Patent Publication Number: US-8537849-B2

Title: Low power telemetry system and method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Non-Provisional application Ser. No. 10/859,910, filed on Jun. 1, 2004, which claims the benefit of U.S. Provisional Application No. 60/474,316, filed on May 30, 2003. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of telemetry systems, and, more specifically, to very low power time division multiplexed analog domain telemetry systems. 
     RELATED ART 
     Conventional telemetry systems typically function in the digital domain, and consume excessive power because of the high clock rates incident to the use of digital circuitry, and the infrastructure needed to support digital domain processing. Dissipated power in such systems is ultimately dominated by this digital infrastructure and, as such, cannot be further optimized for very low power applications, such as autonomous underwater surveillance, very large scale fixed systems, multi-line towed array systems, and other multiplexed sensor systems that are required to operate in “power limited” applications. 
     SUMMARY 
     In a first aspect of this disclosure, a telemetry system is described. The system comprises a bus and a plurality of channels each coupled to the bus. A control subsystem (1) controls the channels so that each one presents to the bus, during a time period designated for the channel, a channel characteristic representative of a signal sampled by the channel, (2) interrogates in the analog domain each of the channels, during the time period designated for a channel, the characteristic presented by the channel, and (3) forms a signal representative of the characteristic presented by the channel during the designated time period. 
     In one embodiment, the control subsystem time division multiplexes each of the channels so that each and only one of the channels presents to the bus during the designated time period for the channel within a cycle a channel impedance proportional to a signal amplitude sampled by the channel. During the time periods in the cycle other than the time period designated for the channel, the channel presents an open circuit equivalent impedance to the bus. The control subsystem in this embodiment interrogates each of the channels by presenting to each of the channels during its designated time period an analog pulse over the bus. The channel impedance presented by the channel creates an impedance mismatch, which causes a modulated version of the pulse to be reflected back to a termination device coupled to one end of the bus. The modulation of the reflected pulse is representative of the channel impedance, and thus the amplitude of the signal sampled by the channel. The control subsystem forms a signal representative of the amplitude of the reflected pulse. In one embodiment, the control subsystem forms a packet from one or more of the signals, and then transmits the packet over a network. 
     Each channel in the telemetry system can include a sample-and-hold circuit, a variable resistor circuit, and a control element. The sample-and-hold circuit is configured to hold a sample of a signal. The variable resistor circuit is configured to present a variable impedance to one or more signal lines during a time period designated for the channel an impedance representative of the sample held by the sample-and-hold circuit. The control element is configured to control the variable resistor circuit to present to the one or more signal lines an open circuit equivalent impedance during times other than the time period designated for the channel. In some implementations, the channel also includes a switch which decouples the sample-and-hold circuit from the variable resistor circuit during times other than the time period designated for the channel. 
     In some implementations, the control element is a programmable control element that is programmable with a unique channel identifier, and may include a timing element which is updated responsive to a control signal. The programmable control element determines that the designated time period for the channel is occurring or will occur based on a comparison of the contents of the timing element with the unique channel identifier. The control element can be a zero power control element. 
     Other systems, subsystems or system components, methods, features and advantages of the invention or combinations of the foregoing will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, advantages and combinations be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1A  is a block diagram of one embodiment of a low power telemetry system according to the invention. 
         FIG. 1B  is a block diagram of another embodiment of a low power telemetry system according to the invention. 
         FIG. 2  is a block diagram of one embodiment of a termination device in the system of  FIG. 1A  or  1 B. 
         FIG. 3  is a block diagram of one embodiment of a channel in the system of  FIG. 1A  or  1 B.  FIGS. 4A-4I  comprise a circuit diagram of one implementation example of the channel of  FIG. 3 , wherein: 
         FIG. 4A  illustrates an input stage to a power conditioning and control signal recovery circuit; 
         FIG. 4B  illustrates a voltage regulation circuit; 
         FIG. 4C  illustrates a voltage-controlled impedance circuit; 
         FIG. 4D  illustrates an output gating circuit; 
         FIG. 4E  illustrates a channel programming port with a plurality of programming inputs; 
         FIG. 4F  illustrates a system synchronization acquisition circuit; 
         FIG. 4G  illustrates a low power sample &amp; hold circuit; 
         FIG. 4H  illustrates a system clock acquisition circuit; and 
         FIG. 4I  illustrates a low power channel control circuit comprising a programmable logic element and interfacing circuitry. 
         FIG. 5  is a flowchart of one embodiment of a method of telemetering one or more channels coupled to a bus. 
