Abstract:
A peak detector circuit that responds rapidly to power transients, and yet is able to avoid interpreting data fluctuations as power transients by generating dual peak signals from an amplifier&#39;s differential output signal, where the dual peak signals have data ripple components that tend to cancel one another. The system and methods permit the peak detectors to be much more responsive to power transients by expanding their bandwith (shortening the time constants) to the point that low frequency data components affect the individual peak detector signals, but the effects are cancelled out when the individual components are added together. The peak detector described herein may be used in an AGC system to provide ripple-free gain control signals, while rapidly following any power transients in transmitted signals.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A. Field of the Invention 
         [0002]    The invention described herein relates to methods and devices for providing a signal peak detector having rapid response to signal envelope variations while reducing ripple due to low frequency data components. 
         [0003]    B. Description of the Related Art 
         [0004]    In communication systems, variations of the amplitude of the received signals require continuing adjustments of the receiver&#39;s gain in order to maintain a relatively constant output signal. For example, power transients caused by switching devices may create power fluctuations as channels are added or dropped. A variable gain amplifier (VGA) with automatic gain control (AGC) is commonly used to automatically maintain a constant signal level at the output of the analog front end of the receiver. AGC systems typically measure the peak value of the waveform of the VGA output to detect any power fluctuations. 
         [0005]    Peak detectors are typically used to measure the peak value of the fluctuating electrical signals. It is desirable to have a peak detector that is capable of tracking amplitude variations due to undesired power fluctuations rather than to interpret amplitude variations that are due to data signal components. Typically the bandwidth of a peak detector is sufficiently low so that frequency components associated with the data signaling is not passed through the peak detector. 
       SUMMARY OF THE INVENTION 
       [0006]    Described herein is a peak detector circuit that responds rapidly to power transients, and yet is able to avoid interpreting data fluctuations as power transients by generating dual peak signals from an amplifier&#39;s differential output signal, where the dual peak signals have data ripple components that tend to cancel one another. The system and methods permit the peak detectors to be much more responsive to power transients by expanding their bandwidth (shortening the time constants) to the point that low frequency data components affect the individual peak detector signals, but the effects are cancelled out when the individual components are added together. The peak detector described herein may be used in an AGC system to provide ripple-free gain control signals, while rapidly following any power transients in transmitted signals. Preferred embodiments may be used in optical communication systems to enable the detection of power fluctuations due to the operation of optical add/drop multiplexers, while simultaneously rejecting apparent signal level drop due to a series of consecutive identical digits (CID). In SONET optical systems, the sequence may extend to  72  CID. 
         [0007]    In one embodiment, the signal envelope detection circuit comprises a pair of peak detectors operating on a differential voltage signal, wherein the peak detectors have bandwidths sufficiently high such that each of the pair of peak detectors has an output which exhibits a voltage ripple associated with low frequency components of a data signal present in the differential voltage signal; and, an active ripple cancellation circuit for adding the outputs of the pair of peak detectors to generate an envelope magnitude signal and to cancel the voltage ripple. In addition, the peak detector bandwidths are sufficiently low such that the voltage ripples associated with the low frequency components are substantially symmetrical to each other. This symmetry is obtained by the presence of a capacitor within each peak detector that is charged and discharged at equivalent rates by the low frequency data components. That is, in the presence of the low frequency components (and the absence of power transients), the capacitors are cyclically charged to a voltage by current flowing through a diode and then discharged by an amount, preferably in a range of between 5 and 25 percent of the voltage, such that the current in subsequent charging cycles remain in a linear region of the diode. Faster discharge rates (again, in the absence of a power transient) would result in a nonlinear charging period due to the diode I-V characteristic. This would result in ripple signals that would not be symmetric, and would therefore not substantially cancel each other. 
         [0008]    Preferably, the envelope magnitude signal is indicative of an error between a desired envelope reference and the peak detector output. A charge pump may be connected to the active ripple canceller for maintaining a cumulative desired gain control signal. A reference voltage representing a desired peak target voltage may be applied to a third peak detector for generating a processed reference voltage. The active ripple cancellation circuit preferably includes adders to compare the outputs of the pair of peak detectors to the processed reference voltage to generate the envelope magnitude signal. The peak detector capacitors are preferably discharged via a base current of an emitter degeneration transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The objects, features and advantages of the present invention will be more readily appreciated upon reference to the following disclosure when considered in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  depicts a block diagram of a system using the fast peak detector and active ripple canceller in an AGC application. 
           [0011]      FIG. 2  depicts a peak detector tuned to a high bandwidth. 
           [0012]      FIG. 3  depicts a diagram showing peak detector response curves. 
           [0013]      FIG. 4  depicts a block diagram of the active ripple canceller circuit. 
           [0014]      FIGS. 5A and 5B  depict flow diagrams of preferred methods. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    With reference to  FIG. 1 , one embodiment of an AGC system  108  with fast peak detection and active ripple cancellation is described. The AGC  108  incorporates three fast peak detectors followed by an active ripple canceller (ARC)  101  to track dynamic power transients while being insensitive to the data-dependent ripple at the output of the peak detectors caused by a long string of CID. In order to achieve ripple cancellation, the peak detectors are tuned so that the ripple components of the signal when run through the ARC offset each other. 
