WO2002037705A1 - Method and device for mitigating the effects of quarter-wave shorts caused by branched wiring - Google Patents

Abstract

A device for reducing effects of quarter-wave shorts caused by branched wiring on communications over a twisted-pair communication link. An external telephone wiring (38) is terminated at a network interface (NID) or junction box (12) located on the exterior of the home or premises. A household telephone wire (10) is routed from the jacks on the NID to a number of telephone jacks (14) within the home. The telephone jacks (14) are located in various rooms around the house, and thus provide connectivity to these jacks. The telephone wiring will have multiple 'tie-ins' or wiring branches (16) of various lengths and configurations. These tie-ins create areas of attenuation at certain frequency ranges. These areas of attenuation are known as RF quarter-wave shorts. Using a synthetic delay line, the device can change a length and delay of the branch, and move the quarter-wave shorts to some other frequency range, and thus avoiding interference with shared-communications over the twisted-pair communication link.

Description

METHOD AND DEVICE FOR MITIGATING THE EFFECTS OF QUARTER- WAVE SHORTS CAUSED BY BRANCHED WIRING

Cross-Reference to Related Applications

5 Embodiments of the present invention claim priority from U.S. Provisional

Patent Application Serial No. 60/240,286 entitled "Synthetic Delay Line for Mitigating the Effects of Quarter- Wave Shorts Caused by Branched Wiring on Communications Over a Shared Communication Link," filed October 13, 2000, and are related to U.S. Utility Patent Application Serial No. 09/687,231 entitled "System 0 and Method for Enabling Simultaneous Multi-Channel Analog Cornmunications with Multiple Customer Premises Devices Over a Shared Communication Link, " filed October 13, 2000, and U.S. Utility Patent Application Serial No. 09/687,620 entitled "System and Method for Enabling Simultaneous Multi-Channel Digital Communications with Multiple Customer Premises Devices Over a Shared 5 Communication Link," filed October 13, 2000. The contents of each of these applications are incorporated by reference herein.

Background of the Invention

1. Field of the Invention

The present invention relates, generally, to communications over a shared 0 communication link and, in preferred embodiments, to systems, methods, and devices for mitigating the effects of RF quarter- wave shorts caused by branched wiring on cornmunications over the shared communication link.

2. Description of Related Art

As illustrated in FIG. 1, external telephone wiring 38 is typically terminated at 5 a network interface device (NID) or junction box 12 located on the exterior of the home or premises. The external telephone wiring 38 attaches to jacks on the side of the NID, which also includes a lightning arrestor. This portion of the NID is inaccessible to the user. The external telephone wiring 38 is then connected to a bank of consumer accessible jacks. Household telephone wiring 10 is typically routed from the jacks on the NID to a number of telephone jacks 14 within the home. The telephone jacks 14 are typically located in various rooms around the house, and thus to provide connectivity to these jacks, the telephone wiring will have multiple "tie- ins" or wiring branches 16 of various lengths and configurations. It should be noted that FIG. 1 is only exemplary, and that a number of wiring configurations are possible, including having every telephone jack 14 directly wired to the NID. As illustrated in FIG. 2, these tie-ins create areas of attenuation 18 at certain frequency ranges. These areas of attenuation are known as RF quarter-wave shorts, or more colloquially, as "suck-outs."

A brief discussion of RF quarter wave shorts will now be provided. Transmission lines may be comprised of different types of wiring such as coaxial cable, single ended wiring, or double ended wiring. Telephone wiring for example, is double ended. When transmitting over double ended wiring, such as in telephone wiring, cornmunications travel from point to point along a main path, but may also travel along additional double ended wiring that branches off from the main path for a certain distance. A null, or frequency of attenuation, will occur at the frequency of a signal having a quarter wavelength that travels the distance of the branch. For example, if the length of the branch was roughly 100 feet, the frequency of a signal having a quarter wavelength that travels 100 feet is about 2.5 MHz, and there may be a null of as much as a 60 dB at 2.5 MHz. In other words, communications operating at frequencies of about 2.5 MHz will essentially see a short in the line.

