relates to a receiver arrangement and, more particularly, to a receiver with increased sensitivity realized by improving the noise performance of the receiver without excessive spreading of the received bits into the neighboring bits (as might be caused, for example, by just reducing the bandwidth). 
     In accordance with the present invention, a receiver is configured to modulate the parameters of the circuit in synchronization with the incoming data signal so as to adjust its sensitivity in a real-time fashion. In particular, a dynamic feedback arrangement is utilized that functions to increase the receiver&#39;s bandwidth at the leading edge of each data bit, thus improving its high speed response. The dynamic feedback arrangement will also decrease the receiver&#39;s bandwidth in the middle of each bit, reducing the noise level present in the signal. By dynamically adjusting the receiver&#39;s bandwidth as a function of location along the “width” of the received bit, the sensitivity of the receiver is significantly increased. A further improvement in operation may be obtained by optimizing the shape of the change in bandwidth curve as a function of time. 
     In accordance with one embodiment of the present invention, a variable feedback resistance element is used to dynamically adjust the bandwidth. A field-effect transistor (FET) may be utilized for this purpose, with a control signal synchronized with the incoming bit rate utilized to match the changing resistance in the feedback path with the location along the width of an incoming bit. Various shunt and series resistance elements may be used in conjunction with the FET to limit the excursions in the feedback resistance value. 
     It is the intent of the present invention to use time variations of circuit elements within the amplifier to approach the theoretically-optimum filter shape that is known to optimize the sensitivity of the receiver. Additionally, overload of the circuit itself is addressed by gradually reducing the amount of feedback as the signal strength improved, thus increasing the receiver&#39;s dynamic range. 
     Other and further features and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, where like numerals represent like elements in different embodiments, 
         FIG. 1  illustrates an exemplary prior art transimpedance amplifier (TIA) receiver gain stage; 
         FIG. 2  contains a plot of sensitivity as a function of effective transimpedance for a both a conventional receiver (graph I) and a variable feedback receiver of the present invention (graph II); 
         FIG. 3  illustrates an exemplary amplifying section  10  of a receiver formed in accordance with the present invention; 
         FIGS. 4(   a )- 4 ( d ) show a set of four different variable resistive feedback arrangements that may be used in the arrangement of  FIG. 3 ; 
         FIG. 5  illustrates various types of waveforms that may be used as the bit-synchronous control signal input C(t) to the variable feedback resistance element in the arrangement of the present invention; 
         FIGS. 6(   a ) and ( b ) illustrate the effect of duty cycle, saturation and cutoff on control signal C(t); 
         FIG. 7  shows an embodiment of the present invention including a DC bias source to control the duty cycle of control signal C(t); 
         FIG. 8  shows yet another embodiment of the present invention, in this case incorporating an input level monitor to adjust and control the magnitude of control signal C(t) as a function of the level of the incoming digital signal; 
         FIG. 9  illustrates an exemplary optical receiver incorporating the bit-synchronous feedback control of receiver sensitivity of the present invention; 
         FIG. 10  depicts a different synchronization method that may be used in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As is known in the art, improving receiver sensitivity is related to maximizing the signal-to-noise ratio (SNR) for the weak signals presented to the receiver. An important aspect to maximizing the SNR is to minimize noise sources at locations where the data signal is weakest, for example, at the input to the receiver gain stage. 
       FIG. 1  illustrates an exemplary prior art receiver gain stage  1  in the form of a transimpedance amplifier (TIA)  2  and a resistive feedback element  3 . Feedback element  3  provides a feedback current I f  in response to the output voltage V out  from TIA  2 . Assuming that the gain of TIA  2  is very large, the effect of this feedback is that output voltage V out  is defined as follows:
 
 V   out   =I   in   *R,  
 
where R is the resistance value associated with element  3  and I in  is defined as the input current applied to TIA  2 .
