Abstract:
A squelch detector, including: an input configured to receive an input signal; a peak detector connected to the input configured to detect a maximum value of the input signal wherein the peak detector includes a refresh input configured to receive a refresh signal to refresh the output of the peak detector, a valley detector connected to the input configured to detect a minimum value of the input signal wherein the valley detector includes a refresh input configured to receive the refresh signal to refresh the output of the valley detector, and a comparator including a first signal input connected to an output of the peak detector, a second input connected to an output of the valley detector, and a first reference input, wherein the comparator is configured to compare a difference between an output of the peak detector and an output of the valley detector and a reference value received at the first reference input and configured to produce an output based upon the comparison.

Description:
TECHNICAL FIELD 
     Various exemplary embodiments disclosed herein relate generally to signal swing squelch detectors and methods. 
     BACKGROUND 
     Squelch is a circuit function that acts to suppress the output of a receiver in the absence of a sufficiently strong desired input signal. Squelch is applied in various communication systems. 
     SUMMARY 
     A brief summary of various exemplary embodiments is presented below. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of an exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. 
     Various exemplary embodiments relate to a squelch detector, including: an input configured to receive an input signal; a peak detector connected to the input configured to detect a maximum value of the input signal wherein the peak detector includes a refresh input configured to receive a refresh signal to refresh the output of the peak detector, a valley detector connected to the input configured to detect a minimum value of the input signal wherein the valley detector includes a refresh input configured to receive the refresh signal to refresh the output of the valley detector, and a comparator including a first signal input connected to an output of the peak detector, a second input connected to an output of the valley detector, and a first reference input, wherein the comparator is configured to compare a difference between an output of the peak detector and an output of the valley detector and a reference value received at the first reference input and configured to produce an output based upon the comparison. 
     Various embodiments further relate to method of producing a squelch indicator, including: receiving an input signal; detecting by a peak detector a maximum value of the input signal; detecting by a valley detector a minimum value of the input signal; comparing the difference between the detected maximum value and the detected minimum value and a reference value; determining a squelch indicator based upon the difference between the detected maximum value and the detected minimum value and a reference value. 
     Various embodiments further relate to squelch detector, including: an input configured to receive an input signal; a peak detector connected to the input configured to detect a maximum value of the input signal wherein the peak detector includes a refresh input configured to receive a refresh signal to refresh the output of the peak detector, wherein the peak detector includes a refresh switch and a peak sampling capacitor, and wherein the refresh switch is configured to discharge the peak sampling capacitor based upon the refresh signal; a valley detector connected to the input configured to detect a minimum value of the input signal wherein the valley detector includes a refresh input configured to receive the refresh signal to refresh the output of the valley detector, wherein the valley detector includes a refresh switch and a valley sampling capacitor, and wherein the refresh switch is configured to discharge the valley sampling capacitor based upon the refresh signal; a comparator including a first signal input connected to an output of the peak detector, a second input connected to an output of the valley detector, and a first reference input, wherein the comparator is configured to compare a difference between an output of the peak detector and an output of the valley detector and a reference value received at the first reference input and configured to produce an output based upon the comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein: 
         FIG. 1  illustrates an embodiment of a squelch detector, 
         FIG. 2  illustrates an input signal, a refresh signal, and the timing of the output signal; 
         FIG. 3  illustrates an embodiment of a peak detector, and 
         FIG. 4  illustrates an embodiment of a valley detector. 
     
    
    
     To facilitate understanding, identical reference numerals have been used to designate elements having substantially the same or similar structure and/or substantially the same or similar function. 
     DETAILED DESCRIPTION 
     The description and drawings illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, “or,” as used herein, refers to a non-exclusive or (i.e., and/or), unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. 
     Communication standards, e.g., universal serial bus (USB) power delivery (PD), require a transceiver to be capable of identifying signal from noise. Traditionally, such task is performed by a squelch detector that includes a peak detector and a comparator. This traditional squelch detector monitors the amplitude of an AC coupled input. However, duty cycle distortion, voltage asymmetry of the signal, and a leaky discharge path significantly degrade the accuracy of the detection. For example, existing squelch detectors for high frequency sinusoidal signals are not effective for arbitrary signals, which have duty cycle distortion, varying frequency, varying common mode, etc. Further, squelch detection methods for differential signals are only suitable for differential input signals with a high accuracy. For such squelch detection methods to work with single ended signal, a differential-to-single-ended conversion is needed, which will degrade the accuracy significantly. In addition, the differential-to-single-ended conversion will be flawed when the common mode varies. 