Thus, for example, if communication signals from the central office or exchange are transmitted at 2.5 MHz at a certain amplitude, by the time it is received within a home, the signal level will be substantially attenuated. Nevertheless, if the signal is still large enough, all of the 2.5 MHz signals may be received. However, if this signal is transmitted over household telephone wiring having an RF quarter-wave short at 2.5 MHz, the signal level will drop down another 60 dB, which may be below the usable threshold. In order for signals to be received, communications may need to be slowed down to avoid the RF quarter-wave short. In households of the past, the only signals that were communicated through the household telephone wiring were Plain Old Telephone Service (POTS) signals operating from 0 to about 4 kHz (see reference character 20 in FIG. 2). Because communications were limited to that frequency range, any RF quarter- wave shorts that appeared at other frequencies had no effect on POTS.

However, with the advent of new technologies that enable communications over a single household telephone line at higher frequencies, the RF quarter- wave shorts can become more of a problem. For example, as illustrated in FIG. 3, in addition to POTS 20, Asymmetric Digital Subscriber Line (ADSL) communications operate at frequencies from about 25 kHz to about 1 MHz (see reference character 22), and Home telephone Network Alliance (HomePNA) communications operate at frequencies from about 4 MHz to about 9.5 MHz (see reference character 24). Furthermore, customer premises gateways (CPGs) and distributed voice systems providing capabilities such as the communication of additional telephone channels over the same telephone line may operate within the frequency band of about 1 MHz to about 4 MHz (see reference character 26). See, e.g., U.S. Utility Patent Application Serial No. 09/687,620 entitled "System and Method for Enabling Simultaneous Multi-Channel Digital Communications with Multiple Customer Premises Devices Over a Shared Communication Link," filed October 13, 2000.

With the proliferation of new communications protocols now occurring over household telephone wiring, RF quarter-wave shorts 18 that occur in a frequency band utilized by one of these communications protocols can cause communications at that particular frequency band to be severely attenuated and essentially disabled. Furthermore, unlike the more uniform twisted pair wiring that may have used in the past, which had about 206 millihenries per 100 feet, newly installed wiring is much less uniform. As a result, the impedance is constantly changing over the frequency ranges of concern.

Using ADSL as an example, the non-ideal conditions presented by branched telephone wiring cause RF quarter-wave shorts that have forced ADSL to operate a speeds of less than 1 MHz, much slower than optimum. Isolation filters may be placed between an ADSL modem and the remainder of the household telephone wiring to isolate the ADSL modem from the complex impedance represented by the remainder of the household wiring. ADSL signals can then be routed straight to the ADSL modem without being affected by the remainder of the household telephone wiring. However, if multiple ADSL modems are needed to provide ADSL service to other computers, then an isolation filter would be needed for each ADSL modem.

Nevertheless, even if the RF quarter- wave shorts are eliminated, the 1 MHz speed limitation on ADSL currently remains. ADSL drivers, located at the telephone company central office or exchange, must drive a certain amount of current through the resistance of the telephone wiring between the exchange and a home in order to sweep out the capacitance of that wiring. Because conventional ADSL drivers can only drive a limited amount of current, and because of the resistance in long lengths of wiring, ADSL speeds are generally limited to about 1 MHz at a distance of not more than two miles of wiring, as dictated by the RC time constant. Homes that are located more than two miles away may encounter further degradation in the form of diminished speed.

Over time, new ADSL drivers may be developed that can sweep a larger amount of current through the telephone lines, and thus future versions of ADSL may be able to theoretically operate at frequencies higher than 1 MHz. However, at frequencies higher than 1 MHz, the RF quarter- wave shorts may become the speed- limiting factor, unless the isolation filters are used. If an isolation filter is added, ADSL should be able to operate at optimum rates because the modem is isolated from the remainder of the household telephone wiring, and the speed of the connection from the exchange to the home has been improved. It should be noted that other presently available filters, designed to be installed at the telephone, do not affect the RF quarter- wave shorts. Household telephone wiring is susceptible to interference from a number of sources. For example, if any non-linear junction exists in the household telephone wiring, such as a non-linear junction that may be the result of a poor wiring connection, nearby AM broadcasts may be rectified and ultimately heard over the telephone line. Conventional telephone line filters are low pass filters that cut off at frequencies just above POTS, and are designed to eliminate these interfering signals.