         [0016]    The AGC  108  receives input from the differential output of the VGA at nodes  106  and  107 . In the example of transmitted signals being  72  CID, the input signals to peak detectors  102  and  104  are single-sided high-frequency signals with opposite polarities. Although the peak detector includes a low pass-filtering characteristic, the time constant is fast enough that data variations associated with low frequency components of the data signal will appear at the output of the peak detectors. This allows maximum tracking of power transients. The output of each peak detector can be modeled as a peak (DC) waveform V p  representing the signal envelope plus a ripple component R[V o+ ] or R[V − ] due to the data-dependent signal. In order to achieve optimal AGC performance, the ripple component of the peak detector output is eliminated by the ARC  101  before the signal is fed back to the gain block  105 . To control the desired signal level, a reference voltage is also provided to the AGC  108 , and the signal envelope amplitude deviation will be measured with respect to this reference. Note that the reference voltage is also passed through a fast peak detector  103  to generate a processed reference voltage that may be used to ensure the reference is subjected to the same diode drop associated with the diode connected transistor to achieve process, temperature insensitivity. 
         [0017]      FIG. 2  is a diagram of the specially tuned peak detector circuit. The peak detector circuit includes transistors  201 ,  203 , and  205 , a capacitor  204 , a resistor  207 , and a current source  202 . The input signal V inp  is fed into the base of the transistor  201  and the output of the peak detector circuit is taken from the emitter of the transistor  205  at node  206 . The output from the emitter of the transistor  201  is input to the base of the transistor  203  and to the current source  202 . The collector of the transistor  203  is fed to its base so that it operates as a diode connecting nodes  208  and  209 . The output from the emitter of the transistor  203  is input to the base of the transistor  205  and to the capacitor  204 . The emitter of the transistor  205  is connected to the resistor  207 . The output of the current source  202 , the capacitor  204 , and the resistor  207  share a common grounding point. The collector of the transistor  201  is connected to the collector of the transistor  205 . 
         [0018]    In order to achieve ripple cancellation, the time constants of the peak detectors are tuned so that the rise and decay characteristics of the ripple components are similar. This is because one peak detector will be charging, while the other one is decaying.  FIG. 3  shows four examples A, B, C, and D of rise and decay characteristics of the ripple components of the output from the peak detector circuit. As the peak detector bandwidth decreases, the rise and decay characteristics of the ripple component become more similar. As  FIG. 3  shows, A is the desired ripple component because of the small variation in rise and decay characteristics that will offset each other when added together. Yet note that the peak detector bandwidth is still much greater than a typical prior art peak detector because the bandwidth is wide enough that the fast peak detector, if used by itself, would generate a peak signal having significant data-dependent ripple that would disrupt the operation of a VGA. However, by tuning the peak detectors and configuring them to operate on both sides of the differential signal, the data ripple components are significantly symmetric such that they are effectively cancelled. 
         [0019]    In order to obtain this characteristic in the ripple components, the transistor  203  in  FIG. 2  preferably operates in high-resistance region so that the rise and decay of the ripple component can be approximated as a first order response. To this end, the capacitance of the capacitor  204 , the effective diode resistance of the transistor  203  and the resistance of the resistor  203  are chosen so that node  206  exhibits similar charge-up and discharge transients in the presence of  72  CID, which is associated with the SONET optical communication specification. Yet the capacitance of the capacitor  204  depicted in  FIG. 2  is kept small enough to enhance tracking performance. 
         [0020]      FIG. 4  depicts the block diagram of a preferred embodiment of the active ripple canceller (ARC)  101  in  FIG. 1 . The ARC receives inputs from peak detectors  102 ,  103 , and  104  and outputs to the gain control  105  through a charge pump  401 . In the case of CID signals, the inputs to the ARC are V in,p =V p +R[V o+ ], V in,n =V p +R[V o− ], and V ref , where V p  is the envelope wave form caused by the power transient, R[V o+ ] and R[V o− ] are the pseudo-differential ripple components with opposite polarities caused by the data-dependent signals, and V ref  is the output of peak detector  103 , representing the desired signal level. The signal V out  is formed by first subtracting V ref  from V in,p  and V in,n  respectively, and then adding the resulting signals together: V out =2 V p −2 V ref +R[V o+ ]+R[V o− ]=2(V p −V ref ). V out  is then run through the charge pump and output to the gain control  105  depicted in  FIG. 1 . 
         [0021]    As  FIG. 4  shows, the ripple components R[V o+ ] and R[V o− ] of the peak detector outputs offset each other due to opposite polarities and the resulting control signal is proportional to the variation in the peak wave form V p  with respect to the reference signal V ref . 
         [0022]    In  FIG. 5 , a preferred method  500  of detecting a signal envelope is set forth. The method includes step  502  of generating a first and second peak signal, each having a ripple component. The first peak signal is generated from a data signal on a positive node of a differential amplifier and a second peak signal from a data signal on a negative node of a differential amplifier. The ripple components are associated with low frequency components of a data signal being amplified by the differential amplifier. At step  504  the first and second peak signals are added to obtain an envelope magnitude signal having a substantially reduced aggregate ripple component. 
         [0023]    An alternative method  520  of detecting a signal envelope is shown in  FIG. 5B . At step  522  first and second peak signals are generated from a data signal on the positive and negative nodes at the output of a differential amplifier. At step  524 , the first and second peak signals are added to obtain an envelope magnitude signal. Preferably, the first and second peak signals are compared to a reference voltage prior to being added. The first and second peak signals are preferably generated by peak detectors having time constants sufficiently fast such that the first and second peak signals will decay in a range of between 5 percent and 25 percent in the presence of a 72 consecutive identical digit data signal. The first and second peak signals also contain ripple voltages associated with low frequency components of a data signal. The ripple components are substantially symmetrical and cancel each other out when the first and second peak signals are added to obtain the envelope magnitude signal. The envelope magnitude signal is preferably applied to a charge pump circuit to generate a gain control voltage, which may then be used to adjust the gain of the differential amplifier. 
         [0024]    The claims should not be read as limited to the described order of elements unless stated to that effect. In addition, use of the term “means” in any claim is intended to invoke 35 U.S.C. §112, paragraph 6, and any claim without the word “means” is not so intended. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.