 
     It is to be understood that the signs of I in  and V out  are selected to be of opposite polarity; that is, if I in  is flowing away from TIA  2 , voltage V out  will be positive (with respect to ground). In this, the feedback provided by resistive element  3  is “negative” and if the value of R is small enough, the associated gain vs. frequency curve will be very flat and extend to very high frequencies. In actual implementation, the gain falls off since at higher frequencies the intrinsic gain of the amplifier falls to a point that even without negative feedback, the transimpedance is insufficient to satisfy the above relationship. 
     It follows that for a given value of I in , making the value of R large will increase the output voltage V out . However, output voltage V out  cannot be increased indefinitely, since the intrinsic gain of TIA  2  is finite and the output voltage can never exceed the supply voltage. Additionally, the bandwidth of the circuit will diminish for large values of R as a result of the gain-bandwidth limitations of TIA  2  (as well as the parasitic circuit capacitances between the input and ground, or between the input and output (i.e., across resistive element  3 ). 
     As alluded to earlier, the SNR is a critical issue. When the value of resistive element  3  is increased, the feedback signal is decreased. The noise that is generated by TIA  2  is also likely to decrease and thus improve the SNR. 
     Noise may be introduced by the presence of resistive element  3  itself, an effect known as “Johnson-Nyquist” noise (or, variously, “thermal noise”, “Johnson noise” or “Nyquist noise”) and is attributed to Brownian (thermal) motion of the charge carriers. Specifically, this type of noise can be defined as follows:
 
ν 2 =4 *k   b   *T*R,  
 
where k b  is Boltzmann&#39;s constant, T is the temperature of resistive element  3  (measured in Kelvin), R is the resistance value of element  3  measure in ohms and ν 2  is mean square noise per unit bandwidth measured in Hz. Inasmuch as the SNR improves with increasing R, the effective bandwidth is decreasing until eventually the bandwidth is no longer sufficient to pass the signal. At this point the SNR ratio no longer improves with increasing R, but reaches a peak and begins to decline with increasing R.
 
       FIG. 2  contains an experimental plot of optical sensitivity as a function of effective transimpedance R, showing how the (non-time-varying) receiver sensitivity varies for selected values of R. At relatively small values of transimpedance (left-hand portion of curve I), the sensitivity is poor since the signal is weak compared to other noise sources in the receiver. As the transimpedance is increased (moving from left to right along the curve), the sensitivity increases to a point P, defined as the optimum sensitivity point. 
     As shown, further increases in R become detrimental; the noise is reduced but the signal becomes badly affected by the continuing reduction in bandwidth. The reduction in bandwidth in turn causes the signal energy during a particular bit period to “spread” increasingly into the neighboring bit periods. This spreading not only reduces the magnitude of the bit during its time interval, but introduces its energy into other bit times, resulting in inter-symbol interference (ISI). ISI results in requiring a larger input signal to overcome the noise, thus once again degrading the receiver sensitivity. In extreme cases, ISI can produce data errors even in the absence of noise. 
     In light of this analysis, the conventional wisdom has been that the best sensitivity to be expected from a receiver is that given by point P on curve I—the ‘best sensitivity’ point. However, as will be discussed in detail below, further improvements in receiver sensitivity are obtained in accordance with the present invention by dynamically adjusting the feedback of the TIA during the bit period. 
     Indeed,  FIG. 3  illustrates an exemplary amplifying section  10  of a receiver, formed in accordance with the present invention, that utilizes a variable feedback resistance element  12  between the output node  14  and input node  16  of a transimpedance amplifier (TIA)  18  to provide a variable feedback current I f (t) at input node  16 . As shown in  FIG. 3 , variable feedback resistance element  12  is controlled by an incoming control signal C(t) in a manner such that the value of the resistance is synchronous with (and changes as a function of) the bit rate of the incoming signal I in . As will be discussed in detail below, there are many suitable circuit arrangements that are capable of providing the dynamic resistive value as a function of bit rate. In this case, the sensitivity of the dynamic system is improved by about 2 dB over the static TIA, shown by the plot II in the graph of  FIG. 2 . 
     Various exemplary arrangements for variable feedback resistance element  12  are illustrated in the diagram of amplifying section  10  as shown in  FIGS. 4(   a )-( d ). In each of these embodiments, a field effect transistor (FET)  20  is used as the basic component in variable feedback resistance element  12 . An FET device is a very attractive way to provide an approximately resistive value that can be rapidly varied and, therefore, is considered as a preferred embodiment of the present invention. 