Summary of the Disclosure

Therefore, it is an advantage of embodiments of the present invention to provide a synthetic delay line for mitigating the effects of RF quarter-wave shorts caused by branched wiring on communications over a shared communication link.

It is a further advantage of embodiments of the present invention to provide a synthetic delay line for mitigating the effects of RF quarter-wave shorts caused by branched wiring that essentially converts the complex and variable multi-branched telephone wiring of any premises into a known and manageable quantity.

It is a further advantage of embodiments of the present invention to provide a synthetic delay line for mitigating the effects of RF quarter-wave shorts caused by branched wiring that is inexpensive and can be installed by a consumer, without a need for expensive truck rolls. It is a further advantage of embodiments of the present invention to provide a synthetic delay line for mitigating the effects of RF quarter- wave shorts caused by branched wiring that is self-adjusting to compensate for the unique wiring characteristics of any customer premises, thereby enabling communications to occur within a larger number of customer premises. These and other advantages are accomplished according to a device for reducing effects of quarter-wave shorts caused by branched wiring on communications over a twisted-pair communication link. The device includes M upper inductors connected in a series arrangement and identified in their serial arrangement as UL_X through UL_M. Each connection between the M upper inductors defines M-1 upper nodes identified in order as UNj through UN_M_, within the serial arrangement of M upper inductors. Inductor UL_! has an unconnected terminal UN₀ and inductor UL_M has an unconnected terminal UN_M.

The device also includes M lower inductors connected in a series arrangement and identified in their serial arrangement as LL_X through LL_M. Each connection between the M lower inductors defines M-1 lower nodes identified in order as LN_! through LN_M__! within the serial arrangement of M lower inductors. Inductor LL_j has an unconnected terminal LN₀ and inductor LL_M has an unconnected terminal LN_M.

The device also includes M-1 capacitors identified as C_t through C^. Each capacitor C_κ is connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1. UN₀ and LN₀ are couplable to a distal end of a branch on the twisted-pair communication link. Furthermore, the values of the M upper inductors, the M lower inductors, and the M-1 capacitors are selected to effectively change a length and delay of the branch to which the synthetic delay line may be coupled and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims.

Brief Description of the Drawings

FIG. 1 illustrates a simplified block diagram of typical household telephone wiring having multiple wiring branches.

FIG. 2 illustrates the frequency spectra of POTS and some RF quarter- wave shorts that may be present on household telephone wiring.

FIG. 3 illustrates the frequency spectra of the communication platforms and some RF quarter- wave shorts that may be present on the household telephone wiring.

FIG. 4 is a circuit diagram of a synthetic delay line according to an embodiment of the present invention. FIG. 5 is a circuit diagram of a synthetic delay line according to an alternative embodiment of the present invention.

FIG. 6 illustrates a simplified block diagram showing the synthetic delay line added to the distal end of the wiring branches of typical household telephone wiring according to an embodiment of the present invention. FIG. 7 illustrates the frequency spectra of some communication platforms that may be present on the household telephone wiring, and some RF quarter-wave shorts that have been pushed down to a region between about 1.1 MHz to 1.4 MHz due to the addition of a synthetic delay line according to embodiments of the present invention.

FIG. 8 is a block diagram of a synthetic delay circuit according to an embodiment of the present invention.

FIG. 9 is a representative reflected signal seen by the synthetic delay circuit according to an embodiment of the present invention. FIG. 10 is a table illustrating the four possible states corresponding to the six- inductor synthetic delay circuit of FIG. 9 according to an embodiment of the present invention.

FIG. 11 is a table illustrating the generation of control signals based on characteristics of the reflected signal, and their effect on a state machine within the synthetic delay circuit according to an embodiment of the present invention.

