Patent Publication Number: US-9404943-B2

Title: System, method, and apparatus for a safe powerline communications instrumentation front-end

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/943,493, filed Nov. 10, 2010, now U.S. Pat. No. 8,773,108, which claims priority to U.S. Provisional Application Ser. No. 61/259,855, filed Nov. 10, 2009. 
    
    
     BACKGROUND 
     Powerline communication (PLC) technologies have greatly increased in complexity and number over the last decade. The difficulties present in the powerline environment, including noise, severe resonances, complex topologies, large attenuation, and time-varying parameters make success with any PLC scheme extremely difficult 
     Testing and analyzing PLC networks can be done with general purpose communications test equipment. However, because PLC signals are carried over power lines, it may difficult to analyze certain parts of a PLC network due to the high voltages carried by the lines that are also carrying the PLC signals. These high voltages may be hazardous to individuals, and may also be unsafe for use with general purpose communications test equipment. This may result in a reduced ability to safely test and analyze a PLC network, as only certain parts of the network may be safely tested with general purpose communications test equipment. 
     SUMMARY 
     In various embodiments, a system, method, and apparatus are provided for adapting general purpose communications test equipment to test PLC signals on power lines. 
     In one embodiment a safe powerline communications instrumentation front end device includes a voltage input for receiving a line voltage from a powerline, a voltage reducer for reducing the voltage of the line voltage, a filtering system for extracting a Power Line Communications (PLC) signal from the line voltage, and an analog output for outputting the PLC signal as an analog signal for communications test equipment 
     In one embodiment, a method for testing and analyzing PLC systems using a safe powerline communications instrumentation front end device includes receiving a line voltage, reducing the voltage of the line voltage, extracting PLC signals from the line voltage, and outputting the PLC signals as analog signals for communications test equipment. 
     In one embodiment, a system for testing and analyzing PLC systems using a safe powerline communications instrumentation front end device includes a front end device installed near a powerline, communications test equipment for testing and analyzing a PLC communications system, and a cable configured for connecting the front end device to the communications test equipment. 
     Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device; 
         FIG. 2  depicts exemplary diagram of a safe powerline communications instrumentation front end device with passive filtering; 
         FIG. 3  depicts an exemplary diagram of a safe powerline communications instrumentation front end device with active signal processing; 
         FIG. 4  depicts an exemplary diagram of a safe powerline communications instrumentation front end device with a field programmable gate array; 
         FIG. 5  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device with wireless communication between physically separate voltage input and processing sections; 
         FIG. 6  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device with wired communication between physically separate voltage input and processing sections; 
         FIG. 7  depicts an exemplary diagram of a voltage input section for a safe powerline communications instrumentation front end device with physically separate voltage input and processing sections; 
         FIG. 8  depicts an exemplary diagram of a processing section for a safe powerline communications instrumentation front end device with physically separate voltage input and processing sections; and 
         FIG. 9  depicts an exemplary procedure for analyzing PLC systems using a front end device. 
     
    
    
     DETAILED DESCRIPTION 
     In various embodiments as illustrated in  FIGS. 1-9 , a safe powerline communications instrumentation front end device processes signals from a powerline and communicates with general purpose communications test equipment. 
       FIG. 1  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device. The safe powerline communications instrumentation front end device  101  may be a bridging device that enables the safe use of existing general purpose communications test equipment  106  in a PLC system. The front end device  101  may accept the line voltage from the powerline  103  through the voltage input  102 , may condition and filter the signal from the powerline  103 , and may provide a low-voltage, isolated, filtered signal on the output  104  across the cable  105  to the communications test equipment  106 . The function of the front end device  101  may be performed by a single physical device, or by multiple physical devices. 
     When the front end device  101  is applied to the powerline  103  in a PLC system, a user may be able use the communications test equipment  106  in a line voltage system where it may not be safe to attach the communications test equipment  106  directly to the powerline  103 . The front end device  101  may accept input(s) to the voltage input  102  at full line voltage, which may be 600V. The line voltage may contain PLC signals from a transmitter located in the power distribution system, or noise signals of interest in PLC communications. The front end device  101  may use circuitry to reduce the line voltage from the powerline  103  to safe levels, and then may apply various filtering and conditioning techniques to the signals from the powerline  103 . The low voltage signals may then be buffered and sent to the output  104 , which may be suitable for direct connection to the communications test equipment  106 . The communications test equipment  106  may be any general purpose communications test equipment, including, for example, spectrum or network analyzer, oscilloscope, data recorder, and so on. The front end device  101  may also provide optical or magnetic isolation between the voltage input  102  and the output  104 , for user safety. 
     The voltage input  102  of the front end device  101  may be standard 4 mm banana jacks, and suitable CAT IV 600V rated leads. The output signals from the output  104  may terminate into the standard Bayonet Neill-Concelman (BNC) inputs of the communications test equipment  106 . It may not be desirable to use BNC jacks on the front end device  101 , so the output  104  may use a weatherproof connector which contains all output signals, and a separate cable assembly may be used to convert the outputs to BNC connectors. This may help to maintain the CAT IV 600V rating, and may be more conducive to weatherproof construction. 
     Various parameters of the filtering and conditioning techniques may be user-configured. Additionally, the front end device  101  may include multiple outputs  104 , which may allow the same voltage input to be viewed with different filters simultaneously. Optional markers, reference level bars, and other special signals may be superimposed on the low voltage signal sent through the output  104 . 
     In one embodiment, the front end device  101  may have a single voltage input  102  connected to a single powerline  103 . In another embodiment, the front end device  101  may include multiple voltage inputs  102 , which may allows for analysis of polyphase systems (e.g. wye, delta circuits). The front end device  101  may be used without the capability to configure the device parameters, or with the ability to configure parameters through a standard port and control device, such as, for example, with a USB port connected to a laptop or PC. 
