Abstract:
An improved digital signal line carrier type detector non-conductively senses and counts voltage pulses on a probed line. Further, the detector produces a human-readable display of the carrier type in use on the probed line, and can detect carriers of various frequencies without the use of multiple oscillators or probes. The detector may be housed in a hand-held portable housing, and may be powered by one or more batteries. A timer circuit is usable with the detector to prevent accidental discharge of the battery for an extended period of time.

Description:
TECHNICAL FIELD 
     This invention relates generally to telephone signal transmission systems and, more particularly, relates to analytical tools for providing non-conductive detection and identification of digital signal types on lines used for transmitting electrical signals. 
     BACKGROUND OF THE INVENTION 
     DSL (Digital Subscriber Line) technology is a relatively new technology that increases the data transmission rate of ordinary telephone lines substantially compared to common V.34 (33600 bps) or V.90 (56 Kbps) modems. DSL systems are either asymmetric or symmetric. Asymmetric systems provide different transmission rates depending on direction. Asymmetric systems are accordingly well suited for Internet access tasks and video on demand operations. Symmetric DSL provides data transmission at the same rate in both directions. DSL uses a packet switching technology that operates independently of the ordinary voice telephone system. The maximum transmission rate of a DSL system decreases as the distance between transmitting and receiving sites increases. This is predominantly due to voltage deterioration and signal spreading experienced by digital signals as a function of distance. 
     Often, service personnel or craftspersons wish to determine which line or lines in a given bundle of lines are carrying DSL signal. Furthermore, because conductive probing of DSL lines can damage wires, cause signal deterioration, and present a serious shock hazard due to high line voltages on some wires, non-conductive probing of such lines is desirable. Known DSL probing methods designed to meet these requirements are lacking in other respects. The devices disclosed in U.S. Pat. Nos. 5,297,167 and 5,140,614 capacitively sense voltage changes on a line, and convert the sensed voltage changes to an audible signal. The strength and frequency of the audible signal are used by the technician to surmise whether the probed line is carrying DSL signals. A relatively weak audible signal emitted by the probe may indicate the presence of electromagnetic coupling with nearby lines rather than an actual DSL signal on the probed line. 
     The probes of the U.S. Pat. Nos. 5,297,167 and 5,140,614 patents both utilize a heterodyning technique. This technique entails combining a capacitively detected high frequency (f 1 ) DSL signal with a locally generated signal of a frequency (f 2 ) that differs by an amount f 3 , where f 3  is audible to the human ear. The resultant signal has both high frequency (f 1 +f 2 ) and low frequency (|f 1 −f 2 |=f 3 ) components. A low pass filter removes the high frequency component leaving an audible signal of frequency f 3 . 
     It can be seen that if the frequency of the probed signal differs from that of the local oscillator by more than the range of human hearing, which spans at most 20 kHz, then the filtered heterodyned signal will not be in the human-audible range. This has necessitated the use of multiple probes for multiple carrier types. For example, one probe is required for T1 lines, which operate at 1.544 Mb/s and another for E1 lines, which operate at 2.048 Mb/s. 
     A digital signal probe is needed that easily, accurately, and non-conductively detects digital signals. 
     SUMMARY OF THE INVENTION 
     The present invention remedies a number of the shortcomings in prior known hand-held detectors. In accordance with the present invention, a detector containing signal detection and processing circuitry counts at least a portion of the signal transitions on a probed line. The detector further contains threshold circuitry that reduces signal identification errors resulting from weak electromagnetic coupling, or “cross-talk.” Because the detector operates by counting transitions rather than heterodyning, there is no similar limitation on the identifiable frequency ranges. For example, a single detector according to the present invention preferably detects different carrier types even when they differ greatly in frequency, such as T1 and E1 carrier types. 
     Additionally, a detector according to the invention may further comprise a human-readable display usable to display an indication of the counted frequency of the probed line. Such indication may be of the carrier type, capacity, or frequency, or other quantifiable indication related to a detected count. 
    
