Abstract:
A system is provided for detecting a fault in a signal transmission path. In one embodiment, the system can include a variable amplitude signal attenuator which is operable to modify an input signal by variably attenuating a signal voltage swing of the input signal. Desirably, the input signal is attenuated only when transitioning from a high signal voltage level towards a low signal voltage level d variably, such that a larger high-to-low signal voltage swing is attenuated more than a smaller high-to-low signal voltage swing. Desirably, a comparator, which may apply hysteresis to the output signals, may detect a crossing of a reference voltage level by the modified input signal. In this way, when the comparator does not detect an expected crossing of the reference voltage level by the modified input signal, a determination can be made that a fault exists in the signal transmission path.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the determination of connectivity faults in communication systems, especially between chips at opposite ends of serial data transmission path. 
     Connectivity faults are of several types including “stuck-at” faults in which a transmitted signal appears stuck at either a low state or a high state and fails to transition between states. Other types of connectivity faults include those in which a coupling capacitor used for alternating current (AC) mode operation is shorted, and those in which the arriving signal appears to “float”, such as when the cable is disconnected from the signal source. Elements which provide connectivity between chips at remote and near ends of a transmission link include at least a transmitter of the chip at the remote end, the package of the chip at the remote end, a card connecting the remote end chip to the chip at the near end, an AC coupling capacitor, the package of the near-end chip, and a receiver implemented on the near-end chip. Connectivity between chips of such system is frequently referred to as “cable connectivity”, whether or not a literal cable (rather than a card or other conductive connection) is provided between the packages of respective chips. One way to test the cable connectivity is to send a stream of test data from a remote transmitter and then verify the data received at the near-end receiver. 
     In AC coupled transmission links, termination is provided at the source (transmitter side) such that no DC current is sunk at the receiver end. In such AC coupling mode, a high-pass filter exists by the combination of the series AC coupling capacitor (also referred to as a “DC blocking capacitor”) and a termination resistor which is shunted to ground. The high-pass filter causes low-frequency test signals to decay with an “RC” time constant determined by the magnitude of the resistance (R) and the capacitance (C) of the respective circuit elements, which can be fixed or variable. 
     Differential signal transmission is frequently favored for the transmission of higher frequency RF signals over signal conductors. An advantage of differential signal transmission is larger peak-to-peak differential signal swing and improved common mode noise rejection. However, differential signal transmission poses particular challenges. Heretofore, robust systems have not been provided for detecting and isolating a single-ended connectivity failure of a differential signal transmission link. For differential signal transmission links, it is not sufficient to detect the presence and/or absence of signals on a pair of signal conductors, a robust cable fault detector must determine which of two cables carrying the paired differential signals is faulty. The challenges of testing are particularly great when detecting a failing signal conductor when communication between chips is provided in an AC coupling mode in which signals must pass through an AC coupling capacitor, because such capacitor blocks direct current (DC) transmission. While some techniques such as AGC (adjusted gain control), DFE (decision feed-back equalization), etc., are available to reconstruct received signals, such techniques are of no use when signals are suddenly ruined by signal interference, or accidental disconnection of a cable from one or two ends of the transmission link. 
     Conventional signal detectors operate by detecting the absence of a valid signal within a specified latency. However, a signal detector cannot determine whether the failure is due to the cable, or the data itself. Neither can it tell which cable has a problem, or what type of defect mechanism is present, i.e., whether the fault is one of stuck-at high, stuck-at low or floating. This is because such signal detectors detect the presence of absence of signal from the difference between levels of a pair of differential signals arriving from the transmission link. 
     The 1149.6 standard of the Institute of Electrical and Electronics Engineers (IEEE) entitled “IEEE Standard for Boundary-Scan Testing of Advanced Digital Networks”, published April, 2003 (hereinafter IEEE 1149.6), specifies a cable fault detector at a receiver end of a transmission link for detecting single-ended cable faults for both AC and DC coupling modes. The published IEEE 1149.6 standard provides an example of using a hysteresis comparator  10  ( FIG. 1 ) to detect loss of signal on the transmission link. In the published example, the hysteresis comparator compares the level of a signal arriving from a single-ended cable of the transmission link with a delayed version of the same signal. The published example does not provide for robust testing, because when testing for a signal in AC-coupled mode, the comparator&#39;s operation destroys the hysteresis effect. For an AC coupled signal having an RC decay, as described above, when the decay drops to a certain level, the output of the hysteresis comparator  10  changes state. The hysteresis comparator  10  of the published example is poorly suited for testing an AC-coupled mode, because it does not maintain state when the AC-coupled signal decays. 