     The front end device  101  may be configured in different ways for different classes of PLC signals. PLC systems may be divided into two classes, low frequency and high frequency. Low frequency protocols, such as, for example, X10, INSTEON, KNX, LonWorks, Turtle TS1 and TS2, TWACS, and so on, may be low-bandwidth systems, which may be used for control and low-data rate applications such as automated meter reading (AMR), or other “smart-grid” applications. Low frequency protocols may generally operate in the DC-500 kHz range, and may often be narrowband. High frequency protocols, such as, for example, HomePlug, HD-PLC, and OPERA, may operate in the 1-30 MHz band, and may have very high data rates. These may be the Broadband over Powerline (BPL) systems. The front end device  101  may be suited only for low frequency or high frequency systems, or may have separate hardware to handle both types simultaneously. 
     The front end device  101  may also perform sophisticated filtering and triggering to allow more efficient use of the communications test equipment  106 . Recording and playback features may also be present in the front end device  101 , which may allow a user to record a signal in the field, and play it back later in a laboratory for analysis. 
     The front end device  101  may come in a variety of configurations depending on the type of signals carried on the powerline  103 . 
       FIG. 2  depicts an exemplary diagram of a safe powerline communications instrumentation front end device with passive filtering. In one embodiment, the front end device  101  may include the voltage input  102 , scaling and isolation circuitry, and the voltage output  104 . The scaling and isolation circuitry may include passive components, including a voltage reducer  201  and a passive filtering network  202 . The voltage reducer  201  may be, for example, any suitable isolation transformer, or any suitable pulse-withstanding divider resistors. No Analog to Digital (A/D) or Digital to Analog (D/A) converters may be necessary with the passive filtering network  202 . The passive filtering network  202  may use a combination of passive bandpass or shelf filters, and an optional line-frequency notch filter. A CAT IV 600V rating may be achieved by suitable spacing and transformer design. Various filters may be switched in and out of the passive filtering network  202  with mechanical switches, or by having multiple outputs  104 , with different filters in the passive filtering network  202  used for each output  104 . 
     The signal from the powerline  103  may enter the front end device  101  through the voltage input  102 . The voltage reducer  201  may reduce the voltage from the signal from powerline  103  down to safer level. The low voltage signal from the voltage reducer  201  may pass through the passive filtering network  202 , which may filter out noise and optimize the signal for use by the communications test equipment  106 . The optimized signal may be sent out through the output  104 , and across the cable  105  to connected communications test equipment  106 . The signal may not be digitized at any point, and may be analog across the entire system. 
     Pulse-withstanding divider resistors may scale the larger voltage input from the powerline  103  down to a reasonable circuit voltage, for example, 1V. A passive RC shelf filter may be used to attenuate the line frequency, reducing the required A/D dynamic range. A typical shelf filter may have a 20 db attenuation at the line frequency. Although the shelf filter may be constructed from passive components, the shelf filter may be adjustable by suitable switching of passive devices via analog switches or relays, or other suitable means. If the front end device  101  is to be used with high-frequency PLC schemes, the front end device  101  may use transformer couplings instead of pulse-withstanding divider resistors as the voltage reducer  201 . This may avoid high frequency rolloff with high-value resistors and stray capacitance. 
     Some signals on the powerline  103  may be better analyzed with active signal processing instead of passive filtering with the passive filtering network  202 . 
       FIG. 3  depicts an exemplary diagram of a safe powerline communications instrumentation front end device with active signal processing. The front end device  101  may include the voltage reducer  201 , an A/D converter  301 , a Digital Signal Processor (DSP)  302 , and a D/A convertor  303 . 
     The front end device  101  may act as an interface between the raw power line signal on the powerline  103 , which may contain both line frequency power and small PLC signals, and the communications test equipment  106 . This interface may be needed due to the dangerous voltages, hazardous locations, and special signal processing involved with PLC communications schemes, all of which may prevent direct use of the communications test equipment  106 . The front end device  101  may attach directly to line voltage conductors of the powerline  103 , reduce the voltage to safe levels, and provides an isolation and safety barrier from high-voltage transients. The input signal from the powerline  103  may be digitized and processed to remove the effects of the large line frequency signal, which may otherwise mask many PLC signals and make it difficult or impossible to use the communications test equipment  106 . The processed signal may then be converted to an analog signal, amplified, and brought out to standard BNC or other connectors, ready to be connected to the communications test equipment  106  for PLC signal analysis. The front end device  101  may generally do little or no communications analysis itself, however, the front end device  101  may perform any suitable communication analysis. 
     The signal from the powerline  103  may enter the front end device  101  through the voltage input  102  and be voltage reduced by the voltage reducer  201 , as in the front end device  101  with passive filtering. The output of the shelf filter may optionally be further conditioned with analog filtering or buffering before being passed to the A/D converter  301 . 
     The A/D convertor  301  may be any suitable device for digitizing an analog signal. The A/D converter  301  may sample the signal at a rate suitable for PLC communications analysis, for example, around 1 MHz for low-frequency PLC schemes, and 80 MHz or higher for high-frequency schemes. The sampling rate may be set to optimize the digital filtering to follow. For example, if a comb filter is performed on the digital samples, the A/D convertor  301  sampling rate may be synchronized to the frequency of the powerline  103 , and also set so that many line frequency harmonic periods are an integer number of A/D samples in duration. The A/D converter  301  may be of sufficient resolution to detect low-amplitude protocols such as Turtle TS1 and TS2, which may be 16 bits or higher. 
     The digital output of the A/D converter  301  may be electrically isolated with an isolation barrier, to allow the front end device  101  to achieve a 600V CAT IV rating. This may be done with optical or magnetically coupled digital isolation circuits between the serial A/D converter  301  output and the rest of the digital circuitry. 