       FIG. 4  ( a ) illustrates the simplest embodiment of element  12 , with an FET  20  utilized to provide the variable resistance and, as a result, a variable feedback signal I f (t). As shown, a gate electrode  22  of FET  20  is used to receive a bit-rate synchronous control signal C(t), with a drain electrode  24  of FET  20  coupled to output node  14  and a source electrode  26  coupled to input node  16  of TIA  18 . Depending upon the voltage applied between gate electrode  22  and gate electrode  26 , FET  20  will exhibit an effective resistance that varies from a relatively few ohms (when in “saturation” mode) to extremely high values (when in “cutoff” mode). Thus, depending upon where FET  20  is between these two extreme modes, the effective resistance and feedback control signal I f (t) can be varied in a somewhat controllable manner. 
     In order to provide some additional control to the range of resistive values, the arrangement of  FIG. 4(   b ) illustrates the addition of a shunt resistor  30  between source  26  and drain  24  of FET  20 . The presence of resistor  30  limits the feedback resistance of variable feedback resistance element  12  so as to not exceed the specific value of shunt resistor  30 , even when FET  20  is in cutoff mode.  FIGS. 4(   c ) and  4 ( d ) illustrate alternative embodiments where a resistor is added in series with FET  20 . In particular,  FIG. 4(   c ) illustrates an arrangement with a series resistance  32  added between source  26  of FET  20  and input node  16 .  FIG. 4(   d ) illustrates an arrangement with a series resistance  34  added between drain  24  of FET  20  and output node  14 . The embodiments of  FIGS. 4(   c ) and ( d ) provide limitations on the other end of the feedback resistance value of element  12 , that is, providing a lower limit on how small the feedback resistance can become, even when FET  20  is in saturation. 
       FIG. 5  illustrates various types of waveforms that may be used as the bit-synchronous control signal input C(t) to variable feedback resistance element  12 . The waveform shown in  FIG. 5(   a ) may be generated from a voltage-controlled oscillator (VCO) that is locked to the bit rate, where the waveform of  FIG. 5(   b ) may be created as a result of passing the signal of  FIG. 5(   a ) through a low-pass filtering arrangement that will “round” the corners of the signal. The waveform of  FIG. 5(   c ) is even more rounded than that of  FIG. 5(   b ), essentially sinusoidal, and can be created through additional low-pass and/or narrowband filtering. Other variations, shown in  FIGS. 5(   d ) and ( e ) may be used. In any embodiment, it is to be understood that the phase of control signal C(t) relative to the incoming signal I L , is very important, and may need to be actively adjusted in some embodiments so that the adjustments in receiver bandwidth remain synchronized with the location along the width of an incoming bit (i.e., higher bandwidth at leading edge of bit, lower bandwidth in middle of bit). 
     In the preferred embodiment of the present invention, the duty cycle of control signal C(t) is not 50%. Indeed, it has been found that improved receiver sensitivity is obtained when variable feedback resistance element  12  is configured to exhibit a relatively “low” resistance at each bit transition and a “high” resistance otherwise (not only in the center in the bit). The plots as shown in  FIG. 6  illustrate this aspect of the present invention. 
       FIG. 6(   a ) contains a plot of an exemplary waveform for a control signal C(t), where dotted line A illustrates the effective of shunt resistor  30  (as shown in  FIG. 4(   b )) on “clamping” the uppermost excursions of the waveform (saturation mode of FET  20 ). Dotted line B similarly represents the effect of series resistors  32  or  34  ( FIG. 4(   c ) or ( d )) on “clamping” the lower limit of waveform C(t), the cutoff mode of FET  20 . When both types of impedance limitations are utilized, the resulting waveform takes the form of bounded control signal C B (t) as shown. 