     Commonly assigned U.S. patent Publication No. 2005/0190828 to Hsu et al. describes a cable connectivity test receiver operable in accordance with IEEE 1149.6. However, that application does not seek to address the problems that are uncovered and addressed in accordance with the below-described embodiments of the invention. Said U.S. patent Publication 2005/0190828 is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the invention, a system is provided for detecting a fault in a signal transmission path. In one embodiment, the system can include a variable amplitude signal attenuator which is operable to modify an input signal by variably attenuating a signal voltage swing of the input signal. Desirably, the input signal is attenuated only when transitioning from a high signal voltage level towards a low signal voltage level and variably, such that a larger high-to-low signal voltage swing is attenuated more than a smaller high-to-low signal voltage swing. Desirably, a comparator, which may apply hysteresis to the output signals, may detect a crossing of a reference voltage level by the modified input signal. In this way, when the comparator does not detect an expected crossing of the reference voltage level by the modified input signal, a determination can be made that the fault is detected in the signal transmission path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block and schematic diagram illustrating operational specifications of prior art IEEE 1149.6 standard. 
         FIG. 2  is a block diagram illustrating a connection arrangement of a cable connectivity test receiver in accordance with an embodiment of the invention. 
         FIG. 3  is a high-level block diagram illustrating a cable connectivity test receiver in accordance with an embodiment of the invention. 
         FIG. 4  is block and schematic diagram illustrating a cable connectivity test receiver in accordance with an embodiment of the invention. 
         FIG. 5  is a schematic diagram illustrating a hysteresis comparator incorporated into cable connectivity test receiver in accordance with an embodiment of the invention. 
         FIG. 6A  is a graph illustrating operation of the hysteresis comparator shown in  FIG. 5  during DC-coupled mode in accordance with an embodiment of the invention. 
         FIG. 6B  is a graph illustrating operation of the hysteresis comparator shown in  FIG. 5  during AC-coupled mode in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     IEEE 1149.6 compatible connectivity test receivers present a problem which has not been addressed heretofore. Signal amplitudes can vary greatly between signaling protocols such as Common Electrical Interconnect (“CEI”), Fiber Channel (“FC”), Infiniband™ (“IB”), and Serial Attached SCSI (“SAS”). The signal amplitudes can range from a few hundred millivolts peak-to-peak differential to 2000 mV peak-to-peak differential. 
     Large test signal amplitudes can cause exceed the limits of a cable connectivity test receiver that is designed for smaller amplitude signals. One problem is seen particularly when the test receiver is connected in a signal transmission path in AC-coupled mode. Due to the presence of the on-chip or off-chip AC-coupling capacitor, the test signal amplitude arriving from the transmitter is superimposed on top of a common mode voltage which is designed according to requirements of the data receiver. For good performance, the common mode voltage at the data receiver is usually set to a level close to the power supply voltage. In such case, an incoming cable test signal which has a large amplitude can exceed the level of the power supply voltage. 
     When the test signal exceeds the power supply voltage level on existing test receivers, it may not be possible for the test receiver for the test receiver to latch the test signal properly. Apart from a problem of insufficient power supply headroom, such condition can cause excessive stress to be placed on gate oxides of transistors included in a comparator stage of the receiver. In addition, under such condition, when handling a large amplitude test signal, some devices of the comparator may be operated in non-optimal regions. 
     Accordingly, a robust system, a cable connectivity test receiver is provided herein for detecting a fault in a signal transmission path used usable for the transmission of data, especially data which is transmitted serially by a differential pair of data signals at transmission rates exceeding one gigabit per second (Gbs). The system is capable to operating over a wide range of test signal amplitudes, e.g., from between about 600 mV peak-to-peak differential to about 2000 mV peak-to-peak differential, and perhaps a greater range. Such signal amplitude range is sufficient to accommodate cable connectivity test requirements of the IEEE 1149.6 standard, and various data communication standards, such as CEI, FC, IB and SAS mentioned above. According to various embodiments of the system, cable connectivity faults can be detected in DC-coupled or AC-coupled modes or both, despite the differences in signal amplitude levels of the various types of input signals. Cable connectivity faults such as stuck-at high, stuck-at low, floating signal and shorted AC-coupled capacitor can be detected in accordance with embodiments of the invention. 