     After the isolation barrier, the digital signal output from the A/D convertor  301  may pass through a programmable logic device that may deserialize the A/D stream. This may allow for the digital signal to be input to a parallel interface for the DSP  302 . Other suitable interfaces may be used. For example, the DSP  302  may have a high speed direct serial interface, which may obviate the need for the digital output of the A/D convertor  301  to be parallelized. 
     The front end device  101  may include only a single input chain, or may include two or more, for polyphase systems. Each input may be identical, and may be isolated from the DSP  302  circuitry for safety. 
     The DSP  302  may compute output signals from the digital signal, and feed these output signals to one or more D/A converters  303 . The D/A convertor  303  may be any suitable device for converting a digital signal to analog. The D/A convertor  303  may be clocked synchronously to the A/D converter  301 , or independently, as required. A programmable logic device may be used to serialize the output signals of the DSP  302  in the case of high-speed serial D/A convertors  303 . The D/A convertor  303  outputs may be filtered by suitable analog reconstruction filters, and then buffered. The signal outputs may be designed for standard 1 Mohm or 50 ohm test equipment inputs. In the case of 50 ohm output impedance, suitable driver circuitry may maintain the required impedance over the entire frequency band. The analog signals output from the D/A convertor  303  may be sent to the output  104 , then to the communications test equipment  106  across the cable  105 . The communications test equipment  106  may then test and analyze the PLC signals. 
     The DSP  302  may be able to correct for hardware inaccuracies such as gain and offset errors, input and output filter frequency response, such as, for example, magnitude and phase, and so on. as necessary. The DSP  302  may use nonvolatile correction factors, programmed at the factory, which may be applied by the DSP  302  as appropriate in the signal chain. These nonvolatile correction factors may include corrections at various temperatures, in addition to room temperature data. Where temperature compensation is needed, the front end device  101  may include one or more internal temperature sensors. The front end device  101  may also include reference broadband noise, DC, or D/A generated signal sources which may be fed into the A/D input chain, and may be used for autocalibration in the field. D/A outputs may be read with an auxiliary A/D to aid in autocalibration of the output signal path. 
     A main power supply may be included in the front end device  101  to power the DSP  302  and D/A convertor  303 , and an isolated power supply may be included to power the A/D convertor  301 . In the case of multiple voltage inputs  102 , the isolated power supply may have multiple outputs, each isolated from the others. The front end device  101  may use power from an internal battery, USB input, or line voltage input from the powerline  103 . If line voltage input power is used, any suitable means of filtering power supply noise from the voltage input  102  may be used. 
     If the front end device  101  is to be used with high-frequency PLC schemes, the front end device  101  may use transformer couplings as the voltage reducer  201 . These transformers may provide the CAT IV 600V isolation, and present a good impedance match to the broadband amplifiers that may drive the A/D converter  301 . Since it may not be practical to create a transformer with a bandwidth from 60 Hz to 30 MHz or more, the transformer may act as a high-pass filter, removing the line frequency and most harmonics. In this case, a separate isolated means of feeding the line frequency signal to the DSP  302  may be needed, for example, an optical isolator or separate A/D system. This may be included so that the DSP  302  may perform triggering in a synchronous fashion, which may be desired even for high-speed PLC signals. The front end device  101  may also include both transformer couplings and pulse-withstanding divider resistors, to allow simultaneous low- and high-frequency analyses. 
     The front end device  101  may also include auxiliary analog inputs, which may allow the powerline  103  currents to be measured. The auxiliary analog inputs may be sampled by a different A/D subsystem, and input into the DSP  302 . The signals from the auxiliary analog inputs may be used for triggering various filters or markers on the analog outputs. External off-the-shelf current transducers such as, for example, Rogowksi coils or iron-core clamp-on transformers may be used to convert the line current into a low voltage suitable for digitizing. The sampling rate and filtering for these auxiliary analog inputs may be different than for the voltage inputs  102 , since the PLC signals themselves may not be present on them. 
     High-frequency PLC analysis, involving sampling rates of 80 MHz or more, may require more processing power than the DSP  302  can achieve. 
       FIG. 4  depicts an exemplary diagram of a safe powerline communications instrumentation front end device with a field programmable gate array (FPGA). The front end device  101  may include an FPGA  304  or other high-speed programmable logic device, which may allow for the processing of high frequency PLC systems at high sample rates. Various input and output filters may be applied using the FPGA  304 , including sample rate conversion/decimation, and FFT computations. The FPGA  304  may receive the raw A/D samples, which may be the digital output of the A/D convertor  301 , pre-process them, and then send them to the DSP  302 . The DSP  302  may output D/A values to the FPGA  304  for post-processing. The FPGA  304  may send the post-processed D/A values to the D/A convertor  303 , for conversion to an analog output signal. In filtering schemes, the DSP  302  may not perform any direct data processing, but may instead perform configuration of the FPGA  304 , such as, for example, calculating and loading filter coefficients, and so on, or secondary processing in parallel with the FPGA  304 . 
     The front end device  101  may include the DSP  302  to apply filtering and processing to the input signals, and compute the output signals. Functions such as time keeping, data storage, battery charging, communications, and so on, may be handled by a separate microprocessor, or they may be performed by the DSP  302  in addition to the signal processing tasks. The DSP  302  may include such auxiliary processors, and may also include the entire processing system, including external SRAM/SDRAM memory, Flash memory, and communications modules, such as, for example, Bluetooth, Ethernet PHY, and so on. 
     The front end device  101  may be separated into two physical devices. One device may connect directly to the powerline  103 , and draw power from the connected voltage. This device may digitize the AC signals, and use WiFi or other wireless means to transmit the raw samples to the second device in real-time. The second device may not be physically connected to the powerline  103 , and thus may not have any extra safety or isolation requirements. The second device may include the main signal processing section, and signal output lines. 