     In the plot of  FIG. 6(   b ), the initial incoming control signal C(t) is shown as being shifted upward in the positive direction by applying a predetermined DC bias voltage to control signal C(t). Saturation and cutoff levels A and B remain the same as for the arrangement of  FIG. 6(   a ) and, as a result, the resulting maximum resistance portion R max  of bounded waveform C B (t) extends over a longer portion of the bit duration, when compared to waveform C B (t) of  FIG. 6(   a ). Similarly, the minimum resistance portion R min  of waveform C B (t) is of shorter duration than that associated with waveform C B (t) of  FIG. 6(   a ). Thus, it is shown that by varying the DC bias associated with the incoming bit-synchronous control signal, the duty cycle of the signal will vary as well, with this variation used to control the feedback resistance of TIA  18 . 
       FIG. 7  illustrates an embodiment of the present invention that includes a variable DC bias source  35  disposed along the signal path of control signal C(t). It is also be to noted that as the input signal level increases away from the sensitivity point, the need for the bit synchronous control signal decreases. Thus, it is considered reasonable to decrease the magnitude of control signal C(t) with increasing input power, and/or shift the DC bias level accordingly.  FIG. 8  illustrates another embodiment of the present invention, incorporating a power monitor  37  that is used to look for a predetermined power threshold and function to decrease the magnitude of control signal C(t) when this threshold is crossed. 
       FIG. 9  illustrates an exemplary optical receiver  40  incorporating the bit-synchronous feedback control of receiver sensitivity of the present invention as described above. As shown, an incoming optical signal O is passed through a photodiode  42  to create a low-level (weak) electrical current output signal, defined above as I in . As described above, signal I in  is applied as an input to, and amplified by, amplifying circuit  10 , which provides amplified output voltage V out . Amplified output voltage V out  is thereafter applied as a first input to a decision circuit  44 . A second input to decision circuit  44  is a threshold voltage, V th , where the determination of whether a digital “zero” or “one” has been received is based upon a comparison of V out  to V th . The stream of digital output signals (of one&#39;s and zero&#39;s) from decision circuit  44  is then applied as the signal input to a data latch  46 , which creates a properly “clocked” stream of output pulses by controlling data latch  46  with a clock signal, denoted CLK in  FIG. 9 . 
     As also shown in  FIG. 9 , a clock delay distribution circuit  48  is utilized to provide bit-synchronized control signals to both variable feedback resistive element  12  (used as control signal C(t) in the manner described above) and data latch  46 . The delay associated with circuit  48  is controlled to align the time-variable feedback applied to element  12  during the most opportune time for the incoming data signal, as well as to generate the different delay required to properly “clock” the output from decision circuit  44  to most accurately recreate the transmitted data signal with a minimal BER. 
     In situations where no local clock exists and bit-synchronous control signal C(t) must be derived from a local clock recovery device, the output of clock delay and distribution circuit  48  is gated “on” or “off” by the state of clock recovery lock detect circuits (not shown). In the event that the clock recovery is not able to lock onto a clock, then control signal C(t) is turned “off”. When the clock recovery circuit asserts a successful lock, then clock delay and distribution circuit  48  is gated “on” and bit-synchronous control signal C(t) is applied to variable resistive feedback element  12 . 
       FIG. 10  depicts a different synchronization method that may be used in accordance with the present invention and that does not require a “loss of clock” inhibit gate circuit. In this case, a master clock  90  is utilized, where master clock  90  may be disposed at any location with respect to a data source  92  and a receiver  94 , where an exemplary receiver  94  includes the bit-synchronous feedback arrangement of the present invention (not shown) for controlling receiver sensitivity. Master clock  90  is shown as applied to clock the incoming data from data source  92  along a first signal path  96 , and similarly propagates along a signal path  98  to be applied as a clocking input to receiver  94 . Because the relative delays in the transmission of the clock over signal paths  96  and  98 , as well as the range of possible delays introduced by a communication channel  100 , it is recommended that there be a provision to vary the delay of the received clock (over a range of, for example, at least one bit interval) in order to maximize performance. For example, a microcontroller that controls an electronic delay circuit can be used. 
     As will be apparent to those skilled in the art, numerous modifications may be made within the scope of the invention, which is not intended to be limited except in accordance with the following claims.