FIG. 12 is a state diagram illustrating the states and transitions of the state machine within the synthetic delay circuit according to an embodiment of the present invention.

Detailed Description of Preferred Embodiments In the following description of preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the preferred embodiments of the present invention.

Embodiments of the present invention generally disclose a delay element which may be added to the end of each wiring branch and to a CPG to move the RF quarter-wave short associated with the wiring branch to a frequency range that will create minimal interference with communications occurring over the premises telephone wiring. In one embodiment, the delay line may comprise a roll of wire at a desired length. However, in preferred embodiments illustrated in FIG. 4, a synthetic delay line 28 is disclosed which is comprised of three capacitors 30 (identified as Cj -C₃) and six inductors 32 (identified as UL_t - UL₃ and LL_t - LL₃). Nodes UN₀ - UN₃ and LN₀ - LN₃ are defined between the inductors. Depending on the desired length of the synthetic delay line, the design may be extended or minimized (see, e.g. , the minimized example illustrated in FIG. 5). In addition, in alternative embodiments not shown in FIG. 4, a capacitor C₀ maybe added between UN₀ and LN₀, or C₃ may be eliminated from the synthetic delay line 28. In FIGs. 4 and 5, UN₀ and LN₀ are connectable to the distal end of a twisted-pair wiring branch, while UN₃ and LN₃ may be connected to a telephone or other device. It should be noted that the synthetic delay line, although a wideband filter by design, is not truly a filter in its performance.

This synthetic delay line 28 may also be built into a device such as a telephone, or a peripheral node interface (PNI) (see e.g., U.S. Utility Patent Application Serial No. 09/687,231 entitled "System and Method for Enabling Simultaneous Multi-Channel Analog Communications with Multiple Customer Premises Devices Over a Shared Communication Link," filed October 13, 2000). In alternative embodiments, the synthetic delay line 28 can be designed into a telephone jack junction box, either external or internal to a wall, so that there is no need to manually connect a synthetic delay line. Because the telephone or PNI has a certain amount of input capacitance and the telephone wiring has a certain amount of capacitance, some of the capacitors on the ends of the delay line may not be required. As illustrated in FIG. 6, in preferred embodiments of the present invention the synthetic delay line 28 is coupled to the distal end of every household telephone wiring branch 16, whether a telephone 34 or other device is connected to the branch or not. However, in alternative embodiments, some of the branches (such as unused telephone jacks) may not be coupled to a synthetic delay line, with some degradation in performance. In addition, if a CPG 70 is present, a synthetic delay line 28 may be coupled to the distal end of the telephone wiring branch 16 to which the CPG 70 is connected. As noted above, branches may create suck-outs that undesirably occur within the frequency band of certain communication channels. For example, given a 100 foot wiring branch, the frequency of a signal having a quarter wavelength that travels 100 feet is about 1.5 MHz, and thus there may be a null of as much as 60 dB at about 1.5 MHz. With such a null, communications in the 1.5 MHz range may be degraded or impossible. The presence of a synthetic delay line on a branch creates a certain amount of additional signal delay, which effectively creates additional line length. This additional line length can move the suck-out to a frequency range outside of existing of communication channels. As FIG. 6 illustrates, because the synthetic delay line 28 is added to existing branches 16, the length of those branches 16 must be taken into account. Empirical studies have found that the average length from the junction box to a telephone jack in a home is anywhere from about 10 feet to up to about 100 feet. With a synthetic delay line added to all extensions, including the shortest extensions and longest extensions, the effective length of each branch may be anywhere from about 110 feet (10 feet + 100 feet) to about 200 feet (100 feet + 100 feet). In the present example, these lengths will push the 1.5 MHz RF quarter- wave short 18 down to a range from about 1.1 MHz to about 1.4 MHz, as illustrated in FIG. 7.