       FIG. 5  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device with wireless communication between physically separate voltage input and processing sections, and  FIG. 6  depicts an exemplary overview diagram of a safe powerline communications instrumentation front end device with wired communication between physically separate voltage input and processing sections. 
     The front end device  101  may be implemented using two separate physical devices. The first device may be a voltage input section  501 , and the second device may a processing section  502 . 
     The voltage input section  501  may include the parts of the front end device  101  up through the isolation barrier for the A/D convertor  301  in one enclosure. The processing section  502  may include the parts of the front end device  101  after the isolation barrier in a separate enclosure. For a wireless connection, a wireless output  503  of the voltage input section  501  may be any suitable wireless communications device, such as, for example, a Wi-Fi or Bluetooth device, and may be used to send the A/D signals from the voltage input section  501  to a wireless input  504  on the processing section  502  using wireless connection  505 . The wireless input  504  may also be any suitable wireless communications device. Both the wireless output  503  and the wireless input  504  may be capable of two-way communication. For a wired connection, a wired output  601  of the voltage input section  501  may send the A/D signals to a wired input  602  of the processing section across a cable  603 , which may be, for example, a fiber optic cable. Both the wired output  601  and the wired input  603  may be capable of two-way communication. 
     Physically separate devices for the voltage input section  501  and processing section  502  may be used when the voltage connections are in a hazardous location, for example, on top of a power pole, in an underground vault, and so on. The communications link itself may be the isolation barrier in this case, if the link is wireless or non-electrical. The same effect may also be accomplished by keeping the voltage reducer  201  and the DSP  302  in one enclosure, and putting the D/A convertor  303  in the separate enclosure. This may be desired if the DSP  302  is performing any triggering or recoding functions, and the front end device  101  is to be left in place for a period of time. 
       FIG. 7  depicts an exemplary diagram of a voltage input section for a safe powerline communications instrumentation front end device with physically separate voltage input and processing sections. 
     The voltage input section  501  may include the voltage input  102  for the powerline  103 , the voltage reducer  201 , the A/D convertor  301 , and the wired output  601  or the wireless output  503 . The voltage input section  501  may be connected directly to the AC power conductors in the powerline  103 , and may meet the full CAT IV 600V safety requirements. The signals from the powerline  103  may be reduced in voltage by the voltage reducer  201 , shelf-filtered or have other analog filtering applied, and digitized with the A/D converter  301 . The digital signals from the A/D converter  301  may then be read by a microprocessor or a DSP, and buffered in memory. The digital signals may then be transmitted to the processing section  502  using the wired output  601  or the wireless output  503 , or other suitable communications means. The voltage input section  501  may be small enough to fit inside an electrical panel or transformer enclosure, so the panel door can be completely closed, with voltage input section  501  inside. 
       FIG. 8  depicts an exemplary diagram of a processing section for a safe powerline communications instrumentation front end device with physically separate voltage input and processing sections. 
     The processing section  502  may include the main DSP  302 , where all digital filtering and other operations may be performed, and the D/A converter  303 . This may provide complete isolation from arc-flash and other hazards associated with high voltage electrical environments, since the user only has to work directly with the processing section  502 . The processing section  502  may receive the A/D signals from the voltage input section  501  with the wired input  602  or the wireless input  504 . The processing section  502  may be connected directly to the communications test equipment  106 , and may output the analog signals from the D/A convertor  303  to the communications test equipment  106  through the output  104  over cable  105 . 
     Instead of being in a separate enclosure, the processing section  502  may be software residing on a PC or laptop. The voltage input section  501  may stream the A/D samples directly to the PC, which may then perform the signal conditioning and filtering functions, and display the data directly on the screen, instead of sending the results to the D/A converter  303  and the communications test equipment  106  for further analysis. The data from the voltage input section  501  may also be simultaneously broadcast to both a PC and a processing section  502  in a separate enclosure. 
     The voltage input section  501  may also be constructed for use in a standard wall outlet, such as, for example, a US 120V receptacle. For a voltage input section  501  for use in a wall outlet, a lower voltage and safety rating, for example, 150V CAT II, may be acceptable. 
     The system may allow simultaneous input from multiple voltage input sections  501 , for example from one or more 120V receptacle devices, and one or more polyphase 600V devices. The A/D streams may be filtered independently, or combined as needed, just as if they had originated from the same voltage input section  501 . Suitable A/D synchronization may be sent from the processing section  502 . 
     The A/D sampling rates may be adjusted as necessary where the communications link limits the data rate. For example, if WiFi is used to connect the voltage input section  501  to the processing section  502 , and the WiFi link cannot support the highest A/D data rate, the sampling rate may be reduced by the voltage input section  501 . Alternatively, one or more channels may be eliminated from the data stream, as needed. The voltage input section  501  may provide bandpass filtering to allow for data reduction or compression techniques, to maximize the effective sample rate through the wireless link. This management process may be performed automatically, or at the direction of the user. One method for automatic selection of A/D inputs to eliminate may be for the voltage input section  501  to automatically detect PLC signals on each phase input, and select the phase containing the signal of interest. 
     The voltage inputs  102  of the front end device  101  may be electrically balanced, and passed through a differential signal chain, to avoid common-mode noise, for example, when the signal outputs are connected to grounded communications test equipment  106 . Even though the voltage inputs  102  are electrically isolated, stray capacitance across the barrier can cause noise across the barrier. This may be minimized by using balanced voltage inputs  102 . Two wires would be used for each voltage input  102 . To reduce the number of connection points, and size of the front end device  101 , the front end device  101  may use differential amplifiers internally, and balanced resistive dividers, but tie one of the two input lines from each voltage input  102  to a common connection. In this configuration, the voltage inputs  102  may all be true balanced, differential inputs, but have one leg of each input connected together. This may result in the voltage inputs not only being isolated from the DSP  302  and the output  104 , but also from each other. In a low-cost front end device  101 , it may be possible to not isolate each of the voltage inputs  102  from the others, if other noise-reducing means are taken, such as, for example, DSP filtering to remove line frequency noise. 
