Burst-mode receiver having a wide dynamic range and low pulse-width distortion and a method

A burst-mode Rx is provided that has a wide dynamic range, low pulse-width distortion and low technological overhead. The Rx is capable of processing signals having levels that range from low noise levels up to high noise levels. In addition, the Rx is capable of quickly and simultaneously adapting the TIA gain and the bit decision threshold level, thereby eliminating the need to transmit and receive a training bit sequence prior to transmitting and receiving actual data. By simultaneously adapting the TIA gain and the bit decision threshold level on the first bit of actual data received in the Rx, the Rx is capable of being used with short packets and with packets of varying lengths transmitted from different types of transmitters located in the same network.

TECHNICAL FIELD OF THE INVENTION

The invention relates to receivers. More particularly, the invention relates to a burst-mode receiver.

BACKGROUND OF THE INVENTION

A typical optical receiver (Rx) includes at least one photodiode that detects an optical signal and converts it into an electrical signal and at least one transimpedance amplifier (TIA) that converts the electrical signal into a voltage signal. The photodetector, which is typically a P-intrinsic-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA converts this electrical current signal into an output voltage signal having some gain, commonly referred to as transimpedance gain. This output voltage signal is further processed by other stages (i.e., a limiting amplifier (LA), clock and data recover (CDR), etc.) of the Rx.

The TIA handles input signals (the photodiode output) of varying optical modulation amplitude (OMA) and average power (Pavg), and therefore needs to have a wide input dynamic range. Wide input dynamic range is typically achieved by incorporating an automatic gain control (AGC) circuit in the RX portion for automatically adjusting the gain of the TIA based on the amplitude of the input signal. If the Rx does not include an AGC circuit, the TIA of the Rx will try to use its transimpedance gain to convert the current into a corresponding output voltage as the amplitude of input current signal increases. When this happens, however, the transimpedance gain is limited by the voltage headroom (the maximum high and low output voltage for linear operation of the TIA) as the output voltage swing increases, which results in the output signal becoming distorted. Hence, an AGC circuit is needed in order to lower the gain of the TIA as the amplitude of the input signal grows so as to prevent the TIA from saturating and producing distortion at its output.

Burst-mode optical Rxs are used in networks in which optical signals of various optical power levels and phases (timeslots) are transmitted from various sources. The TIA used in a burst-mode optical Rx should be capable of handling such optical signals. Although it is known to use AGC circuits in burst-mode optical Rxs for automatically adjusting the gain of the TIA based on the incoming signal, existing solutions generally have large pulse-width distortions and limited dynamic range, especially for the first bit received after a long period of quiescence.

Moreover, existing solutions typically require transmission of a training bit sequence to the Rx prior to the data being transmitted. The training bit sequence is then processed in the Rx to set the TIA gain and the decision threshold value. Use of the training bit sequence increases processing overhead and reduces the effective data rate of the optical link.

Accordingly, a need exists for a burst-mode Rx that is capable of adapting both the gain and the decision threshold simultaneously and very quickly to obviate the need to transmit and receive a training bit sequence.

SUMMARY OF THE INVENTION

The invention is directed to a burst-mode Rx and methods for use in a burst-mode Rx. The burst-mode Rx has a wide dynamic range and low pulse-width distortion. In accordance with an illustrative embodiment, the Rx is configured to adapt a gain of a TIA circuit of the Rx and to adapt a decision threshold voltage of a bit decision circuit of the Rx substantially simultaneously based on a first bit of data of a data transmission received in the Rx, thereby obviating a need to transmit a training bit sequence to the Rx prior to transmitting the data transmission to the Rx.