       FIG. 2  is a block and schematic diagram illustrating the interconnection of a pair of cable fault detectors  50 ,  60  in a chip  100  according to an embodiment of the invention. As shown therein, signals arrive from a transmission link on a pair of cables  13 P and  13 N carrying differential signals, and are coupled by coupling capacitors  21 ,  22 , respectively, to signal conductors  14 P and  14 N, which, in turn are coupled to the pads  110 ,  120  of the chip  100 . In turn, the pads  10 ,  20  are coupled to a pair of internal differential signal conductors  15 P and  15 N, respectively. The internal differential signal conductors  15 P,  15 N are coupled as input to a signal detector  70  and a receiver  80 . In turn, receiver  80  is connected to a receiver (RX) core  90  for outputting a latched data signal thereto. 
     As shown in  FIG. 2 , a first cable fault detector  50  is coupled to receive input from the first internal differential signal conductor  15 P and a second cable fault detector  60  is coupled to receive input from the second internal differential signal conductor  15 N. Each of the cable fault detectors  50 ,  60 , when enabled by a TEST_EN input, outputs a detection signal which is latched by a respective scan latch  30 ,  40 . Cable discharge devices  52 ,  62  are provided to controllably reset voltages on internal differential signal conductors  15 P,  15 N in accordance with a Cable Discharge signal, as appropriate before or during testing. Devices  52 ,  62  can also be used for ESD protection, as well, such as when connecting a cable to the chip  100 . When the Cable Discharge signal is active, the voltages on the signal conductors  15 P,  15 N are discharged to ground. After the cable is connected to the chip  100 , the Cable Discharge signal is deactivated, which then allows signals on conductors  15 P and  15 N to be input normally to the cable fault detector again. During testing of the cables, signal detector  70 , the receiver  80 , and other receiver components are turned off to reduce noise on signal conductors  15 P and  15 N, because single-ended cable fault detectors are more susceptible to noise. 
       FIG. 3  is a block and schematic diagram illustrating a cable connectivity test receiver in accordance with an embodiment of the invention. As illustrated therein, a pair of differential data signals MBDP and MBDN, arriving from a remote transmitter  102  through a signal transmission channel  110  on respective signal conductors  112 ,  114  is input to a package  120 . The outputs MBDN and MBDP of the transmitter  102  can be coupled to the channel  110  in alternating current (“AC”) coupled mode through decoupling capacitors (not shown). In AC coupled mode, the package generally is also coupled to the channel  110  through additional decoupling capacitors (not shown). Otherwise, the transmitter outputs MBDN and MBDP can be coupled directly to the channel  110  in direct current (“DC”) coupled mode. In AC coupled mode, only AC components of transmitted signals are propagated through the channel  110 . In DC coupled mode, the signals arriving from the channel  110  on conductors  112 ,  114  can have DC components. 
     After traveling through the package  120 , the input signals, now called RXDP and RXDN, are input to a signal conditioning circuit referred to as a receiver “front end” circuit  130 . Electrostatic discharge protection circuits (“ESD”)  140 A and  140 B are coupled, respectively, to the conductors carrying the signals RXDP and RXDN for protecting circuits connected to the same conductors from damage due to electrostatic discharge. The front end circuit  130  also includes 50 ohm termination resistances  160 ,  170 . Each of the termination resistances is coupled between one of the input signals RXDP and RXDN and a common mode voltage level (“VCM”) node connected to a common mode voltage bias circuit (“BIASCMV”)  150 . Ideally, the common mode voltage level VCM remains fairly steady over many cycles of the pair of differential signals. For AC-coupled signals, VCM will be at about 200 mV below the power supply voltage level. For DC-coupled signals, VCM reflects the DC component of the signals RXDP and RXDN. 