     The front end device  101  may include a USB port for communications with a device such as a PC or laptop. PC software may be used to configure the front end device  101  through the USB port. The USB port may also be used to power the front end device  101 . The USB port may be an On-The-Go port which may allow the front end device  101  to be a USB peripheral when connected to a PC, but a USB host when connected a USB Flash drive. When a Flash drive is connected, the front end device  101  may stream the input and output signals to the connected drive in real-time, for analysis later. Configuration data from a PC to the front end device  101  could also be transferred via USB Flash card. The front end device  101  may also include other wired or wireless communications devices, such as, for example, Bluetooth, WiFi, Ethernet, or a cell modem, in addition to, or in place of, the USB port. In some embodiments, multiple front end devices  101  may be linked together through their communications port, allowing for cross triggering. 
     Since powerline conductors such as the powerline  103  are often outdoors, the front end device  101  may be rugged and weatherproof. A NEMA-4X or IP-65 rating may be required, and may be achieved through suitable enclosure design, connectors, and internal sealing and potting as needed. 
     The front end device  101  may include an audio output, such as a headphone or speaker output. The signal for the audio output may be generated by the DSP  302 , and may be suitably processed so the entire band or signal of interest is in the audio band. The audio output may be a standard ⅛″ or ¼″ stereo jack on the front end device  101 , or the signal may be brought out through the same cable harness as the BNC outputs through the output  104 . An onboard speaker may be used to play the output directly inside the front end device  101 . Alternatively, the front end device  101  may include an integral Bluetooth module, programmed to support the Bluetooth headset or wireless speaker profile. In that case, audio information may be sent via Bluetooth, and no speaker or headphone jack may be required. 
     The audio output may allow a user with headphones or a speaker to listen to PLC signals and noise. The DSP  302  may route applicable signals to special audio D/A outputs. For low-frequency PLC schemes, the entire band may already be inside the audio band, and no further processing may be necessary, except for scaling, bandpass filtering, possible time-domain blanking, and so on. Some PLC schemes are narrowband, but the band may be above the human audio range. In this case, the DSP  302  may shift the frequency band to the audio band with standard signal processing techniques, such as, for example, mixing with an internal oscillator and filter. If the signal bandwidth exceeds the audio band, sample rate conversion or other DSP  302  techniques may be applied to compress the bandwidth so that the entire band is audible. 
     PLC signals may be brought in to the front end device  101  through auxiliary inputs instead of through the voltage input  102 . In this case, the signals may have already been lowered to small voltage levels. The front end device  101  may use an external electric or magnetic field transducer to acquire the raw signal, which may be fed into the system. For example, a magnetic field transducer may be used to examine radiated PLC signals from a nearby powerline  103 . The transducer output may be fed into the same signal chain as the voltage inputs  102  of the front end device  101 , except for the resistive divider network. 
     The output  104  may be a high-voltage output, for example 120 or 240V nominal. The output  104  may be used to feed a filtered or conditioned signal into a PLC device for further testing. For example, the front end device  101  may be connected to the powerline  103 , set to filter out noise, and output the resulting conditioned signal at a 240V level. A PLC revenue meter may be attached to the signal for testing while various filters were tried in the front end device  101  to see the effects. Alternatively, an external amplifier may be used to boost the low-voltage output of the front end device  101  to line voltage levels. 
     Although configuration and control of the system has been described as occurring through a communications link to an external PC, PDA, or cell phone, other suitable control mechanisms may be used with the front end device  101 . For example, physical switches may be used to enable or disable preset filters and triggers, allowing changes in configuration in the field without another device. In a hybrid front end device  101 , the actions of the physical switches could be configured by a communications link to a PC beforehand. 
     The front end device  101  may include a GPS time-sync input, or a GPS or WWVB receiver to generate accurate timing information, or the front end device  101  may use the Network Time Protocol (RFC 1305), or IEEE 1588 to generate accurate time information or clock synchronization. This time information may be used for accurate sample rate measurement, and also for synchronization with external devices. For example, if a PLC transmission is known to occur at certain precise times, the front end device  101  may use the accurate time signal to perform special processing, or generate output markers at those times, as programmed by the user. Once time synchronization has been achieved, it may be possible to maintain synchronization by counting line frequency cycles, instead of continued use of a GPS or other time source. 
     The front end device  101  may be designed such that, with no power applied, a passive voltage reducer  201  and passive shelf filter are still operational, and an isolated signal is still present on one or more of the outputs  104 . Isolation in this case may be provided with a passive transformer. 
     The front end device  101  may perform one or more digital signal conditioning and filtering operations, to help adapt the communications test equipment  106  to the unique PLC signal and noise characteristics. Although these functions will be described as being performed by the DSP  302  with digital samples, the functions may also be implemented with special-purpose devices such as the FPGA  304 , or with analog hardware. 
     Each of the filtering operations may be configurable by the user, and have the ability to be enabled or disabled. Many of the filtering operations may be linear, so that the order of application is immaterial. Where this is not the case, the operation order may also be set by the user. In the case of multiple outputs  104 , different filter combinations may be performed on the same input signal, and presented on separate outputs  104  simultaneously. The following are operations that the front end device  101  may perform. 