In accordance with an illustrative embodiment, the burst-mode Rx comprises a detector, a TIA circuit, an AGC circuit, a threshold generation circuit, a bit decision circuit, and a timing adjustment circuit. The detector is configured to receive an input signal and to output an electrical detection signal. The TIA circuit receives the electrical detection signal output by the optical detector. The TIA circuit comprises at least a first variable resistor having a resistance value that can be varied to cause a gain of the TIA circuit to be varied. The TIA circuit outputs an output voltage signal, VOUT, having a value that is based at least in part on the gain of the TIA circuit. The AGC circuit receives VOUT. The AGC circuit has a controller circuit that causes an AGC output voltage signal, VAGC, having a value that is set based at least in part on VOUTto be output from the AGC circuit to the TIA circuit. The resistance value of the variable resistor is varied based on the value of VAGC. The threshold generation circuit receives VOUTand produces a bit decision threshold value, VTHDEC, based on VOUT. VTHDECis output from the threshold generation circuit. The threshold generation circuit varies VTHDECbased on variations of VOUT. The bit decision circuit receives VOUTfrom the TIA circuit, receives VTHDECfrom the threshold generation circuit, compares VOUTto VTHDEC, produces a bit decision signal that is based on the comparison of VOUTto VTHDEC, and outputs the bit decision signal from the bit decision circuit. The timing adjustment circuit receives the bit decision signal from the bit decision circuit, adjusts a pulse width of the bit decision signal to reduce or eliminate pulse-width distortion in the bit decision signal, and outputs a data output signal.

In accordance with another embodiment, the burst-mode Rx comprises a detector, a TIA circuit, a threshold generation circuit, a bit decision circuit, and a timing adjustment circuit. The detector is configured to receive an input signal and to output an electrical detection signal. The TIA circuit receives the electrical detection signal output by the detector. The TIA circuit comprises at least a first variable nonlinear resistor having a resistance value that varies over a range of resistance values to cause a gain of the TIA circuit to be varied. The TIA circuit outputs an output voltage signal, VOUT, having a value that is based at least in part on the gain of the TIA circuit. The gain variations caused by the resistance value variations provide an AGC function for the TIA circuit that obviates the need for an AGC circuit. A threshold generator receives VOUTand produces a bit decision threshold value, VTHDEC, based on VOUT. VTHDECis output from the threshold generator. The threshold generator varies VTHDECbased on variations of VOUT. The bit decision circuit receives VOUTfrom the TIA circuit, receives VTHDECfrom the threshold generator, compares VOUTto VTHDEC, produces a bit decision signal that is based on the comparison of VOUTto VTHDEC, and outputs the bit decision signal from the bit decision circuit. The timing adjustment circuit receives the bit decision signal from the bit decision circuit, adjusts a pulse width of the bit decision signal to reduce or eliminate pulse-width distortion in the bit decision signal, and outputs a data output signal.

In accordance with an illustrative embodiment, the method comprises:

receiving an input signal with a detector and outputting an electrical detection signal;

with a TIA circuit having a variable gain, receiving the electrical detection signals produced by the detector;

outputting an output voltage signal, VOUT, from the TIA circuit, where VOUThas a value that is based at least in part on the gain of the TIA circuit;

with an AGC circuit, producing an AGC output voltage signal, VAGC, having a value that is set based on VOUT;

outputting VAGCfrom the AGC circuit to the TIA circuit;

in the TIA circuit, varying a resistance value of a variable resistor of the TIA circuit based on the value of VAGCto cause the gain of the TIA circuit to vary;

in a threshold generation circuit, producing a bit decision threshold value, VTHDEC, based on VOUT, where the threshold generation circuit varies VTHDECbased on variations of VOUT;

in a bit decision circuit, comparing VOUTto VTHDECand producing a bit decision signal that is based on the comparison; and

with a timing adjustment circuit, adjusting a pulse width of the bit decision signal to reduce or eliminate pulse-width distortion in the bit decision signal and outputting a data output signal.

In accordance with an illustrative embodiment, the method comprises:

with a detector, receive an input signal and outputting an electrical detection signal;

with a TIA circuit comprising at least a first variable nonlinear resistor having a resistance value that varies over a range of resistance values to cause a gain of the TIA circuit to be varied, receiving the electrical detection signal outputted by the detector, where the TIA circuit;

outputting an output voltage signal, VOUT, from the TIA circuit, where VOUThas a value that is based at least in part on the gain of the TIA circuit, and where the gain variations that are caused by the resistance value variations provide an AGC function for the TIA circuit that obviates the need for an AGC circuit;

in a threshold generator, receiving VOUTand producing a bit decision threshold value, VTHDEC, based on VOUT, where the threshold generator varies VTHDECbased on variations of VOUT;

in a bit decision circuit, comparing VOUTto VTHDECand producing a bit decision signal that is based on the comparison of VOUTto VTHDEC; and

with a timing adjustment circuit, adjusting a pulse width of the bit decision signal to reduce or eliminate pulse-width distortion in the bit decision signal and outputting a data output signal.