         FIG. 6  is a diagram illustrating one example of the time periods within a cycle. 
         FIG. 7  is a flowchart of one implementation example of a method of telemetering one or more channels coupled to a bus. 
     
    
    
     DETAILED DESCRIPTION 
     As utilized herein, terms such as “about” and “substantially” and “near” are intended to allow some leeway in mathematical exactness to account for tolerances that are acceptable in the trade. Accordingly, any deviations upward or downward from the value modified by the terms “about” or “substantially” or “near” in the range of 1% to 20% or less should be considered to be explicitly within the scope of the stated value. 
     The term “logic” refers to implementations in hardware, software, or combinations of hardware and software. 
     The term “packet” means a grouping of digital data and control elements which is switched and transmitted as a composite whole, wherein the data and control elements and possibly error control information are arranged in a specified format. The term “packet” includes a frame. 
       FIG. 1A  illustrates one embodiment of a telemetry system  100  according to the invention. The system comprises a bus  102 , and a plurality of channels  104   a ,  104   b ,  104   c ,  104   d ,  104   e , each coupled to the bus  102 . Note that the physical limitation on-the number of channels is determined by the combination of the sample rate and the round trip propagation time associated with the strobe-and-probe process. Thus, a telemetry system according to the invention is not limited to the number of channels described in this exemplary embodiment. A control subsystem  106 , which includes termination device  106   a , is configured to (1) control the channels  104   a ,  104   b ,  104   c ,  104   d ,  104   e  so that each one presents to the bus  102 , during a time period within a cycle designated for the channel, a channel characteristic representative of a signal sampled by the channel, (2) interrogate, during the time period designated for a channel, the characteristic presented by the channel, and (3) form a signal representative of the characteristic presented by the channel during the designated time period. 
     Each of the channels is assumed to sample (or have previously sampled) a signal provided by a sensor accessible to the channel. In the embodiment of  FIG. 1A , for example, channel  104   a  samples a signal provided by sensor  110   a , channel  104   b  samples a signal provided by sensor  110   b , channel  104   c  samples a signal provided by sensor  110   c , channel  104   d  samples a signal provided by sensor  110   d , and channel  104   e  samples a signal provided by sensor  110   e . Since it is contemplated that any type of sensor may be used in the system, these sensors are not considered part of the system  100  as broadly described. 
     In one embodiment, the control subsystem  106  time division multiplexes the channels so that each and only one channel presents to the bus a characteristic of the channel during a time period designated for the channel within a cycle. 
     The channel characteristic presented by the channel during its designated time period may be any measurable parameter such as a channel voltage, current, or a channel impedance representative of the amplitude of the signal sampled by the channel. In one embodiment, where the measured parameter is an impedance, control subsystem  106  may control each of the channels to present to bus  102  an open circuit equivalent output impedance during time periods in the cycle other than the time period designated for the channel. Control subsystem  106  may synchronize the channels so that each one samples the amplitude of the output of its corresponding sensor at about the same time. 
     Bus  102  may comprise one or more signal lines. In the embodiment of  FIG. 1A , bus  102  comprises three signal lines  108   a ,  108   b ,  108   c , but it should be appreciated that examples are possible where more or less than three signal lines are included in the bus. For example, the embodiment shown in  FIG. 1B  includes a bus  102  having four signal lines  108   a ,  108   b ,  108   c , and  108   d.    
     Control subsystem  106  may control and provide power to each of the channels through the same one or more signal lines in the bus. In the embodiment of  FIG. 1A , for example, control subsystem  106  may control and provide power to each of the channels through a differential signal transmitted over signal lines  108   b  and  108   c  which lines may be implemented, e.g. as a twisted pair or as a coaxial cable. In the embodiment of  FIG. 1B , control subsystem  106  may provide power to each of the channels over any two signal lines, such as  108   b  and  108   c , while a dedicated signal line, such as  108   d , may be used exclusively to transmit control signals to the channels. 
     In one example, where a pair of signal lines carries a differential signal that provides power and control, the differential signal has first and second states. In the first state, the differential signal provides power to the channels. In the second state, the differential signal performs a control function at one or more of the channels. A power storage element (such as a capacitor) at each channel provides power to the channel when the differential signal is in the second state. 
     A control subsystem  106  may comprise a termination device  106   a  at one end of bus  102 , and a distributed control element (not shown) at each of the channels. The distributed control element in a channel may identify the time period designated for the channel within a cycle. 