     As further shown in  FIG. 3 , the differential pair of signals RXDP and RXDN and their common mode voltage level VCM are input to the cable connectivity test receiver  210 . Desirably, the cable connectivity test receiver includes three components connected together in series. The differential pair of signals RXDP and RXDN and the common mode voltage VCM are provided as input to a variable amplitude attenuator  180 . The outputs INP and INN of that attenuator  180  are input to a reference voltage generator  180 . The reference voltage generator  180 , in turn, generates reference voltages Vrefp and Vrefn for input to a hysteresis comparator stage  200 , along with the differential signals DP and DN passed thereto from signals INP and INN. 
       FIG. 4  is a more detailed block and schematic diagram illustrating a cable connectivity test receiver according to one embodiment of the invention. As shown in  FIG. 4 , the purpose of the variable amplitude attenuator is to produce conditioned signals INP and INN from the respective input signals RXDP and RXDN, in which the signal swing is reduced when the signals transition between the high (voltage) level and the low level. In such way, the low voltage level of the outputs INP and INN remains sufficiently high to avoid the above-mentioned problem of drastically reduced tail current in the subsequent comparator stage. The variable amplitude attenuator  180  receives the common mode voltage VCM and applies it to the midpoint of a voltage divider formed by equal resistances R 1  and R 2  arranged between respective ones of the signals RXDP and RXDN. 
     The variable amplitude attenuator includes two PFETs P 1  and P 2  and two resistors R 1  and R 2 . The two PFETs P 1  and P 2  can be disabled when the cable connectivity test receiver is powered down, so that all parasitic and device loading resulting from operation of the test receiver can be effectively isolated by these devices. 
     In operation, when the two PFETs P 1  and P 2  are on, P 1  and P 2  can be modeled as linear resistors Ron_P 1  and Ron_P 2 , respectively. Ron_P 1  and resistor R 1  form a resistive divider between the signal RXDP and VCM. Similarly, Ron_P 2  and resistor R 2  form a resistive divider between the signal RXDN and VCM. From these two equivalent resistive dividers, it can be established that INP=R 1 /(Ron_P 1 +R 1 ) and INP=R 2 /(Ron_P 2 +R 2 ). In general, in AC-coupled mode, desirably, VCM is set to be slightly below the power supply voltage for high-speed data receiver performance, in terms of gain, bandwidth and linearity. During operation, when the input signals RXDP and RXDN have small amplitude, devices P 1  and P 2  are strongly on, their gate voltages being tied to ground, and Ron_P 1  and Ron_P 2  being very small. In such case, the outputs INP and INN of the variable attenuator are approximately the same as the inputs RXDP and RXDN, respectively. When the amplitudes of the input signals RXDP and RXDN increase, no effect is apparent when each input signal transitions from low to high signal level. The variable amplitude attenuation is felt most strongly for large amplitude input signals RXDP, RXDN during transitions from high to low signal levels. For example, when the input signal RXDP is transitioning from a high voltage level to a low voltage level, the device P 1  initially is turned on strongly. However, as the signal RXDP continues to fall lower, nearing ground, the resistance Ron_R 1  becomes very large. By operation of the resistive divider, In such way, the amplitude of the output signal INP=R 1 /(Ron_P 1 +R 1 ) is reduced at that time in relation to the input signal RXDP. Similarly, during high to low signal transitions for the input signal RXDN, the amplitude of the output signal INN=R 2 /(Ron_P 2 +R 2 ) is reduced at that time in relation to the input signal RXDN. Thus, the amount of attenuation is directly dependent on the amplitudes of the input signals RXDP and RXDN and the resistance values of R 1  and R 2 . 
     The outputs INP and INN of the variable amplitude attenuator are applied to a reference voltage generator  190 . A function of the reference voltage generator is to generate reference voltages (Vrefp and Vrefn) and pass the reference voltages with the differential pair of signals INP and INN as input signals DP and DN to the hysteresis comparator stage  200 . The reference voltage generator includes two low pass filter multiplexers Mux 1  and Mux 2 . Each of the multiplexers accepts either a self-generated reference voltage by way of a low-pass filter R F  and C F  or a fixed reference voltage Vbias. To reduce area required to implement the low pass filter on the chip, the on-resistance of a pass-gate inside each multiplexer can be function as the resistive element of the low-pass filter. Each low-pass filter multiplexer selects the self-generated reference voltage in AC-coupled mode and selects a fixed reference voltage Vbias in DC-coupled mode. The reference voltage generator  190  passes the input signals INP and INN directly to the hysteresis comparator stage  200 , desirably, without modification. 