These and other features of the invention will become apparent from the following description, drawings and claims.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

In accordance with embodiments of the invention, a burst-mode Rx is provided that has a wide dynamic range, low pulse-width distortion and low technological overhead. The Rx is capable of processing signals having levels that range from noise levels on the low end up to high signal levels (i.e., generally no noise) on the high end. In addition, the Rx is capable of quickly and simultaneously adapting the TIA gain and the bit decision threshold level, thereby eliminating the need to transmit and receive a training bit sequence prior to transmitting and receiving actual data. In accordance with illustrative embodiments described herein, the Rx is configured to simultaneously adapt the TIA gain and the bit decision threshold level on the first bit of actual data received in the Rx, which is not possible in the aforementioned known solutions. This feature also allows the Rx to be used with short packets and with packets of varying lengths transmitted from different types of transmitters located in the same network.

These features also make the burst-mode Rx an ideal solution for plastic optical fiber (POF) networks that employ passive optical star couplers. In such networks, the transmit power can be high due to the large overdrive capability of the back-to-back Rx configurations used in the networks. The wide dynamic range of the optical Rx makes it well suited for use in such networks. In addition, because the burst-mode optical Rx has very high sensitivity, it is well suited for use in networks that have very large numbers of ports, long fibers, or links that employ many connectors. The Rx, however, is not limited to being employed in such networks, as will be understood by those of skill in the art in view of the description being provided herein.

Embodiments of the invention combine known peak detection and fast gain setting techniques with techniques for post processing of the pulse width. Decision threshold and gain management circuits of the Rx ensure that the bit decision threshold value is maintained at an optimum level for the complete dynamic range of the Rx. Because the use of adaptive threshold techniques result in the detected bits being enlarged in time, a post-processing circuit of the Rx processes the pulse width after the bit decision is made in order to shorten the pulse width to thereby reduce or eliminate pulse width distortion over the entire dynamic range of the Rx. In accordance with illustrative embodiments, the dynamic range of the Rx ranges from the natural noise level on the low end of the range to several milliamperes (mA) of photocurrent on the high end of the range.

Due to the various approaches that can be taken for providing fast gain settling, two different illustrative topologies are presented herein, which are referred to herein as Topologies A (FIGS. 1 and 2) and B (FIGS. 3 and 4). In topology A, the feedback resistors of the TIA circuit are actively steered by an AGC circuit to keep the output voltage of the TIA circuit at a constant level for input currents above the AGC threshold level. In topology B, a nonlinear resistive element is provided in parallel with the conventional linear feedback resistor of the TIA circuit to reduce the gain of the TIA circuit for increasing photocurrents. In both topologies, these TIA gain adjustment features are used in combination with features for fast adaptation of the bit decision threshold level and post processing of the pulse width to achieve the aforementioned goals and advantages. The different topologies have different advantages that make them suitable for use in different types of network configurations or in networks that have different bandwidth ranges, as will be described below in more detail. Illustrative, or exemplary, embodiments will be described below with reference toFIGS. 1-4, in which like reference numerals represent like components, elements or features.

FIG. 1is a schematic diagram of the burst-mode optical Rx1in accordance with a first illustrative embodiment in which a single-ended, Topology A configuration is employed. An optical input signal2stimulates an electrical current in a photodiode3of the optical Rx1. For the case where the input signal2is small, an AGC circuit4of the optical Rx1is in an inactive state. When the AGC circuit4is in the inactive state, a TIA circuit5of the optical Rx1is operating with its highest gain, and a voltage-controllable or current-controllable variable resistor6of the TIA circuit5is in a high-impedance state. In accordance with Topology A, the resistance value of the voltage-controllable or current-controllable variable resistor6is steered by the AGC circuit4. An amplifier7of the TIA circuit5has a fixed resistor8that has a fixed resistance value. The resistance values of the resistors6and8provide the TIA circuit5with a transimpedance value, Z0, that linearly translates the electrical current signal produced by the photodiode3into an output voltage, VOUT, which is output from the TIA circuit5.