     In one example, the termination device clocks each of the distributed control elements through a clocking signal sent over bus  102 . In this example, each of the distributed control elements includes a timing element (such as a counter) which is updated responsive to the clocking signal. The distributed control element may then identify the time period designated for the channel based on a comparison of the contents of the timing element with a predetermined value for the channel. 
     The distributed control elements may be programmable with the predetermined value for the channel. In one example, each of the distributed control elements is remotely programmable with the predetermined value for the channel. 
     In one implementation, the distributed control element for one or more of the channels is implemented as a field programmable gate array (FPGA). Alternatively or in addition, the distributed control element for or more of the channels is implemented as a complex programmable logic device (CPLD). In one example, the CPLD is a CMOS CPLD. The CMOS CPLD may have a power consumption which varies directly with its clocking rate. In one example, the CMOS CPLD has about a zero power consumption at about a zero clocking rate. 
     In one embodiment, each channel includes a voltage controlled impedance circuit for presenting to bus  102  during the designated time period for the channel an impedance representative of the signal amplitude sampled by the channel. Control subsystem  106  may control this circuit so that an open circuit equivalent output impedance is presented to bus  102  during time periods in the cycle other than the time period designated for the channel. 
     The impedance presented by a channel to bus  102  during the designated time period may give rise to an impedance mismatch condition at the channel. Control subsystem  106  may interrogate a channel during its designated time period by transmitting from termination device  106   a  an analog pulse over bus  102 . The impedance mismatch at the channel may cause at least a partial reflection of the pulse to return to termination device  106   a , with the amplitude of the reflected pulse being representative of the impedance presented by the channel. 
     Control subsystem  106  may perform at least partial equalization of the transmitted pulses to compensate for variable attenuation caused by different distances between termination device  106   a  and each of the channels. In the embodiment of  FIG. 1A , for example, the interrogation pulse sent to channel  104   e  may experience greater attenuation than that sent to channel  104   a  since the distance between the termination device  106 A and the channel  104   e  exceeds that for channel  104   a . This is particularly true for configurations having very long cable arrays, for example, 1000 m or more. Accordingly, control subsystem  106  may perform equalization on the two pulses used to interrogate these channels so that the pulses will have about the same amplitude at the time they are received at their respective channels. 
     Control subsystem  106  may also comprise a linearization circuit to correct any nonlinearities introduced in the reflected signal by the voltage controlled impedance circuit. In one embodiment, these nonlinearities may be corrected by post-linearization of the reflected signal using a processor (not shown) in control subsystem  106 . For example, a digital value of a reflected signal may be read by means of the processor, and mapped into a lookup table to retrieve a corrected value. 
     Termination device  106   a  may be configured to combine the signals representative of the characteristic presented by one or more of the channels into a packet, and then transmit the packet over a network. In the embodiments illustrated in  FIGS. 1A and 1B , for example, the packet may be transmitted to the network over one or more egress lines  112 , which may be wireline or wireless links. In one example, the one or more egress lines  112  are wireline modem connections. Termination device  106   a  may also comprise a trunk interface or a network interface. In one example, the termination device is a backbone network interface. In another example, it comprises an ethernet interface. In a third example, it comprises an Internet interface. 
     Termination device  106   a  may be configured to process the one or more signals using a procedure which may be programmed into termination device  106   a . In one example, the procedure is remotely programmed into termination device  106   a . In the embodiments illustrated in  FIG. 1A  or  1 B, for example, the procedure may be programmed into termination device  106   a  over one or more ingress lines  114 , which again may be wireline or wireless links. In one example, the one or more ingress lines  114  are wireline modem connections. These one or more ingress lines  114  may also provide a mechanism for remotely programming the control element in the termination device to allow dynamic selection of preprogrammed sensor modes. These specific channel selection or sample rate modes are contained in the distributed control elements located at each of the channels. These mode values may be provided to the ingress lines  114 , and then routed over bus  102  to the individual distributed control elements. 
     The procedure may comprise a transformation (such as a Fourier transform) applied to one or more of the signals. The procedure may also comprise a feature extraction procedure. In one example, the procedure combines one or more of the signals into a beam. The procedure may also comprise a search procedure. In one example, the procedure comprises a predetermined mode of operation, such as a test mode, or another mode that continuously samples a selected channel while the other channels remain unsampled. In another example, the procedure comprises a schedule for interrogating more than one of the channels, or all of the channels. A skilled artisan will appreciate that many different modes and sequences may be realized by programming a sampling procedure into a termination device  106   a.    