    
     Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments which proceeds with reference to the accompanying figures. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a perspective external view of a detector according to an embodiment of the invention; 
     FIG. 2 is a schematic drawing of circuitry usable in an embodiment of the invention; and 
     FIG. 3 is a flow chart summarizing the steps followed to identify an unknown carrier signal in an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to FIG. 1, in an embodiment of the invention, certain elements are housed in a hand-held ergonomic housing  1 , having handle  3  and probe  5  portions. The handle  3  is dimensioned so as to provide a grip for a human hand, and may have a depressible switch  7  for operation located thereon in an accessible manner. In order to facilitate non-conductive (i.e., no current is drawn) probing of a digital line  13 , the probe portion  5  is shaped as an open torroidal circular curve, or polygon so that it can be situated in a substantially surrounding relationship to the line  13 , as will be more fully described hereinafter. For capacitive rather than inductive sensing, the probe portion could be shaped as two opposing plates to be situated between line pairs. However, inductive sensing is preferred, because it detects predominantly what is within the inductive element, while a capacitive probe requires a line pair for sufficient signal, and may be excessively influenced by nearby lines which are not being probed. 
     In a preferred embodiment of the invention, the handle  3  supports a display  9  for visually conveying to the craftsperson an indication of the type of signal detected in the form of a name, symbol, and/or frequency, as well as any other messages for operation. Other messages include low battery warnings, error codes, and so on. Additionally or alternatively. the housing  1  includes an attachment point  11  such as a loop for attachment to a lanyard, cord, or strap. 
     In operation the craftsperson situates the housing  1  such that probe portion  5  partially encompasses the line  13  to be probed, as set forth in step  201  of the flow chart of FIG.  3 . During step  203 , when the unit is activated via switch  7  or otherwise, the unit detects at least a portion of logical transitions (e.g. positive transitions, negative transitions, etc.) in a signal stream on line  13  during a period to establish a detected pulse count of the detected signal stream. Next, at step  205 , the unit correlates the detected pulse count with a set of pulse count ranges representative of and corresponding to expected count value ranges for various digital signal transmission conventions. Lastly, during step  207  the detector displays to the craftsperson via display  9  an alphanumeric code corresponding to the type of digital signal detected. If the count derived in step  203  does not correlate in step  205  with a pulse count range in the set, the display in step  207  may consist simply of the count value, or of an error signal such as a question mark. 
     FIG. 2 illustrates in greater detail circuitry usable in an embodiment of the invention to accomplish detection and notification functions. Detector coil  101  located within the probe portion  5  of the housing  1  is arranged to inductively sense proximate fluctuating magnetic fields such as those generated by line  13  while carrying a high frequency digital signal. Common mode choke  107 , coupled to detector coil  101 , filters high frequency noise generated by electronic circuitry of the hand-held detector that could effect the line  13  under sense. A voltage follower  103 , coupled to the common mode choke  107  via line  105 , amplifies the current received on line  105  from the common mode choke  107 . Voltage follower  103  is preferably a unitary gain amplifier. 
     In operation, the current output from voltage follower  103  varies proportionally with the current supplied by coil  101 . The voltage follower  103  receives and conveys the voltage from line  105 , increasing the current to the received signal to sustain more current drain. The voltage follower  103  is constructed from an op-amp such as those supplied in the LM324A low power quad operational amplifier IC produced by SGS-Thomson Microelectronics by connecting the op-amp output to the negative op-amp input. This will cause the op-amp output voltage to be substantially identical to the input voltage at the positive input of the op-amp. 
     The output of follower  103  is connected via line  109  to resistor  111  and inverter  113 . Resistor  111  is a 10 kOhm or similar resistor. The output of resistor  111  is connected to line  115 , which is in turn connected to the negative input of op-amp  117 . The positive input of op-amp  117  is tied to ground, with the output connected to line  119 . Capacitors  121 ,  123  of 22 pF capacitance and a resistor  125  of 49.9 kOhm are connected in parallel between lines  115  and  119 . The output of op-amp  117  connected as described is an inverted, amplified, and low-pass filtered signal corresponding to the output of follower  103 . The low pass filtering is desirable if there are stray high frequency signals on the probed line, caused perhaps by electromagnetic coupling with nearby lines. 
     Inverting amplifier  131  receives the output of op-amp  117  on line  119 , producing an inverted amplified signal on line  135 . Amplifier  133  receives the output of inverting amplifier  131  via line  135  and supplies an amplified output via line  137  to Schmitt trigger  139 . Schmitt trigger  139  supplies an inverted digital output to a data input of microcontroller  129  via line  141 . 
     Staging amplification as described above is preferred, but not required to practice the present invention. In an alternative embodiment a single stage amplifier is used. However staging amplification provides certain benefits given current electrical component technology. For example, staging lower gain amplifiers (instead of using a single high gain amplifier) achieves desired high gain, while avoiding frequency response reduction that typically results from using a single higher gain amplifier. 
     Inverter  113  supplies an output on high power bypass line  127  to a second data input of microcontroller  129 . Thus if the unamplified output of voltage follower  103  is sufficient for the microcontroller  129  to register digital transitions, the microcontroller  129  is able to receive and correctly interpret the signal inverted directly from that point without further amplification or filtering. In turn, microcontroller  129  preferably ignores signals received via line  141  during the same time period because the amplifiers are likely saturated and unable to accurately transmit the received signal stream. The microcontroller  129  is, by way of example, an Atmel AT89C52 or AT89C55, with the 0, 1, and 2 counters assigned to record time, count on line  141 , and count on line  127  respectively. An oscillator such as crystal oscillator  163  is preferably utilized to provide the clock signal to the microcontroller  129 , and to counter  0 . Counters  1  and  2  are triggered by a high-to-low transition, so that the falling edge of each detected pulse is preferably counted by the microcontroller  129 . 
     In practice selection of the proper line for purposes of counting transitions is accomplished by assigning separate counters within microcontroller  129  to each one of the two distinct data input lines. A first counter within microcontroller  129  counts pulses received via line  141 , and a second counter counts pulses received via line  127 . The higher count value is used in further processing, and the lower count value is ignored. 