A linear peak detector9of the optical Rx1detects the maximum VOUTand generates a threshold voltage, VTH, based on the maximum VOUT. A linear peak limiter11of the optical Rx1limits VTHand outputs a limited threshold voltage VTHLIM. The linear peak detector9and the linear peak limiter11together comprise a threshold generation circuit10. The minimum limit for VTHLIMis VTHMINand the maximum limit for VTHLIMis VTHMAX. The minimum limit VTHMINis typically set to a value that is greater than zero to prevent sporadic toggling of the data output signal12when there is no optical input signal2. A threshold voltage that is one-half of the maximum VOUT, VTHOUTMAX, is used as a decision threshold voltage, VTHDEC, by a decision circuit13to decide whether a bit of the incoming binary data stream is a logic 1 or a logic 0. The decision circuit13compares the output voltage VOUTof the TIA circuit5with the decision threshold voltage VTHDECto make the bit decision. The decision circuit13may be a simple comparator circuit, with or without hysteresis.

For larger levels of the optical input signal2, when VOUTreaches an AGC threshold voltage, VTHAGC, a controller circuit14of the AGC circuit4that compares VOUTto VTHAGCdelivers an output signal to a peak-hold circuit15of the AGC circuit4that causes it to output an AGC output voltage, VAGC. The AGC threshold voltage VTHAGCis generated by a voltage source17of the AGC circuit4. In the case of larger increasing optical input signals2, the output voltage VAGCcauses the resistance value of the variable resistor6to decrease, which decreases the transimpedance value of the TIA circuit5. The decrease in the transimpedance value causes VOUTto decrease until it reaches VTHAGC. Therefore, after stabilization, VOUT=VTHAGCcorresponds to a logic 1 bit and VOUT=0 corresponds to a logic 0 bit. Because the linear peak detector9outputs a much higher threshold voltage during the stabilization phase, the maximum threshold voltage output from the linear peak limiter11, VTHMAX, is limited to VTHAGC/2 in order to provide a precise decision between a High bit (logic 1) and a Low bit (logic 0). The AGC control loop timing constant is set to match the bit duration in order to achieve a correct bit decision for the first and subsequent bits of the incoming data stream.

In order to properly discharge the linear peak detector9and the AGC peak-hold circuit15, an activity detector16of the AGC circuit4detects whether or not the AGC circuit4is active based on the output of the AGC peak-hold circuit15. In the case of small optical input signals2(i.e., when the AGC circuit4is inactive), the linear peak detector9should be slowly discharged. In the case of large optical input signals2(i.e., when the AGC circuit4is active), the linear peak detector9should be kept charged under all circumstances in order to ensure that the maximum threshold voltage VTHMAXis used by the decision circuit13. The peak-hold circuit15of the AGC circuit4is slowly discharged over time when no input signal2is present. If the AGC circuit4falls below its activation level based on the comparison performed by the controller circuit14, then the linear peak detector9starts to be discharged until the minimum threshold limit VTHMINis reached. At that point in time, the Rx1is operating in its normal burst-mode condition for no optical signal2being detected by the photodiode3and waits for new incoming optical signals2to be detected.

A function generator18generates a correction signal that is based on the VTHLIMsignal output by the linear peak limiter11and based on the output voltage VAGCoutput by the AGC circuit4. The correction signal steers a timing adjustment circuit20. A High (logic 1) output signal output from the decision circuit13is always longer than the corresponding bit in the input optical signal2due to the adaptive threshold process performed by components9and11, which can result in pulse-width distortion. In order to prevent pulse-width distortion from occurring, or to reduce the amount of distortion that occurs, the timing adjustment circuit20“shrinks” the corresponding pulse duration by either accelerating the falling edge of the pulse, or, alternatively, by phase shifting the entire signal, which has the effect of pulse shrinking. In this way, the incorporation of the function generator18and the timing adjustment circuit20into the Rx1reduces the occurrence of systematic pulse enlargement that might otherwise occur if the decision circuit13outputs a wrong decision based on the rising edge of VOUT.

Because the timing correction factor that is applied by the function generator18depends in a nontrivial way on various parameters of the Rx1,100(i.e., photodiode bandwidth, TIA bandwidth, AGC bandwidth enhancement, and therefore dependence on pulse amplitude, transmitter pulse shape), the optimization of the function generator18is preferably performed as a last step after the Rx signal chain has already been modeled and optimized (and available as an exact representation in the simulation environment). Optimization is then performed by adjusting, in an iterative way, various parameters of the function generator18, which steers the timing adjustment circuit20to achieve minimum pulse-width distortion over the full input range of optical signals.