       FIG. 2  is a block diagram illustrating one embodiment  200  of the termination element. In this embodiment, the termination element comprises a controller  202  coupled to a probe pulse driver/receiver  204  and a power &amp; control driver  205 . One or more bidirectional pulse probe signal lines  206  extend from the probe pulse driver/receiver  204 . One or more unidirectional differential power &amp; control signal lines  208  extend from the power &amp; control driver  205 . In addition, one or more unidirectional programming lines  214 , such as JTAG compliant programming lines, extend from the power &amp; control driver  205 . 
     The receiver side of the probe pulse driver/receiver  204  is coupled to A/D converter  210 . The output of the A/D converter  210  is coupled to the controller  202 . One or more JTAG compliant programming lines  212  form an input to the controller  202 . In addition, one or more signal lines  216  for outputting digital packet information forms an output of the controller  202 . 
     The one or more pulse probe signal lines  206 , and the one or more power &amp; control signal lines  208  together comprise one embodiment of the bus  102  previously described in relation to system  100 . 
     The controller  202 , through suitable signals sent and received by the probe pulse driver/receiver  204  over the one or more signal lines  206 , interrogates the channels in the analog domain. Similarly, the controller  202 , through suitable signals sent by the power &amp; control driver  205  over the one or more signal lines  208 , provides power to and controls the one or more channels in the previously described manner. In addition, the controller  202 , through the one or more signal lines  214 , programs the distributed control elements in each of the channels in the previously described manner. 
     The signals received from the channels during the interrogation process are routed to the A/D converter  210 , which converts them to digital values. The controller  202  combines one or more of the digital values into a packet, and routes the packets to a network over one or more signal lines  216 . 
     In one implementation example, controller  202  may comprise a single chip  8051  type microcontroller, probe pulse driver/receiver  204  may comprise a series of low resistance CMOS switches to form a differential line driver, power and control driver  205  may be implemented via a single pole double throw CMOS switch that toggles between the two states of the control circuit, and A/D circuit  210  may comprise an Analog Devices AD7677 A/D converter selected for its low power and high speed. In this example, controller  202  clocks the system at about 25-50 kHz, which is much lower than the MHz clock rate typically used to clock digital systems. The combination of the lowered clock rate, the time division multiplexing of the channels, the analog domain probing of the channels (which avoids the need to clock the data out at a high rate), and the use of a zero power CPLD (to be discussed), enables the telemetry segment of a subsystem deploying this technique to achieve a reduced power consumption compared to digital systems in the 5 milliwatt per channel range. 
       FIG. 3  is a block diagram of one embodiment of a channel  300  according to the invention. In this embodiment, it is assumed that the one or more probe pulse signal lines  206  form a twisted pair for the transmission and reception of a differential mode probe pulse  306 . It is also assumed that the one or more power &amp; control signal lines  208  likewise form a twisted pair for the transmission of a differential mode power &amp; control signal  308 . 
     In this embodiment, channel  300  comprises a programmable logic element  318 , such as a field-programmable gate array (FPGA). FPGA  318  in turn comprises a timing element  320  and a storage element  322  for holding a unique channel value in memory which may be remotely programmed into channel  300 . 
     A clocking circuit  324  receives differential power &amp; control signal  308 , and, in response, forms separate sync, strobe and power signals  326 ,  328 , and  330 , provided over separate sync, strobe, and power signal lines, respectively. A power storage element  332  is also provided as part of the clocking circuit  324 . When differential power &amp; control signal  308  is in a first state, it provides power to FPGA  318  over signal line  330 . It also provides power to storage element  332  which stores the power. When differential power &amp; control signal  308  is in a second state, it provides either sync signal  326  or strobe signal  328  to FPGA  318  over, respectively, the sync and strobe signal lines. During this time, power storage element  332  provides power to FPGA  318 , and also to support circuitry within the telemetry electronics of channel  300 . 
     The FPGA  318  includes a storage element  332  which holds a unique channel value previously programmed into FPGA  318  over one or more JTAG compliant signal lines  314 . In addition, FPGA  318  includes a timing element  334 , which, in response to strobe signal  328 , either counts up or down depending on the specific implementation. The contents of timing element  334  are compared with the channel value held by storage element  332 . When the two are equal, FPGA  318  asserts a signal on signal line  336 . Otherwise, signal line  336  remains unasserted. 
     A sample &amp; hold circuit  338  is also provided. This circuit receives a sensor input  340 . When sync signal  326  is asserted, FPGA  318 , responsive to signal  326 , sends a control signal  327  to sample &amp; hold circuit  338 . Upon receiving control signal  327 , sample &amp; hold circuit  338  samples the amplitude of sensor input  340 , and then holds the sampled value. This functional element allows the sensor output to be captured during a system quiet period (the bus power is off during this sampling period) as well as allows simultaneous capture of sensors where skew in samples will violate the processing requirements. 