     Adjustable bandpass filters may allow a user to narrow in on the band of interest. Many PLC schemes are narrow band, and isolating that band may be very helpful. Although spectrum analyzers may isolate signals and bands in the frequency domain, the extreme dynamic range presented by the presence of line frequency voltage may make this difficult without prefiltering in the DSP  302 . Additionally, preset bandpass filters designed especially for each PLC scheme may facilitate rapid analysis. In some cases, a custom filter may be desirable, especially one designed to match the input filtering of a targeted PLC receiver, including in imperfections in that filter. User software may allow arbitrary filter shapes, and also allow user-specified filter coefficients. In some cases, the user may specify an all-pass filter, or time-delay filter, for specialized applications. The user may also use the arbitrary filter feature to download custom filters into the front end device  101  for noise whitening or channel equalization. 
     A comb filter may remove the line frequency component, and all of its harmonics. This may be important for low-frequency protocols, which may coexist in the same band as the harmonics. Filter options include using the line frequency as the fundamental, or submultiples. For example, in a 60 Hz system, a 60 Hz comb filter will remove 60 Hz and all 60 Hz harmonics (e.g. 120 Hz, 180 Hz, etc.). Using the submultiple of 30 Hz also removes 90 Hz, 150 Hz, etc. The front end device  101  may use a phase-lock loop (PLL) or other scheme to adjust either the A/D sampling rate, or the precise filter coefficients so that the fundamental and most harmonics are optimally canceled. This may require that as many large harmonics as possible have periods that are an integer number of A/D samples. 
     A Shift/Subtract filter may also be implemented to remove synchronous noise. In a Shift/Subtract filter, a representative period of the powerline  103  waveform is used as a reference, and this waveform is subtracted from subsequent periods of data. To the extent that the synchronous noise is constant from one period to the next, it may be removed exactly. This may result in no zeros or other distortions being introduced to the PLC signals themselves. Since the synchronous noise is only relatively constant over a short time, typically 5 to 10 line frequency periods, a new reference waveform may need to be acquired often. This may be done with every period, unless a trigger condition is met which extends this time. The algorithm parameters to control the acquisition of new reference waveforms may be user-adjustable. As with the comb filter, the reference waveform length may be adjusted to a multiple of the line frequency period, for better cancellation. 
     The voltage reducer  201  may include a shelf-filter, to reduce the power line signal amplitude. This may reduce the required A/D dynamic range to practical values. Once the signal has been digitized, the dynamic range issue may be removed. In some cases, it may be desirable to restore the powerline  103  signal by reversing the analog shelf-filter. The DSP  302  may perform the inverse of this filter digitally. In other cases, it may be desired to perform a further shelf filter operation, and the DSP  302  may apply a standard shelf filter to further reduce the line frequency signal. In the case of multiple outputs  104 , the shelf-filtered signal may be applied to one output  104 , while the restored signal is applied to another output  104  for comparison. 
     The front end device  101  may optionally employ notch filters to remove narrowband interference. These may be fixed in frequency and bandwidth, adjustable by the user, or dynamically adjusted by the DSP  302  to automatically detect and track narrowband noise sources. The comb and shift/subtract filtering may remove most line frequency and harmonic components, but other non-synchronous narrowband interferes may still be present, which may require the use of notch filtering. 
     The DSP  302  may introduce pure time delays in the signal chain. This may be useful when using multiple outputs  104 , each with different filtering applied. If the filters introduce different amounts of time delay, the outputs  104  may be adjusted with explicit delays, designed to equalize the delay across all the outputs  104 . This may optionally be performed automatically. Manual delays may be desired when the signals are being examined in conjunction with other external signals or events, for example, when feeding the signals into an oscilloscope which is also connected to a PLC transmitter. 
     In front end devices  101  that perform single phase processing, the voltage inputs  102  for both 120V legs and the neutral may be provided. The two 120V signals may be combined by the DSP  302  to compute the 240V waveform, which may then be filtered and sent to the output  104  as if it had been measured directly. Other processing, specific to single-phase systems, may be performed on the signals before being sent to the output  104 . For example, in 120V PLC systems, a signal may be transmitted on one 120V leg, and it may be necessary to measure the coupling into the other 120V leg, by normalizing the output as compared to the first 120V input. 
     In front end devices  101  that perform polyphase processing for polyphase systems, e.g. 3-phase wye or delta systems, further processing may be present. A transformation matrix may be computed by the DSP  302 , which may maximize the noise independence among the phases, and minimize PLC signal leakage from one phase to the others. The transformation matrices may be user-configured, or automatically generated by the DSP  302  based on standard minimization or training techniques. Delta-wye and wye-delta conversion may also be performed in the DSP  302 , and the resultant internal signals made available for further processing, and sent as output signals. 
     Some PLC systems, especially low-bandwidth systems, may only use certain parts of the line frequency waveform. For example, the INSTEON system may transmit on 60 Hz, 120V lines, in bursts lasting 1.823 milliseconds. Each burst may only occur in a 1.823 ms window starting 800 microseconds before a 60 Hz zero-crossing. Thus, only this 1.823 ms time period may be of interest, out of an 8.333 ms 60 Hz half-cycle. The remainder of the 60 Hz waveform may be much larger in amplitude, and may obscure the window of interest if viewed on an oscilloscope, making it impossible to trigger on a transmitted signal. The front end device  101  could optionally zero out all samples outside of this time window, performing time domain masking. Since this is a nonlinear operation, it may be desirable to control when in the signal chain this occurs. The signal may be separated into to two streams, one with the signal window present and the remainder blanked out, and the opposite, for comparison. In polyphase systems, it may be desirable to mask all phases when any phase is outside the signal window, or unmask all phases when any phase is inside the signal window. Instead of zeroing samples outside the signal window, the front end device  101  may optionally just reduce the amplitude of the samples such that the waveform is still visible, but lower in amplitude than the signal window. This may allows the user to view the waveform data, but without obscuring the signal of interest. 