     Take for example a high power signal such as an ADSL signal. Such a signal will likely saturate one or more amplifiers leading to line  141 , causing the count from that line to be lower (due to missed high-to-low signal transitions) than it would otherwise be if the amplifier(s) did not saturate. The same, high power, ADSL signal does not saturate the inverter  113  on the high power bypass line. Line  127  thus provides the more accurate count signal for the relatively high power ADSL signal. The higher count established by the counter assigned to the signal received on line  127  is used for further processing. 
     The described circuitry is powered by any convenient electrical power source. The illustrated circuit preferably operates on a supply voltage of 5V, and may accordingly be powered by a DC-to-DC converter  143  (including a MAXIM MAX1705 circuit) connected via lines  142  and  144  to a 1.5V battery  145 , such as a standard “C-cell” or “D-cell.” In an embodiment of the invention, the power circuitry includes a timer (and discharge) circuit such as a pull-up resistor  151  connected to the supply voltage (5 volts) and (via line  154 ) to capacitor  153  in series. The pull-up resistor  151  and capacitor  153  have values chosen to give a desired delay before the converter  143  is disabled. Line  154  is connected to an “on control” input for the converter  143  such that a high voltage at that input disables the converter  143 , shutting off the converter  143  when the capacitor  153  has received a sufficient charge through resister  151 . 
     The illustrated timer circuit also includes a discharge resistor  155  that is also connected to line  154  between the pull-up resistor  151  and capacitor  153 . The other terminal of the discharge resistor  155  is connected to line  156 . Line  156  terminates at the “off” terminal of switch  158 . Line  142  connects switch  158  to the low voltage input of converter  143 . Switch  158  selectively connects line  142  to the negative battery terminal via line  146  (when the spring loaded button is depressed), and to line  156  (when the button is released). 
     In operation, a craftsperson manipulates the switch  158  (for example by depressing the spring loaded button) to disconnect the discharge line  156  and connect to line  146  enabling the battery  145  to supply power to the converter  143 . In turn the converter  143  powers the entire circuit described to this point by supplying 5 volts to the microcontroller  129 , display  161 , and the high voltage inputs to the amplifiers and buffers. The converter  143  supplies precision 2.5 volts to the reference inputs to the detector coil  101  and amplifiers  117 ,  131  and  135 . 
     With the supply voltage at high voltage, the voltage on line  154  becomes high in an amount of time determined by the values of resistor  151  and capacitor  153 , disabling the converter  143  and preventing further drain on the battery  145  after that amount of time. This avoids draining of the battery by inadvertent continuous activation of the switch  158 . Release of the spring loaded button causes the switch  158  to disconnect power to converter  143  and to connect discharge resistor  155  through line  156  to the ground input of the converter  143  thereby draining the accumulated charge on capacitor  153 . 
     Preferably, a power on reset circuit  159  is provided to reset microcontroller  129  and blank the display  161  when the switch  158  is initially toggled to the activated state. The power on reset signal is transmitted through inverters  162  and  164  to the blanking input of the display  161 . The power on reset signal is also transmitted through inverter  162  to line  160  which connects to a “reset” input on the microcontroller  129 . After the microcontroller  129  executes a start-up sequence it clears the display with a clear signal transmitted on line  166  which resets all the values stored in the display  161 . Thereafter, data lines  168  and address lines  170  reload the registers of the display  161 . 
     While the circuitry is powered, it is operable to count and display the carrier frequency on a probed line as will now be described in more detail. Each pulse detected by detector  101  appears as a voltage pulse of relatively large or small magnitude on line  109  as described above. If the signal is large enough to cross a voltage threshold of the controller  129  data inputs, it is counted as a transition by microcontroller  129  upon receipt of the inverted signal via line  127 . For very large and very small detected signals, the counts on the counters assigned to lines  127  and  141  will differ. For very large signals, the staged amplifiers leading up to line  141  will saturate causing a low count on line  127 . However, if the detected signal on line  109  is very small, but is large enough to trigger Schmitt trigger  139  once amplified by amplifiers  117 ,  131 , and  133 , then a transition is counted by microcontroller  129  as received via line  141 , but not at the counter assigned to line  127 . 
     Controller  129  is programmed in a manner familiar to one of skill in the art to count the number of transitions received in a given period of time. For example, the controller may count the number of transitions occurring in a ¼ second sample period, as the transitions are received via lines  127  or  141 . A table is then used to correlate the quarter-second transition count with a carrier type. 
     It is desirable that the table correlate carrier types to a range of count values, rather than to a specific count value, since the detected carrier transitions count varies with carrier signal strength. The signal strength varies as a function of the distance between the probed point and the transmitter. In particular, the carrier signal strength decreases the further the probed point is from the originating signal source. Thus, a single carrier type may yield a different detected frequency depending upon where along the transmission path the line is probed. On a signal line carrying information in both directions, each signal stream will be strongest, and hence most detectable, at its transmitter, and weakest, and hence least detectable at its receiver. At the midpoint of the line, both signals may be detectable. Accordingly, it has been determined that a given carrier may exhibit a detected count rate at the midpoint which is approximately double the detected count rate at either end. 
     At the end of every sample period after having determined the type of signal detected, the microcontroller  129  writes an indication of the signal type to a human-readable display  161 . Display  161  may be any type of visual display and the indication of signal type may be either an indication of the magnitude of the count, the frequency itself, or of the type of signal calculated by the controller. Display  161  is preferably, but not necessarily, an intelligent display such as the Siemens SLG2016 Alphanumeric Intelligent Display. In an embodiment, the microcontroller  129  altematingly writes frequency-equivalent carrier types to the display  161 . Thus, instead of simply displaying “T1”, the display could intermittently also display for example “PRIS” for Primary Rate ISDN, which operates at the same frequency as T1, and so on. 
     Any carrier type can be sensed and displayed in the same manner. For example, carriers such as 56 Kb/s,64 Kb/s, DDL, HDSL,  2 B+D and Basic Rate ISDN are detected via the same method, their transitions counted and correlated to a carrier type, and a visual indication given of that type or of the count related to that type as discussed above. Table 1 gives a non-exhaustive list of carrier types and their empirically determined range of operation in counts per quarter second. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 CARRIER 
                   