     Embodiments of a squelch detector will be described that detect the swing of an input signal. This squelch detector may include a peak detector, a valley detector, and a differential comparator. The input of the squelch detector may be input to the peak and valley detectors with or without a level shifter. The outputs of the peak and valley detectors are then applied to the differential comparator, where the voltage difference of the peak and valley detectors are compared to a reference voltage difference. In order to achieve accurate detection, there is no constant discharge path in either the peak or valley detector. The signal swing is updated using a refresh mechanism, which discharges the sampling capacitors of the peak and valley detectors periodically. 
     One example of where the embodiments described herein may be applied is to USB PD communication using a biphase mark code (BMC) on the configuration channel (CC) lines. These embodiments further may be applied to any other data communication requiring a squelch detector that monitors the AC swing of its input. 
     Using both a peak detector and a valley detector to sample the input signal results in a measurement of the swing that is not sensitive to the duty cycle distortion and asymmetry of the signal. Further, the input may be DC coupled to the squelch detector instead of AC coupled. In addition, the proposed refresh mechanism eliminates a trade-off between accuracy and working frequency. Also, the input swing is sampled and refreshed periodically to avoid the inaccuracy due to a constant discharge path connected to the sampling capacitor. 
       FIG. 1  illustrates an embodiment of a squelch detector. The squelch detector  100  may include a level shifter  110 , a swing detector  115 , and a differential comparator  130 . The swing detector  115  may include a peak detector  120 , a valley detector  125 , and a refresh input  150 . 
     The squelch detector  100  receives an input signal  105  at the level shifter  110 . The level shifter may shift the level of the input signal  105 . As will be further described below the level shifter  100  is optional. The input signal  105  (whether level shifted or not) is input to the swing detector  115 . The swing detector  115  inputs the input signal  105  to both the peak detector  120  and the valley detector  125 . The peak detector  120  outputs the maximum value of the input signal. The valley detector  125  outputs the minimum value of the input signal. In order to keep the maximum and minimum input values current, a refresh signal may be received at the refresh input  150  to reset the maximum and minimum values of the input signal. 
     The swing detector  115  outputs the maximum input value from the peak detector  120  and the minimum input value from the valley detector to a differential comparator  130 . The differential comparator  130  may also receive reference values refH and refL at a reference high input  140  and a reference low input  145  respectively. The differential comparator  130  may then compare the difference between the outputs from the peak detector  120  and the valley detector  125  to the difference between the refH and refL values, and the differential comparator  130  produces an output or indicator that indicates whether the difference between the outputs from the peak detector  120  and the valley detector  125 , i.e., the swing, is greater than or less than the difference between the refH and refL values. When the swing is less than the difference between the refH and refL values, then squelch may be applied. 
     It is noted that the differential comparator may alternatively include only a single reference input that receives a reference value that is compared to the swing. 
     As described above, use of a level shifter  110  is optional. The level shifter may be used if: 1) the inaccuracy resulting from the circuit non-idealities, e.g., negative-bias temperature instability (NBTI) effects of the p-channel metal-oxide-semiconductor field-effect transistor (PMOS) differential pairs in the detectors and the comparator is greater than inaccuracy due to the level shifter, because the peak detector  120 , valley detector  125 , and differential comparator  130  may adopt n-channel metal-oxide-semiconductor field-effect transistor (NMOS) differential pairs rather than the PMOS pairs; or 2) the input pin has a stringent leakage requirement, as in USB Type-C and PD specifications. 
       FIG. 2  illustrates an input signal, a refresh signal, and the timing of the output signal. The input signal  205  is shown as a pulsed signal, but could be any type of input signal. The refresh signal  210  has a refresh period. When a refresh signal  210  is received the peak detector  120  and valley detector  125  are refreshed by resetting the minimum and maximum measured values of the input signal  205 . The output signal may have a data ready time (as indicated by the plot  215 ) during which the output signal does not have a valid value as the outputs of the swing detector  115  have to settle after the refresh occurs. The refresh period has a minimum value based upon the data ready time. The maximum refresh period would be driven by the specific input communication signal  205  received by the squelch detector  100 . Further, the output of the squelch detector  100  may maintain its output value at a refresh and during the data ready time until the output value of the swing detector  115  has settled. 