FIG. 7 illustrates that RF quarter-wave shorts 36 occurring between 1J MHz and 1.4 MHz avoid the frequency spectra of some communication platforms that may be present on the household telephone wiring. In addition, another reason for designing the synthetic delay line 28 to push the RF quarter-wave shorts down to a region between about 1J to 1.4 MHz is that these frequencies fall within the frequencies of AM radio stations (530 kHz to 1.7 MHz). As described above, cold solder joints, poor connections or other dissimilar materials can all cause intermodulation from nearby AM radio stations. This intermodulation interferes with ADSL, HomePNA, intercom functionality, and other communications occurring over the household telephone lines. Because of this intermodulation, communications over household telephone wiring are designed to avoid the AM radio station frequency bands. Because these frequency ranges are generally utilized less than other frequency ranges, it makes sense for the RF quarter- wave shorts to be pushed into these areas.

For the communication platforms employed in FIG. 7, preferred synthetic delay line embodiments will effectively create approximately 180-200 nanoseconds of synthetic delay, the equivalent of about 100 feet of additional telephone wiring. In preferred embodiments, the synthetic delay line 28 uses six 4.7 microhenry inductors 32 for ULj - UL₃ and LL_X - LL₃, one 330 picofarad capacitor for C_l5 one 470 picofarad capacitor for C₂, and one 680 picofarad capacitor for C₃. These component values will produce a synthetic delay line that will push the quarter-wave shorts for most branches down to a region between about 1J to 1.4 MHz. Because of the signals that can be present on the telephone line, the capacitors 30 should be rated to at least 270 volts. In addition, because of the current required to ring telephones, the inductors 32 should be rated to at least 50 milliamperes.

More generally, however, in alternative embodiments of the present invention, the synthetic delay line components can be chosen to move the RF quarter-wave short to other frequencies. Thus, for example, if there is an unused frequency band at about 2.5 MHz, in alternative embodiments a synthetic delay line may be designed that would create RF quarter- wave shorts at around 2.5 MHz.

Although the preceding discussion disclosed a fixed synthetic delay circuit, in alternative embodiments of the present invention illustrated in FIG. 8, a synthetic delay circuit 40, which includes the synthetic delay line 28, can be self-tuning or adjusting. In alternative embodiments, each synthetic delay circuit 40 includes a pulse generator 42 in a bridged configuration which sends pulses out onto the shared communication link 10 and records time-domain reflectometry information on the reflected pulse in a time-domain reflectometer 44. From the observed rise time of the reflected pulses, an open line, short circuit, change in impedance, and other data can be inferred. Furthermore, with this information the synthetic delay circuit 40 can be configured to move the notch or suck-out for any given extension to a frequency range wherein it will not affect communications over the communications over the link, as described in detail above. The pulse generator 42 according to embodiments of the present invention may comprise a "555-type" timer configured to generate a square wave. However, in alternative embodiments, any pulse generation circuit may be employed. The generated pulses travel out over the communication link 10 and are eventually reflected back to the time-domain reflectometer 44.

FIG. 9 is an example of the reflected signal that may be observed by the time- domain reflectometer 44 as a result of the generated pulses, normalized to the impedance of the line, for example 250 ohms, by setting a bridge circuit within the time-domain reflectometer 44. In preferred embodiments, the bridge is factory-set to the impedance of a particular type of wiring. However, in alternative embodiments, the bridge may be adjustable to adapt to any line impedance. The initial pulse 46 is initially seen starting at time t₀, and rising to some voltage v_t. Depending on the normalized impedance, the reflected signal will drop to some intermediate value v_n and remain essentially constant for some time. At some later point in time t_z, the reflected pulse may be observed by a change in the reflected voltage. If the reflected signal goes down (see reference character 48), a lower impedance exists at the end of the line, and if the reflected signal goes up (see reference character 50), a higher impedance exists at the end of the line. For example, if the reflected signal drops to zero volts, the line is shorted to ground. If the reflected signal abruptly stops and rises straight up, the line is open. In addition, the effect of quarter- wave shorts caused by branches or extensions in the communication link can be seen in the reflected signal. These quarter- wave shorts will appear as dips or discontinuities in the reflected signal, such as the dips 52 and 54 shown in FIG. 9. Furthermore, by observing the point in time at which the signal changes, the location on the line where the change in impedance occurs can be estimated.