               
               
                   
                 TYPE 
                 COUNTS/SEC/4 
               
               
                   
                   
               
             
             
               
                   
                 DDL, ISDN 
                 5,500-7,250 
               
               
                   
                 DMLQ 
                  8,250-11,250 
               
               
                   
                 MDDL, HDSL 
                 23,750-35,000 
               
               
                   
                 ADSL 
                 38,000-60,500 
               
               
                   
                 T1 
                 94,000-98,000 
               
               
                   
                 E1 
                 125,000-131,000 
               
               
                   
                 56K 
                 3,300-3,750 
               
               
                   
                 64K 
                 4,000-4,500 
               
               
                   
                   
               
             
          
         
       
     
     Preferably, the microcontroller  129  is also programmed to supply a signal to the display  161  when signal type is unknown. Thus, for example, the microcontroller  129  may write to the display  161  a signal such as “?” when a signal is unknown or is not a digital signal at all. Alternatively, the display may simply show a pulse count or frequency in such cases. Additionally, the controller also preferably supplies a signal to display  161  to indicate to the craftsperson an actual or imminent low battery condition. Such a signal could be “batt” written to the display intermittently between indications of signal type. Such a low battery signal may be the result of receipt by the microcontroller  129  of a low battery indication signal from a power chip such as those made by Maxim which may house the DC-to-DC converter  143  as well. 
     In an alternative embodiment, the visual indicator of carrier type consists of an array of LED&#39;s or lights located proximally to a listing of carrier types. In this embodiment, the microcontroller  129  causes the LED or light nearest to the list item corresponding to the detected carrier type to light. Thus, the list may include “T1”, “E1”, and so on, and when a T1 carrier is detected, the LED or light nearest “T1” would light. 
     In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that the shape or external appearance of the probe are not critical as long as a probe element is able to detect signals present on the probed line. Additionally, it will be appreciated by those of skill in the art that pulse detection and counting apparatuses and methods not utilizing a microcontroller may equivalently be used to count detected pulses. Accordingly, the invention is not limited to the illustrated embodiment. Rather, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.