     The hysteresis comparator stage  200  includes a first hysteresis comparator (Comp 1 ) which is used to compare one of the signals of the differential signal pair, for example the signal DP, with the corresponding reference voltage level, Vrefp. In that way, it can be determined whether connectivity is present between the transmitter  102  and the receiver  210  along the corresponding path for the signal DP. The hysteresis comparator stage also includes a second hysteresis comparator (Comp 2 ) which compares the signal DN with the corresponding reference voltage level, Vrefn. In that way, it can be determined whether connectivity is present between the transmitter  102  and the receiver  210  along the corresponding path for the signal DN. 
       FIG. 5  is a schematic diagram illustrating a transistor-level structure of a hysteresis comparator  300  according to one embodiment of the invention. One such hysteresis comparator  300  can function as comparator Comp 1  and another such comparator  300  can function as comparator Comp 2  of the hysteresis comparator stage  200  ( FIG. 4 ). The output of each comparator  300  is a rail-to-rail logic signal which swings between the high state at the power supply voltage level VTR and the low state at ground. 
     Where one of the comparators functions as Comp 1 , the data signal input DP is applied to the INP input terminal, and the reference voltage Vrefp is applied to the INN input terminal. The output ZP of comparator  300  as Comp 1  is latched to a high rail-to-rail state when the DP signal rises sufficiently above Vrefp and the output ZP is latched to a low rail-to-rail state when the DP signal falls sufficiently below Vrefp. Where the other one of the comparators functions as Comp 2 , the data signal input DN is applied to the INP input terminal, and the reference voltage Vrefn is applied to the INN input terminal. In this case, the output ZP of comparator  300  as Comp 2  is latched to a high rail-to-rail state when the DN signal rises sufficiently above Vrefn and the output ZP is latched to a low rail-to-rail state when the DN signal falls sufficiently below Vrefn. 
     The comparator  300  includes a current source formed by tail device TT and controlled bias input BIAS, a pair of input NFET devices TP and TN to which the input signals are applied, and active current mirror loading devices T 1 , T 2 , T 3  and T 4 . The comparator further includes protection devices T 5  and T 6  and latching elements T 7 , T 8 , T 9  and T 10 . The comparator further includes push-pull comparison devices T 11 , T 12 , T 13  and T 14 . 
     The comparator  300  has two operational modes: DC-coupled and AC-coupled. In this way, the degree, i.e., the amount of hysteresis applied during AC-coupled mode can be different from the degree of hysteresis applied during DC-coupled mode. Desirably, the degree of hysteresis applied during AC-coupled mode is greater than the degree of hysteresis applied during DC-coupled mode. The hysteresis comparator includes a transistor T 71  and a passgate PG 1  which switchably connects the gates of transistors T 71  and T 7  together during the AC-coupled mode. Similarly, a passgate PG 2  switchably connects the gates of transistors T 71  and T 7  together during the AC-coupled mode. AC-coupled mode operation is enabled when the DC_Mode signal is low. 
     During DC-coupled mode, the DC_Mode signal goes high to switch off the passgates PG 1  and PG 2  and disconnect transistors T 71  and T 81  from transistors T 7  and T 8 , respectively. Transistors T 71  and T 81  are turned off by operation of the transistors TA and TB. During DC-coupled mode, when the transistors T 71  and T 81  are disconnected, transistors T 7  or T 8  turn on more quickly such that the hysteresis comparator reaches the latched state soon. As a result, the degree of hysteresis is relatively small. 
     On the other hand, during AC-coupled mode, transistor T 71  operates in parallel with transistor T 7  and transistor T 81  operates in parallel with transistor T 8 . In that case, during AC-coupled mode, each respective pair of transistors, i.e., the pair of transistors T 7  and T 71 , or the pair of transistors T 8  and T 81  takes longer to turn on, causing the degree of hysteresis to be greater than for DC-coupled mode. 