FIG. 2is a schematic diagram of the burst-mode optical Rx100in accordance with a second illustrative embodiment in which a differential configuration of Topology A is employed. As in the embodiment described above with reference toFIG. 1, the resistance value of the variable resistor6is steered by the AGC circuit4, but with a differential configuration as will be described below in detail. Here again, as in the embodiment represented byFIG. 1, an input optical signal2is fed to a photodiode3, which converts the input optical signal2into a corresponding electrical current signal. The photodiode3, and for symmetry reasons, a light-shielded photodiode3a, are connected to a full differential TIA circuit101that converts the electrical current signals output from the photodiodes3and3ainto a differential voltage signal VOUT. The TIA circuit101comprises a differential amplifier102, fixed resistors8and8a, and voltage-controllable or current-controllable variable resistors6and6a.

For small optical input signals2, the AGC circuit4is in an inactive state and a linear peak detector109is in an active state. In the active state, the linear peak detector109generates, through the linear peak limiter111, an optimum decision threshold voltage VTHDECfor the decision circuit113that is half of the VOUTsignal level for a High bit. The linear peak detector109and the linear peak limiter111comprise a threshold generation circuit110. For large optical input signals2(i.e., VOUTgreater than VTHAGC), the controller circuit14begins adjusting the resistance values of the variable resistors6and6avia the AGC peak-hold circuit15to cause the voltage level of the High-bit VOUTsignal levels to be kept at VAGCTB. Therefore, the High-bit VOUTsignal levels are kept at constant levels and the decision threshold level VTHDECis limited by the linear peak limiter111to VAGCTH/2 for optimum bit decision making. The AGC threshold voltage VAGCis set by a fixed level shifter112that sets VAGCbased on the voltage signal that is output from the positive terminal of the differential amplifier102. The decision threshold voltage, VTHDEC, of the decision circuit113is set by a variable level shifter114based on the voltage signal that is output from the positive terminal of the differential amplifier102.

The AGC activity detector16determines whether or not the AGC circuit4is active based on VAGCand manages the discharging of the AGC peak-hold circuit15and the linear peak detector109. In the absence of an optical input signal2, the AGC circuit4is active and the AGC peak-hold circuit15is discharged while the linear peak detector109is kept charged. If the AGC peak-hold circuit15falls below VAGCTH, then the AGC activity detector16begins discharge the linear peak detector109. The linear peak limiter111may have a lower limit that is greater than 0 Volts (V) to prevent sporadic toggling of the data output signal12when there is no optical input signal2. As in the embodiment ofFIG. 1, the function generator18generates a correction signal that steers the timing adjustment circuit20to prevent or reduce pulse-width distortion.

FIG. 3illustrates an embodiment of a single-ended configuration of the burst-mode optical Rx200based on Topology B. In accordance with this illustrative embodiment, the TIA circuit210has a nonlinear resistor211that is used to compress the large dynamic range of the electrical current signal produced by the photodiode3. The nonlinear resistor211has a nonlinear transimpedance that acts as an instantaneous AGC circuit, which obviates the need for the AGC circuit4shown inFIGS. 1 and 2. Eliminating the AGC circuit4provides the optical Rx200with certain speed advantages over the optical Rx1and the optical Rx100of the embodiments ofFIGS. 1 and 2, respectively, due to the extremely fast reaction time of the nonlinear resistor211. When designing and implementing the Rx200, care should be taken to match the functions performed by a threshold generation circuit220, a discharge current generation circuit230, and a timing correction generator260in order to ensure proper post processing of the output voltage VOUTof the TIA circuit210by a timing adjustment circuit270.

The optical input signal2that is incident on the photodiode3is converted by the photodiode3into a corresponding electrical current signal. For small electrical current signals, the nonlinear resistor211is in a high impedance state and the TIA circuit210operates in a linear mode with a linear transimpedance value. A linear peak detector240and the threshold generation circuit220together comprise a threshold generator that generates, in a linear manner, a threshold voltage VTHDECto be used by a bit decision circuit250for making the bit decisions. The linear peak detector240outputs the peak detected voltage, VPK, that it detects, which is then halved by the threshold generation circuit220to produce VTHDEC.