     In cases where time-domain masking (as described above) is enabled, the samples outside the signal window may normally be zeroed. Instead, the DSP  302  may apply time warping techniques to create a continuous time series out of just the signal window samples. For example, in the INSTEON case, 1.823 ms of data may be kept for every 8.333 ms of samples. The 1.823 ms of data may be stretched to cover the entire 8.333 ms of time, and inserted into the DSP  302  signal chain for further processing. This may be accomplished either by suitable interpolation and decimation techniques to internally resample the data at a new data rate, or by actually changing the D/A sample rate appropriately. For example, if the data were originally sampled at a 1 MHz rate, the signal window may be 1823 samples long, and may need to cover the time taken by 8333 samples. The 1823 samples may be upsampled and downsampled internally to create an 8333 sample equivalent series, suitable for sending to the D/A at a 1 MHz rate. Alternatively, the D/A sampling rate may be changed from 1 MHz to 218.769 kHz, and the original 1823 samples used. In either case, the zero samples that would be present with just time-domain masking may be removed, facilitating analysis with a standard spectrum analyzer. The time warping timing algorithm may track the line frequency to insure correct sample rate conversion. 
     To aid in detecting PLC signal transmission, the expected time-domain signal of a received transmission may be loaded into the DSP  302 . This reference waveform may used to continuously compute the cross-correlation with a voltage input. The cross-correlation signal may then be inserted into the DSP  302  signal chain for optional further filtering, and sent to a D/A output for examination with an oscilloscope or spectrum analyzer. This signal may also be used as a trigger for the communications test equipment  106 , indicating that a transmission was sent. Since the cross-correlation peak may occur in the center of the received waveform, time delays maybe used to move this peak to the beginning of the transmission, relative to the other outputs. This cross-correlation could be performed on any signal in the DSP  302 , including time-masked signals, and could also have any DSP  302  operation performed on it, as if it had been measured on the voltage input  102 . 
     Arbitrary math functions, such as invert, add, multiple, square, absolute value, integrate, etc. may be performed on any signal in the DSP  302  at any stage in the signal chain. For example, if signals of interest may occur in any half-cycle of the line frequency, it may be desirable to take the absolute value of the signal so that every half-cycle has the same polarity. If signal energy rather than magnitude is of interest, the signal may be squared, etc. Many modern oscilloscopes can also perform these functions, but since some of these may be nonlinear, it may be necessary to perform them before other filtering operations in the DSP  302 , which may require the DSP  302  to have this functionality. Sample rate conversion and decimation/interpolation could also be performed. 
     The DSP  302  may optionally compute the spectrum of a signal, via Fast Fourier Transform (FFT). The resulting magnitude or phase information may be sent as a time-domain output, repeating at the FFT update rate. This may allow viewing the spectrum of a signal with a standard oscilloscope, instead of requiring a spectrum analyzer. 
     Various useful markers and reference levels may be sent on D/A channels as separate signals, or superimposed on any output signals. For example, line frequency period or signal window markers may be created by replacing samples at the applicable spots with large values, designed to stand out from the data, appearing as vertical lines in the signal trace. These may be used to trigger an oscilloscope or spectrum analyzer, or just used for visual reference. Additionally, markers may be inserted to represent noise or other events in the masked portion when using time-domain masking. DC markers could be used to indicate reference levels for various PLC transmitter levels, signal thresholds, and so on. These DC reference levels may be generated inside the DSP  302 , and may include all applicable DC gains that are being applied to the other signals, to facilitate easy calibration with absolute levels after many internal DSP  302  filters have been applied. 
     If a coordinated time source is available, markers may indicate various programmed times, to mark known transmissions or other external events. For example, if a transmission or test occurs every 15 minutes on exact 15 minute boundaries, the GPS or other accurate time source may be used to generate a marker each time. 
     The DSP  302  may also synthesize sine waves, reference waveforms or DC levels, or other special signals on auxiliary D/A outputs. For example, a “perfect” received PLC signal could be loaded by PC software into front end device  101 , and this waveform sent out a D/A for oscilloscope comparison with the actual received signal. The reference waveform may either be repeated continuously, or only sent based on a trigger condition, such as a cross-correlation or signal level trigger, preprogrammed event time, and so on. Reference waveforms used for correlation computations may also be sent as an auxiliary output. The front end device  101  may include memory to record signals, and these may also be sent to the D/A as an auxiliary output. 
     The front end device  101  may include a memory buffer which may allow the D/A outputs to be “paused”. When paused, the input data may continue to be processed, and D/A output values streamed to memory instead of to the D/A converters. When “unpaused,” the outputs may resume, with their values read from memory, reading the earliest data first. From this point forward, D/A values may be read from memory, and new D/A values computed from incoming data streamed to the memory buffer in a First In First Out manner, with the outputs continuing to be delayed from the inputs by the pause time. The D/A data may be continuously buffered for a period of time, for example, 30 minutes, allowing a user to pause, “rewind”, and replay signals. PC control may allow the user to repeatedly play D/A values in an endless loop, to facilitate repeated analysis of the same signals with the communications test equipment  106 . The front end device  101  may include nonvolatile memory, which may allow users to replay the D/A values in a laboratory, after recording them in the field. 
     The front end device  101  may also allow time shifting of the A/D inputs instead of the D/A outputs. This may allow the user to adjust filter settings and signal chain parameters repeatedly on the same input data. In this case, all filter state information may be buffered, in addition to the A/D readings, to allow proper “rewinding” of the internal state of the front end device  101 . Time shifting of both A/D inputs and D/A outputs may be provided for in the same front end device  101 . 