     The following description of operation is described first for DC-coupled mode, after which differences relating to AC-coupled mode will be explained. Passgates PG 1  and PG 2  are turned off and transistors TA and TB and turned on by the DC_Mode signal during DC-coupled mode. During operation, the comparator  300  senses the input signals at transistors TP and TN. The input signals are then reflected inversely at nodes QP and QN by the action of active current mirror loading devices T 1  and T 2 . The signal information captured at nodes QP and is then latched onto the nodes OUTP and OUTN by operation of NFET latching elements T 7 , T 8 , T 9 , and T 10 . In this way, a large amplitude input signal applied to INP or INN which exceeds the power supply voltage VTR is only sensed by the PFET sensing elements T 1  and T 2 . The NFET latching devices T 7 , T 8 , T 9  and T 10 , each having a drain connected to ground, are impervious to large amplitude signals which rise above the level of the power supply voltage VTR. In such way, the signal information can be reliably latched even when the input signal exceeds VTR. 
     The PFET current mirror devices T 1 , T 2 , T 3  and T 4  and additional amplification provided by the NFET latching elements T 7 , T 8 , T 9  and T 10 , give the hysteresis comparator  300  quick response time. Quick response time is an advantage when the cable connectivity test receiver is AC-coupled to the signal transmission path due to decay of signals in AC-coupled mode. For this reason, it is also desirable to design the PFET active current mirror devices T 1 , T 2 , T 3  and T 4  to have a thin or intermediate thickness gate oxide or other gate dielectric material. Desirably, each of the devices T 1 , T 2 , T 3  and T 4  has the same design and the same thickness of gate dielectric to avoid mismatch among these current mirroring devices. During operation, one of the nodes OUTP or OUTN can drop as low as ground. In that case, the active PFET current mirror devices T 3  and T 4  could become overstressed. To help address this problem, PFET devices T 5  and T 6  can be made to incorporate a thicker gate oxide or other thick gate dielectric material. The thicker gate dielectric can increase the voltage drop Vds between the drain and the source of devices T 5  and T 6 , which in turn, reduces the voltage drop across transistors T 3  and T 4 . 
     Referring to  FIG. 6A , during DC-coupled mode testing, the degree of hysteresis is relatively small. The hysteresis threshold levels are close together: 0.79 V and 0.81 V, respectively. That is, the output of the comparator transitions to a high level when the input signal exceeds 0.81 V, but the input signal must fall below 0.79 V before the output transitions to a low level. 
     During AC-coupled mode, operation is the same as described above for DC-coupled mode, except that the DC_Mode signal goes low and is disabled. As a result, the passgates PG 1  and PG 2  are turned on, transistors TA and TB are turned off and transistors T 71  and T 81  operate in parallel with transistors T 7  and T 8 , respectively, such that greater hysteresis is applied during operation. For example, as shown in  FIG. 6B , during AC mode testing, the hysteresis threshold levels are 0.75 V and 0.85 V, respectively. That is, the output of the comparator transitions to a high level when the input signal exceeds 0.85 V, but the input signal falls below 0.75 V before the output transitions to a low level. 
     The outputs of each hysteresis comparator (Comp 1  and Comp 2 ) are conditioned, e.g., by inverters (not shown) and temporarily stored as samples, such as in shift registers, for further analysis. Detection of various fault modes can then be performed using the samples stored in the shift register. In one particular example, a shorted decoupling capacitor used in an AC-coupled transmission path can be detected from the stored samples. In AC-coupled mode, a functioning (nonshorted) decoupling capacitor causes the signal arriving at the receiver side to decay rather quickly. A shorted decoupling capacitor can be detected when stored samples at least one RC time constant after the transition of the transmitted signal still show the presence of the signal. 
     Stuck-at faults, i.e., a problem in the cable which makes the signal appear to remain constant, i.e., “stuck” at a certain level, can also be detected by the failure of the stored samples to show transitions in the incoming signal. A third kind of fault, referred to as “floating,” occurs when the signal appears not to have a determinate value. Such fault can be detected by the failure of the stored samples to reflect the pattern of the transmitted test data. 
     Various combinations and modifications can be made to the above-described embodiments of the invention. For example, in one embodiment, a cable connectivity test receiver can be provided which includes the above-described variable amplitude attenuator, but which includes elements different from the above-described reference voltage generator  190  and hysteresis comparator stage  300 . Alternatively, another embodiment of the invention can include the hysteresis comparator without the variable amplitude attenuator  190 . 
     While the invention has been described in accordance with certain preferred embodiments thereof, many modifications and enhancements can be made thereto without departing from the true scope and spirit of the invention, which is limited only by the claims appended below.