As described in detail above, in one example application the intent of the synthetic delay circuit is to introduce enough effective length into each branch or extension such that each branch will appear to be at least 110 feet. This will push the quarter-wave short associated with each branch down to a frequency range of around 1J to 1.4 MHz, thereby avoiding interference with communications occurring over the communication link. The effective length of each branch can be estimated from the location of the dips in the reflected signal. The location of a dip corresponding to a branch length of about 110 feet and a quarter- wave short frequency range of about 1J to 1.4 MHz is indicated by time t-. in FIG. 9, which has been empirically determined to be about 180-200 nanoseconds. (It should be noted that because 180-200 nanoseconds corresponds to about 110 feet, the actual reflection time is about 360-400 nanoseconds. However, the time-domain reflectometer 44 is calibrated such that the apparent time of interest t_n, as illustrated in FIG. 9, is about 180-200 nanoseconds.) The present invention utilizes the calibrated t_n value, as it corresponds to a particular branch length, to determine when a change in the delay of the synthetic delay line is advantageous, and the direction of that change (either more or less delay). However, the determination of a precise value for t_n corresponding to a particular branch length is beyond the scope of the present invention.

More generally, embodiments of the present invention can move the quarter- wave shorts associated with branched wiring to frequencies other than 1J to 1.4 MHz, depending on the frequencies of the communications occurring over the communication link. Thus, the desired branch length and the time t_n may vary accordingly. However, for purposes of explanation only, the following discussion will continue the example presented above, and presume a desired branch length of at least 110 feet and a t,. value of 180-200 nanoseconds. In view of the above empirical determinations, dips occurring before time t_n are caused by branches that can be estimated as being less than 110 feet. For example, in FIG. 9, the branch associated with dip 52 can be estimated as shorter than the branch associated with dip 54, although both can be estimated as less than 110 feet, because the dips occur sooner than t_n. Because any dips occurring before time t_. indicate quarter-wave shorts that may interfere with communications over the communication link, preferred embodiments of the present invention can reconfigure the synthetic delay circuit to push the dip to about the 180-200 nanosecond region, subject to the effective length limitations of the synthetic delay circuit. Similarly, because any dips occurring after time t_n indicate quarter-wave shorts that may interfere with communications over the communication link, preferred embodiments of the present invention can reconfigure the synthetic delay circuit to push the dip down to about the 180-200 nanosecond region, subject to the effective length limitations of the synthetic delay circuit. Note that the scenario of shortening the synthetic delay line to push dips down to the 180-200 nanosecond region is only applicable when the synthetic delay line is presently configured for some amount of delay. Under such circumstances, the delay can be reduced until the synthetic delay line has effectively no delay.

In preferred embodiments of the present invention, the reflected is signal is captured by a voltage sensor with the time-domain reflectometer 44, which continually samples the reflected signal over time to determine when a dip occurs. If a dip is detected prior to a window about 180-200 nanoseconds (see reference character 64 in FIG. 9), an active "length increase" control signal 58 may be generated by the voltage sensor (see FIG. 8). If a dip is detected after the window about 180-200 nanoseconds (see reference character 66 in FIG. 9), an active "length decrease" control signal 60 may be generated (see FIG. 8). If a dip is detected within the pre-defined window around 180-200 nanoseconds (see reference character 68 in FIG. 9), no active length increase control signal 58 or length decrease control signal 60 may be generated.

In preferred embodiments of the present invention, these control signals are communicated to a adjustment control circuit 56. In preferred embodiments, the adjustment control circuit 56 is a state machine that may be fabricated in a single device, such as a field-programmable gate array (FPGA). The state machine can cycle through a fixed number of states, in either direction, depending on the state of length increase control signal 58 and length decrease control signal 60. Each state represents a particular configuration of symbolic switches, which are part of a symbolic switch bank 62 illustrated in FIG. 8. The exemplary six-inductor synthetic delay circuit of FIG. 8 illustrates three upper switches identified as USj through US₃, and three lower switches identified as LS_X through LS₃.