For larger input current signals (i.e., larger optical input power levels), the nonlinear resistor211lowers its impedance and thereby keeps the TIA circuit210below saturation. For example, the nonlinear resistor211should have logarithmic behavior to simplify the threshold generation process performed by components220and240. In the logarithmic case, the decision threshold voltage VTHDECis generated by the threshold generation circuit220by subtracting a constant voltage based on the constants of the logarithmic TIA conversion process from the output voltage of the linear peak detector240. The constants of the logarithmic conversion process should be matched to the linear range of the linear peak detector240to achieve a seamless threshold generation operation that avoids having to take steps to avoid functional gaps.

The discharge current generation circuit230ensures that the linear peak detector240is correctly discharged for the different ranges (i.e., linear vs. logarithmic). In the linear range, the discharge current generation circuit230discharges the linear peak detector240exponentially. In the logarithmic range, the discharge current generation circuit230discharges the linear peak detector240linearly with time. The threshold generation circuit220may also have a lower limit of the threshold output voltage to ensure quietness of the digital output12in the case of no optical input power. The decision circuit250may be a comparator circuit, or alternatively, a Schmitt-trigger circuit to avoid chatter during slow transients. The timing correction generator260generates a timing correction signal that steers a timing adjustment circuit270to correct, dependent upon the input level, the pulse-width of the voltage signal output from the decision circuit250for timing error minimization.

Because the timing correction factor that is applied by the timing correction generator260depends in a nontrivial way on various parameters of the Rx200(i.e., photodiode bandwidth, TIA bandwidth, AGC bandwidth enhancement, and therefore dependence on pulse amplitude, transmitter pulse shape), the optimization of the timing correction generator260is preferably performed as a last step after the Rx signal chain has already been modeled and optimized (and available as an exact representation in the simulation environment). Optimization is then performed by adjusting, in an iterative way, various parameters of the timing correction generator260, which steers the timing adjustment circuit270to achieve minimum pulse-width distortion over the full input range of optical signals.

FIG. 4illustrates another embodiment of the burst-mode optical Rx300having a differential configuration and being based on Topology B. In accordance with this embodiment, the signal chain is fully differential and the function is very similar to the single-ended Rx200shown inFIG. 3. The main difference between the embodiments ofFIGS. 3 and 4is that the nonlinear differential TIA circuit310ofFIG. 4includes an additional correction current generation circuit320for the nonlinear mode of operations that injects current in the path in which the shielded photodiode3aresides. Also, a variable level shifter330is used to generate the VTHDECto be used by decision circuit250based on the differential signal VOUT.

The correction current generation circuit320prevents current charge/discharge effects in the shielded photodiode3a. The circuit320is only active in the nonlinear region. In the linear region, this circuit320is inactive. The circuit320includes an amplifier321and a current source322. In order to provide seamless activation of circuit320, it has a nonlinear characteristic that is matched to the nonlinearity of the differential TIA102. The nonlinear resistors211and211aare used to compress the large dynamic range of electrical current produced by the photodiodes3and3a, respectively. The nonlinear resistors211and211ahave nonlinear transimpedances that behave as an instantaneous AGC circuit, as described above with reference to nonlinear resistor211ofFIG. 3. The functions of the components220-270is as described above with reference toFIG. 3.

It should be noted that although the illustrative embodiments have been described with reference to an optical Rx, the principles and concepts of the invention may also be applied to an electrical Rx. In such cases, the photodiode is replaced by some other type of detector (not shown) that converts some other type of input signal into an electrical detection signal. Persons of skill in the art will understand how the principles and concepts of the invention can be applied in such non-optical environments.

It should be noted that the invention has been described with respect to illustrative embodiments for the purpose of describing the principles and concepts of the invention. The invention is not limited to these embodiments. The circuits described with reference toFIGS. 1-4are merely examples of suitable configurations that demonstrate the principles and concepts of the invention. For example, many of the components that are used in the Rxs described above may be replaced by different components that perform the same or similar functions, as will be understood by persons of skill in the art. For example, although the components3and3ahave been described as being photodiodes, any suitable detector may be used for this purpose. Also, some of the components that are shown as being separate components may be combined into a single component that performs all of the functions associated with the separately-depicted components. For example, the functions of the linear peak detector9and the linear peak limiter11may be performed by a single linear peak detector9that has a built-in limiting structure or an inherent limiting function. As will be understood by those skilled in the art in view of the description being provided herein, many modifications may be made to the embodiments described herein without deviating from the goals of the invention, and all such modifications are within the scope of the invention.