     As stated, the front end device  101  may include nonvolatile memory, such as battery-backed SRAM or Flash memory. If the front end device  101  has a USB On-The-Go port, a Flash USB memory device may be used. Regardless of the nonvolatile memory source, the front end device  101  may allow streaming of A/D values to memory, and also to D/A outputs. This may facilitate time shifting and other device functions, but also may allow the readings to be transferred to a PC or other external device for further analysis. Additionally, the front end device  101  may be configured to play back previously recorded signals, or other signals placed into nonvolatile memory by the PC or other external device. Simultaneous playback of live data on some outputs  104 , alongside recorded or synthetic data on other outputs  104  may be provided for in the same front end device  101 . 
     The front end device  101  may include the ability to emulate the filtering and signal conditioning performed in off-the-shelf PLC equipment. For example, a user investigating signals using the INSTEON protocol may wish to enable the INSTEON emulation signal chain in the front end device  101 . This may cause the device to emulate the characteristics of an INSTEON reference design receiver, and output the resulting signal to a D/A channel. This may allow the user to use the communications test equipment  106  to view the result of normal PLC equipment filtering. This output may be viewed in parallel with other signal paths, for example, signal paths with wider bandwidth, or other different characteristics. The emulation parameters may be loaded by the PC software, or programmed into the front end device  101  with fixed parameters. 
       FIG. 9  depicts an exemplary procedure for analyzing PLC systems using a front end device. In block  901 , a line voltage may be received from a powerline. For example, the powerline  103  may be connected to the voltage input  102  of the front end device  101 . 
     In block  902 , the voltage of the line voltage may be reduced. For example, the front end device  101  may have a voltage reducer  202 , such as a transformer or a pulse-withstanding divider resistor, connected to the voltage input  102 . The line voltage may pass through the voltage reducer  202  and end up with a reduced voltage. 
     In block  903 , the line voltage may be converted into digital samples. For example, the front end device  101  may have the A/D converter  301  connected to the voltage reducer  202 . After the voltage of the line voltage has been reduced, the line voltage may be digitized by the A/D converter  301  and output as a digital signal with digital samples. 
     In block  904 , the digital samples may be filtered to extract PLC signals. The line voltage may carry PLC signals for a PLC system. The digital samples created from the reduced voltage line voltage may be sent to the DSP  302 , which may be connected to the A/D converter  301  in the front end device  101 . The DSP  302  may apply any suitable filtering techniques to the digital samples to extract the PLC signals, or noise that may be of interest in analyzing a PLC system. 
     In block  905 , the PLC signals may be converted into analog signals. The PLC signals extracted by the DSP  302  may be digital, as the PLC signals may be extracted from the digital samples. To allow the PLC signals to be analyzed by the communications test equipment  106 , the front end device  101  may have the D/A converter  303  attached to the DSP  302 . The D/A converter  303  may receive the digital PLC signals output by the DSP  302 , and may convert the digital PLC signals to an analog signal. The analog signal may include the PLC signals in analog form. 
     In block  906 , the analog signals may be output to communications test equipment. For example, the communications tests equipment  106  may be connected by the cable  105  to the output  104  of the front end device  101 . The analog signal output from the D/A converter  303  may be sent to the output  104 , where it may be received across the cable  105  at the communications test equipment  106 . The communications test equipment  106  may then perform any suitable tests and analyses on the analog signal, and the PLC signals included in the analog signal. 
     As used herein, a “computer” or “computer system” may be, for example and without limitation, either alone or in combination, a personal computer (PC), server-based computer, main frame, server, microcomputer, minicomputer, laptop, personal data assistant (PDA), cellular phone, pager, processor, including wireless and/or wire line varieties thereof, and/or any other computerized device capable of configuration for receiving, storing and/or processing data for standalone application and/or over a networked medium or media. Examples of communication media that can be employed include, without limitation, wireless data networks, wire line networks, and/or a variety of networked media. 
     Computers and computer systems described herein may include operatively associated computer-readable media such as memory for storing software applications used in obtaining, processing, storing and/or communicating data. It can be appreciated that such memory can be internal, external, remote or local with respect to its operatively associated computer or computer system. Memory may also include any means for storing software or other instructions including, for example and without limitation, a hard disk, an optical disk, floppy disk, DVD, compact disc, memory stick, ROM (read only memory), RAM (random access memory), PROM (programmable ROM), EEPROM (extended erasable PROM), and/or other like computer-readable media. 
     In general, computer-readable media may include any medium capable of being a carrier for an electronic signal representative of data stored, communicated or processed in accordance with embodiments of the present invention. Where applicable, method steps described herein may be embodied or executed as instructions stored on a computer-readable medium or media. 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements. Those of ordinary skill in the art will recognize, however, that these and other elements may be desirable. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. It should be appreciated that the figures are presented for illustrative purposes and not as construction drawings. Omitted details and modifications or alternative embodiments are within the purview of persons of ordinary skill in the art. 
     It can be appreciated that, in certain aspects of the present invention, a single component may be replaced by multiple components, and multiple components may be replaced by a single component, to provide an element or structure or to perform a given function or functions. Except where such substitution would not be operative to practice certain embodiments of the present invention, such substitution is considered within the scope of the present invention. 
     The examples presented herein are intended to illustrate potential and specific implementations of the present invention. It can be appreciated that the examples are intended primarily for purposes of illustration of the invention for those skilled in the art. The diagrams depicted herein are provided by way of example. There may be variations to these diagrams or the operations described herein without departing from the spirit of the invention. For instance, in certain cases, method steps or operations may be performed or executed in differing order, or operations may be added, deleted or modified. 
     Furthermore, whereas particular embodiments of the invention have been described herein for the purpose of illustrating the invention and not for the purpose of limiting the same, it will be appreciated by those of ordinary skill in the art that numerous variations of the details, materials and arrangement of elements, steps, structures, and/or parts may be made within the principle and scope of the invention without departing from the invention as described in the following claims.