     When signal  336  is asserted, a switch  342  is activated, allowing the sampled value held by sample &amp; hold circuit  338  to be received by a voltage controller impedance circuit  344 . In response, voltage controller resistor circuit  344  presents an impedance to the twisted pair carrying signal pulse  306  which is representative of the sampled amplitude held by the sample &amp; hold circuit  338 . 
     When signal  336  is not asserted, voltage controlled resistor circuit  344  presents an open circuit equivalent impedance to the twisted pair carrying pulse  306 . In addition, switch  342  is opened, thus decoupling sample &amp; hold circuit  338  from voltage controller resistor circuit  344 . 
     When a differential probe pulse  306  is received over the twisted pair, if the voltage controlled impedance circuit  344  is presenting an open circuit equivalent impedance to the twisted pair, pulse  306  terminates in the channel, and is not reflected. The decoupling provided by switch  342  also helps eliminate the introduction of noise and channel cross-talk into the telemetry electronics of channel  300 . 
     However, when a differential probe pulse  306  is received over the twisted pair, and voltage controlled resistor circuit  344  is presenting an impedance to the twisted pair that is representative of the sampled amplitude held by the sample &amp; hold circuit  338 , an impedance mismatch is created at the channel. Accordingly, an amplitude-modulated version of probe pulse  306  is reflected back over the twisted pair, with the degree of amplitude modulation representing the impedance presented by the channel (and hence the sampled amplitude held by sample &amp; hold circuit  338 ). The reflected pulse is thus sent back to termination device  200 . 
       FIGS. 4A through 4I  illustrate one implementation example of a channel according to the invention. These figures make up a single circuit diagram for electronics that comprise a channel such as channel  300  shown in  FIG. 3 . Interconnections between figures are represented by an upper case letter within a circle; for example, the encircled “A” in the upper right corner of  FIG. 4A  defines a direct electrical connection to the encircled “A” in the lower right corner of  FIG. 4I . Typical values for various electronic components comprising the circuit are as shown in the figures. However, these values are exemplary only, and should not be thought of in any limiting sense. Skilled artisans will recognize that many other component values and/or combinations of components may be used to achieve the same or similar results. 
       FIG. 4A  shows the input stage to the power &amp; control input conditioning circuit that is represented by block  324  in  FIG. 3 . A signal received over input line  401  sets one of a plurality of programming modes. Input line  406   a  functions as a system ground. Input line  406   b , shown on the voltage controlled impedance circuit of  FIG. 4C , provides a source for an interrogation pulse, corresponding to input  306 . Input line  408   a  provides a “sync” input, which is a combination of power and signaling corresponding to input  308   a  of  FIG. 3 . Input line  408   b  provides a “strobe” input corresponding to input  308   b . Signals  408   a  and  408   b  may originate from a power &amp; control driver circuit such as block  205  shown in  FIG. 2 . Power regulation for the power &amp; control input conditioning circuit is provided by 6V regulator  454  and 3.3 V regulator  456 , shown in  FIG. 4B . Regulators  454  and  456  may be conventional voltage regulators, for example, National Semiconductor adjustable type LP2980 or other low dropout equivalent. Additional configurations of this power circuit may eliminate the need for the 6 volt supply in as much as the true telemetry system is able to run strictly on a 3.3 volt supply. 
     Signal lines  406   a ,  406   b ,  408   a  and  408   b  may form a twisted quad in this particular example, with signal lines  406   a  and  406   b  forming a twisted pair within this quad, and with signal lines  408   a  and  408   b  forming a second twisted pair within this quad. Implementations utilizing differential mode signals transmitted over twisted pairs enable extended length busses, i.e., up to 500 meters or more, as well as enhanced reliability and low cost compared to single-ended or coax implementations. 
     Probe pulse signal lines  406   a  and  406   b  deliver a differential mode probe pulse to the channel, as well as return a modulated differential mode reflected probe pulse to the termination end. Power &amp; control signal lines  408   a  and  408   b  deliver a differential mode power &amp; control signal which has one of two states. In the first state, it provides power to the channel, and also power to charge capacitor  446 . In the second state, it provides either a sync or strobe signal to the channel, and capacitor  446  provides power to the channel. Diodes  448   a  and  448   b  prevent leakage from capacitor  446  when it is providing power to the channel. Thus, capacitor  446 , in combination with diodes  448   a  and  448   b , functions as an uninterruptible power supply, preventing any power perturbations in the system when signal lines  408   a  and  408   b  are in the second state. In this example embodiment, diodes  448   a  and  448   b  may be Panasonic type MA2 ZD1400 L or equivalent. 