A table illustrating the four states corresponding to the exemplary six-inductor synthetic delay circuit of FIG. 8 is given in FIG. 10, including the configuration of each switch for each state, and the effect of the switch configurations on the effective length of the synthetic delay circuit. Referring again to FIG. 8, each switch, when closed, effectively removes the inductor that it spans from the circuit, and shortens the effective length of the synthetic delay circuit. When the switch is open, the inductor becomes part of the circuit, increasing the effective length of the synthetic delay circuit. Although switches are symbolically illustrated in FIG. 8, in preferred embodiments of the invention, a field effect transistor (FET) may be used for each switch. In alternative embodiments, other transistors, transmission gates, and current switch devices may be employed.

FIG. 11 is a table illustrating the effect of length increase control signal 56 and length decrease control signal 58 on the states of the state machine. FIG. 12 is the state machine corresponding to FIGs. 10 and 11 according to a preferred embodiment of the present invention. In preferred embodiments, the voltage sensor periodically generates new control signals after enough of a time delay has been provided to allow the switches to be reconfigured, and to allow a new pulse to be generated, reflected, captured, and analyzed. In preferred embodiments, the state machine may be implemented using an up/down counter with a disable feature. It should be noted, however, that the control signals of FIG. 8 and 11, and the state machine of FIG. 12, may be replaced with other control mechanisms without departing from the scope of the present invention. For example, in alternative embodiments, a microprocessor and lookup table may also be employed.

In one embodiment of the present invention, a CPG (see e.g., the U.S. utility patent application entitled "System and Method for Enabling Simultaneous Multi- Channel Analog Communications with Multiple Customer Premises Devices Over a Shared Communication Link," attorney docket no. 080632/0113, filed October 13, 2000) may periodically send a command out to each PNI which triggers the synthetic delay circuit to 40 to perform the previously described adjustment process. However, because of the transmission and reflection of pulses, it may be undesirable for the PNIs to simultaneously perform the adjustment process. Therefore, in preferred embodiments, the customer premises gateway can sequence the adjustment process so that each PNI performs the adjustment process at different times. It should be also understood that the complete configuration process for all PNIs coupled to a communication link may take several iterations (repetition of the sequenced adjustments), because each time one synthetic delay circuit is adjusted, it has an effect on the apparent length of the branch that is seen by another PNI. Thus, in preferred embodiments of the present invention, the sequenced adjustment process may be repeated multiple times. After a certain number of iterations, the synthetic delay circuits for each PNI should be tuned to a reasonably optimal configuration, and the iterations can cease. For example, assume that FIG. 9 illustrates the reflected signals seen by a synthetic delay circuit on a branch A, and that dip 54 is caused by a branch B, and dip 52 is caused by a branch C. If the synthetic delay circuit on branch A is configured to move dip 54 into desired region 68, dip 52 will likely move in the same direction, but may not move by the same amount, and may not reach desired region 68. However, if a synthetic delay line on branch C is subsequently configured, the dip 52 seen by the synthetic delay circuit on a branch A may then move into region 68. In general, as each synthetic delay line on a communication link is configured in a sequential and iterative fashion, the reflected signals seen by each synthetic delay circuit will change until eventually, all of the dips reach an equilibrium position within region 68.

In an alternative embodiment of the present invention, every time the synthetic delay circuit 40 is powered on, the synthetic delay circuit will perform the adjustments described above. In a further alternative embodiment, each synthetic delay circuit may have a pushbutton or other triggering device which allows the adjustment process to be manually performed.

As with the synthetic delay line 28 of FIG. 4, the synthetic delay circuit 40 may be a stand-alone device, or it may also be built into a device such as a telephone or a PNI. In alternative embodiments, the synthetic delay circuit 40 can be designed into a telephone jack junction box, either external or internal to a wall, so that there is no need to manually connect a synthetic delay circuit. In further embodiments, the synthetic delay circuit 40 could be placed between the NID and a smart device, such as a computer which receives ADSL signals. As the computer is moved from room to room, or indeed any time the synthetic delay circuit is installed at the end of another branch, it can perform the above-described adjustment process to optimize the synthetic delay circuit for that particular branch.