       FIG. 3  illustrates an embodiment of a peak detector. The peak detector  120  may include an input  305 , a peak switch  310 , a peak comparator  315 , a resistor  320 , a peak sampling capacitor  325 , a discharge switch  330 , and a peak output  335 . The peak detector  120  receives the input signal at its input  305  which is then fed to the positive input of the peak comparator  315  and the peak switch  310 . The peak switch  310  connects the input  305  to the resistor  320  and is controlled by the output signal of the peak comparator  315 . The resistor  320  further connects to the peak sampling capacitor  325 . As shown, the resistor  320  and the peak sampling capacitor  325  are also connected to the negative input of the peak comparator  315 . The discharge switch  330  is connected to the peak sampling capacitor  325 . 
     The peak detector  120  operates as follows. The input signal is applied to the peak comparator  315 . The peak comparator  315  compares the input voltage to a voltage on the peak sampling capacitor  325 . When the input voltage is greater than the voltage on the peak sampling capacitor  325 , the peak comparator  315  outputs a signal to the peak switch  310  that causes the peak switch  310  to close. When the peak switch  310  closes the input voltage is then charged onto the peak sampling capacitor  325 . Once the input signal voltage becomes less than the voltage on the peak sampling capacitor  325 , the peak comparator  315  outputs a signal that opens the peak switch  310 . As a result, the voltage on the peak sampling capacitor  325  indicates the maximum voltage from the input signal. As the peak detector  120  operates, when a new peak input voltage is received, it will be larger than the voltage on the peak sampling capacitor  325  so that the peak comparator  315  closes the peak switch  310  so that the input voltage can be applied to the peak sampling capacitor  325 . As a result the peak sampling capacitor  325  now indicates the new maximum value of the input voltage. When a refresh signal is received, the refresh switch  330  closes to discharge the peak sampling capacitor  325  to a reference level which may also be the ground level or the DC level of the input signal. Accordingly, the discharge path of the peak sampling capacitor  325  is controlled by the refresh signal. 
       FIG. 4  illustrates an embodiment of a valley detector. The valley detector  125  may include an input  405 , a valley switch  410 , a valley comparator  415 , a resistor  420 , a valley sampling capacitor  425 , a discharge switch  430 , and a valley output  435 . The valley detector  125  receives the input signal at its input  405  which is then fed to the negative input of the valley comparator  415  and the valley switch  410 . The valley switch  410  connects the input  405  to the resistor  420  and is controlled by the output signal of the valley comparator  415 . The resistor  420  further connects to the valley sampling capacitor  425 . As shown, the resistor  420  and the valley sampling capacitor  425  are also connected to the positive input of the valley comparator  415 . The discharge switch  430  is connected to the valley sampling capacitor  425 . 
     The valley detector  125  operates as follows. The input signal is applied to the valley comparator  415 . The valley comparator  415  compares the input voltage to a voltage on the valley sampling capacitor  425 . When the input voltage is less than the voltage on the valley sampling capacitor  435 , the valley comparator  415  outputs a signal to the valley switch  410  that causes the valley switch  410  to close. When the valley switch  410  closes the input voltage is then charged onto the valley sampling capacitor  425 . Once the input signal voltage becomes greater than the voltage on the valley sampling capacitor  425 , the valley comparator  415  outputs a signal that opens the valley switch  410 . As a result, the voltage on the valley sampling capacitor  425  indicates the minimum voltage from the input signal. As the valley detector  125  operates, when a new minimum input voltage is received, it will be smaller than the voltage on the valley sampling capacitor  425  so that the valley comparator  415  closes the valley switch  410  so that the input voltage can be applied to the valley sampling capacitor  425 . As a result the valley sampling capacitor  425  now indicates the new minimum value of the input voltage. When a refresh signal is received, the refresh switch  430  closes to discharge the valley sampling capacitor  425  to a reference level which may also be the ground level or the DC level or the input signal. Accordingly, the discharge path of the valley sampling capacitor  425  is controlled by the refresh signal. 
     Other peak and valley detectors that may be refreshed may be used in the swing detector  115  as well. 
     Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be effected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.