     The input on signal line  408   a  (sync) and the input on signal line  408   b  (strobe) are each fed to a comparator circuit to determine if a sync or strobe signal is being received over these signal lines.  FIG. 4F  shows the sync input to a system synchronization acquisition circuit  462 .  FIG. 4H  shows the strobe input to a system clock acquisition circuit  463 . The strobe input corresponds to the optional fourth signal line  108   d  shown in the alternative embodiment of  FIG. 1B , and also to input  308   b  shown in  FIG. 3 . 
     When comparator  462  receives a sync input, comparator  462  outputs a logic signal  426  which provides a reset, or synchronization signal to FPGA  418  on the low power channel control circuit of  FIG. 4I . Similarly, when comparator  463  receives a strobe input, comparator  463  outputs a logic signal  428  which provides a strobe signal to FPGA  418 . In one embodiment, sync signal  426  and/or strobe signal  428  may be provided to FPGA  418  via redundant input lines for enhanced reliability. FPGA  418  may comprise any suitable programmable logic device, such as a XILINX type XCR3064XL-10VQ44 C. 
     If comparator  463  outputs a strobe signal  428 , strobe signal  428  increments a timing element, or counter, maintained by FPGA  418 . If comparator  462  outputs a sync signal  426 , it clears the counter maintained by FPGA  418 . A value unique to the channel is pre-programmed into FPGA  418  through programming signal lines  414   a ,  414   b ,  414   c , and  414   d , shown on the channel programming port of  FIG. 4E . In this embodiment, signal lines  414   a ,  414   b ,  414   c , and  414   d  correspond to JTAG compliant ports TDO, TCK, TDI, and TMS, respectively, for FPGA  418 . Within FPGA  418 , a storage element comprising, for example, EEPROMs, holds this unique value in memory. This value indicates the time period within a cycle that the channel should be active, i.e., presenting an impedance to the probe pulse signal lines which represents a sampled amplitude held by sample &amp; hold circuit  438 . When the contents of the counter equal the unique channel value, output gate signal  436  is asserted. Otherwise, signal  436  remains unasserted. 
     Refer now to the low power sample &amp; hold circuit of  FIG. 4G . When FPGA  418  receives a sync signal  426 , it sends out a sample &amp; hold control signal  470  to a high-impedance switch  472  in sample &amp; hold circuit  438 . As long as sync signal  426  is asserted, switch  472  closes, allowing sample &amp; hold circuit  438  to hold a sensor input signal P by storing a voltage in capacitor  474 . Sample &amp; hold circuit  438  holds this value until the next assertion of the control signal  426 . Until then, the circuit  438  outputs, through op amp  476 , a signal representative of its held value that is proportional to the sampled value of sensor output  440 . Switch  472  may comprise an Analog Devices ADG801 BRM low voltage CMOS switch, or equivalent. Op amp  476  may be any suitable device, such as a Texas Instruments type TLV2221 CDBVR, or equivalent. 
     In an alternative embodiment, sample &amp; hold circuit  438  may be configured with a means for shorting sensor output  440  to the output of op amp  476 , for example, by installing zero-ohm resistors, as shown. Bypassing sample &amp; hold circuit  438  allows further reduction in system power consumption where skewing effects are not critical. In such a case, the channel will acquire whatever sensor output  440  exists at the time a sync signal  426  is asserted. 
     When FPGA  418  receives a sync signal from the termination device, it asserts sample &amp; hold control signal  426 . The assertion of sample &amp; hold control signal  426  causes the sample &amp; hold circuit  438  to sample the amplitude of the signal present on input line  440 , and store the same in capacitor  474 . Sample &amp; hold circuit  438  holds this value until the next assertion of the control signal  426 . Until then, the circuit  438  outputs a signal representative of its held value on signal line  468 . 
     Refer now to  FIGS. 4C and 4D . Element  444  comprises a FET which functions in this particular example to provide the voltage controlled impedance for the channel. FPGA  418  controls whether FET  444  is turned on, thereby outputting an impedance representative of a sensor output level, or whether FET  444  is turned off, thereby outputting a high impedance. When output gate signal  436  is asserted, indicating that the designated time period for the channel is not present, switch  442  couples output gate  436  to the non-inverting input of op amp  478 , effectively shorting the input to op amp  478  and turning FET  444  off, i.e. causing FET  444  to present an open circuit impedance across its output. When output gate signal  436  is not asserted, indicating that the designated time period for the channel is present, the sample &amp; hold output signal  458  drives the non-inverting input to op amp  478 , allowing op amp  478  to display a voltage level representing the amplitude of a signal previously sampled from sensor output  440 . This voltage becomes a bias voltage at the gate of the FET  444 , turning FET  444  on and thereby presenting an impedance at the output of FET  444 . FET  444  operates in a linear range such that the output of the FET, which is coupled to probe pulse signal lines  406   a  and  406   b , presents to these signal lines an impedance which is representative of the sensor output  440 . In one exemplary embodiment, FET  444  may comprise a type VCR2 N or equivalent. In another embodiment, op amp  478  may comprise a Texas Instruments OPA704NA type device, or equivalent. 
     In one aspect of the invention, the output of FET  444  presents a resistance across probe pulse signal lines  406   a  and  406   b  representative of a sensor output  440 . The value of this resistance presents an impedance mismatch across  406   a  and  406   b , causing a reflected signal to occur whenever lines  406   a  and  406   b  transmit an interrogation pulse. The amplitude of that reflection is proportional to the resistance, thus, the reflected interrogation pulse comprises a signal modulated in proportional to sensor output  440 . 
     Referring again to  FIG. 4G , sensor output  440 , which is the input to sample &amp; hold circuit  438 , comprises an output signal of a sensor corresponding to any of sensors  110  through  110   e  as shown in  FIG. 1 . Any sensor, or sensor/transducer combination, may be used including but not limited to an acoustic sensor such as a hydrophone, or a magnetic, electromagnetic, or Hall effect sensor, or any other sensor or sensor/transducer that is capable of measuring a physical parameter and providing an output signal representative of the measured parameter, such as devices that measure flow, temperature, pressure, voltage, current, light, radiation, velocity, acceleration, etc. In one application, the sensor is part of a sensor array configured for underwater surveillance. 
     In one embodiment, sensor output  440  comprises the output of a signal conditioning circuit that provides a proper interface to sample &amp; hold circuit  438 . For example, a signal conditioning circuit may provide any of various signal processing functions well known in the art such as pre-amplification, anti-alias filtering, etc. and may also provide a desired signal-to-noise ratio through lowpass filtering techniques, etc., for a signal comprising sensor output  440 . Proper conditioning may also require sensor output  440  to be scaled to within a desired voltage signal range compatible with the operational characteristics of the telemetry system. 
       FIG. 5  is a flowchart illustrating one embodiment of a method  500  of telemetering one or more channels coupled to a bus. Step  501  comprises controlling the channels so that each channel presents a channel characteristic to the bus during a designated time period for the channel. The next step  503  comprises interrogating in the analog domain each of the channels during its designated time period. The next step  505  comprises forming a signal representative of the characteristic presented by the channel during its designated time period. Optional step  507  comprises combining the signals from one or more of the channels to form a packet, and then transmitting or outputting the packet. From optional step  507 , the method branches back to step  501  for another cycle. 
     Step  501  may comprise time division multiplexing the channels onto the bus so that each and only one channel presents a channel characteristic to the bus during a time period designated for the channel.  FIG. 6  illustrates a typical cycle  600  divided up into N time slots, one for each of the channels in a telemetry system. Time slot  601  is designated for channel  1 , time slot  602  is designated for channel  2 , and time slot  603  is designated for channel N. During time slot  601 , channel  1  alone presents its characteristic to the bus. During time slot  602 , channel  2  alone presents its characteristic to the bus. During time slot  603 , channel N alone presents its characteristic to the bus. The bus may have one or more signal lines, and method  500  may further comprise controlling and providing power to each of the channels through the same one or more signal lines of the bus. 
     In the case in which the channel characteristic presented by the channel is a channel impedance representative of the amplitude of a signal sampled by the channel, assuming time division multiplexing is in effect, each channel presents its impedance to the bus during its designated time period. However, at all other time periods in the cycle, each channel presents an open circuit equivalent impedance to the bus. Thus, in  FIG. 6 , during time period  601 , channel  1  presents its impedance to the bus, and all the other channels present an open circuit equivalent impedance to the bus. During time period  602 , channel  2  presents its impedance to the bus, and all the other channels present an open circuit impedance to the bus. During time period  603 , channel N presents its impedance to the bus, and all the other channels present an open circuit impedance to the bus. 
     The method  500  in  FIG. 5  may further comprise synchronizing the channels so that each one samples the amplitude of a signal at about the same time. This helps prevent clock skew and unwanted phase differences between channels in coherently processed high frequency applications. In one example, this synchronization is provided by a sync signal provided at the inception of a cycle. The sync signal causes a sample &amp; hold circuit at each of the channels to perform a sample &amp; hold function at about the same time. This sync signal may perform other synchronization functions, such as resetting counters maintained at each of the channels for purposes of locating the designated time period for the channel. 
     Method  500  may also comprise controlling and providing power to each of the channels through a differential signal transmitted over the same one or more signal lines. This differential signal may have first and second states, and the differential signal may provide power to the channel in the first state and perform a control function at the channel in the second state. While the differential signal is in the second state, a power storage element at the channel may be switched in to provide power to the channel. 
     Method  500  may further comprise identifying the time period designated for the channel by clocking a timing element or counter at each of the channels through a clocking or strobe signal sent over the bus. The timing element or counter is updated responsive to the clocking signal, and the contents thereof may then be compared with a unique channel identifier. If the two are equal, the time period designated for a channel is determined to be present. 
     Method  500  may further comprise programming one or more of the channels with a predetermined value for the channel. In one example, the method further comprises remotely programming the one or more channels with the predetermined value for the channel. 
     The impedance presented by a channel to the bus during the designated time period may give rise to an impedance mismatch condition at the channel. The channel may then be interrogated during its designated time period by transmitting an analog pulse over the bus. The impedance mismatch at the channel causes at least a partial reflection of the pulse to return to a termination device coupled to one end of the bus, where the amplitude of the reflected pulse is representative of the impedance presented by the channel. 
     Method  500  may also comprise performing at least partial equalization of the transmitted pulses to compensate for variable attenuation caused by different distances between the termination device and each of the channels. 
     The signals for one or more of the channels may be combined into a data packet at the termination device. The packet may further be transmitted to a local controller, to a storage device, or to a remote location over a network. 
     The one or more signals may be processed using a programmable procedure. In one example, the procedure is remotely programmable. In another example, the procedure comprises a transformation applied to the one or more signals. In third example, the procedure comprises a feature extraction procedure. In a fourth example, the procedure combines the one or more values into a beam. 
     The procedure may comprise a search procedure. It may also comprise a predetermined mode or schedule for interrogating the channels. In one example, the procedure changes the order, timing, sequence, or frequency at which the channels are interrogated during a cycle. For instance, an example array may comprise 32 channels. In a first mode of operation, each of the 32 channels may be interrogated during a cycle. However, in a second remotely programmed mode of operation, only some of the channels may be interrogated during a cycle at a higher frequency of interrogation than in the first mode of operation. 
       FIG. 7  is a flowchart of an implementation  700  of a method of telemetering a plurality of channels coupled to a bus. In step  701 , at the inception of a cycle, a sync signal is transmitted over the bus to each of the channels. The sync signal may cause a sample &amp; hold circuit at each of the channels to sample at about the same time the amplitude of a sensed signal input to the circuit at the channel. In addition, the sync signal may cause a counter at each of the channels to sync. 
     In the next step  703 , at the inception of or during a discrete time period within the cycle, a strobe signal and a probe pulse is transmitted over the bus to each of the channels. The strobe signal may cause a counter located at each of the channels to increment or decrement depending on the implementation. If the counter indicates that the designated time period for the channel has arrived, the channel may present an impedance to the bus which is representative of the sampled amplitude held by the sample &amp; hold circuit at the channel. Each of the other channels may present an open circuit equivalent impedance to the bus such that the impedance presented by the one channel gives rise to an impedance mismatch. An analog probe pulse may be used to interrogate the channels. The impedance mismatch at the one channel causes a reflected version of the probe pulse, with the amplitude thereof modulated based on the impedance presented at the channel, to reflect back to the termination device which originated the probe pulse. The open circuit equivalent impedance presented by the other channels causes the pulse to terminate at these other channels. 
     In step  705 , a signal is formed which is representative of the modulation of the reflected pulse from the one channel. This signal may be formed at the termination device. 
     In inquiry step  707 , an inquiry is made whether there are still more time periods within the cycle. If so, a branch is made back to the beginning of step  703 . If not, a branch is made to optional step  709 . In optional step  709 , the signals for one or more of the interrogated channels are combined into a digital packet. This packet may then be transmitted to a local controller or over a network. A branch may then made back to the beginning of step  701  for another cycle. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.