It should also be understood that synthetic delay lines according to embodiments of the present invention may also be useful in HomePNA computing systems communicating over the household telephone lines. The synthetic delay lines would be located between each computer and the telephone jack, and if necessary, on each unused telephone jack. Furthermore, alternative embodiments of the present invention are also applicable to shared communication links other than household telephone wiring, such as telephone wiring in businesses, or communications occurring over the AC power wiring.

In any of these implementations, the synthetic delay circuit may be either connectable to AC power or battery operated. The synthetic delay line and circuit described herein is simple enough to install that it can be mailed to consumers for self-installation, reducing truck rolls and overall costs. Therefore, embodiments of the present invention provide a synthetic delay line for mitigating the effects of RF quarter-wave shorts caused by branched wiring on communications over a shared communication link. A synthetic delay line is disclosed that essentially converts the complex and variable multi-branched telephone wiring of any household into a known and manageable quantity. Embodiments of the present invention also provide a synthetic delay line that is inexpensive and can be installed by a consumer, without a need for expensive truck rolls. Embodiments of the present invention also provide a synthetic delay circuit for mitigating the effects of RF quarter-wave shorts caused by branched wiring that is self-adjusting to compensate for the unique wiring characteristics of any customer premises, thereby enabling communications to occur within a larger number of customer premises.

Claims

CLAIMSWhat is claimed is:

1. A device for reducing effects of quarter- wave shorts caused by branched wiring on communications over a twisted-pair communication link, comprising:

M upper inductors connected in a series arrangement and identified in their serial arrangement as UL_! through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UNj through UN_M-_! within the serial arrangement of M upper inductors, inductor ULi having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M;

M lower inductors connected in a series arrangement and identified in their serial arrangement as LL_t through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LN_t through LN_M.j within the serial arrangement of M lower inductors, inductor LLj having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M; and

M-1 capacitors identified as through C_M._l5 each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1; wherein UN₀ and LN₀ are couplable to a distal end of a branch on the twisted-pair communication link; and wherein values of the M upper inductors, the M lower inductors, and the M-1 capacitors are selected to effectively change a length and delay of the branch to which the synthetic delay line may be coupled and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

2. A device as recited in claim 1, further including a capacitor C_M coupled between UN_M and LN_M, a value of C_M selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M__! to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

3. A device as recited in claim 2, further including a capacitor C₀ coupled between UN₀ and LN₀, a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

4. A device as recited in claim 2, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

5. A device as recited in claim 2, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 100 feet of length to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

6. A device as recited in claim 2, the values of the M upper inductors, the M lower inductors, and capacitors Cj through C_M selected to move the quarter- wave short caused by the branch to a frequency of between about 1 J to 1.4 MHz.

7. A device as recited in claim 2, wherein M=3, each of the M upper inductors and M lower inductors has a value of about 4.7 microhenries, C_x has a value of about 330 picofarads, C₂ has a value of about 470 picofarads, and C₃ has a value of about 680 picofarads.

8. A device as recited in claim 2, wherein UN_M and LN_M are couplable to a device capable of communicating over the twisted-pair communication link.

9. A device as recited in claim 3, wherein UN_M and LN_M are couplable to a telephone jack.

10. A system for reducing effects of quarter- wave shorts caused by branched wiring on communications over a communication link, the system comprising at least one delay line, each delay line couplable to a distal end of a different branch on the communication link; wherein each delay line is selected to effectively change a length and delay of the branch to which the delay line is coupled, and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

11. A system for reducing effects of quarter-wave shorts caused by branched wiring on communications over a twisted-pair communication link, the system comprising: at least one synthetic delay line, each synthetic delay line couplable to a distal end of a different branch on the twisted-pair communication link, each synthetic delay line comprising

M upper inductors connected in a series arrangement and identified in their serial arrangement as ULj through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UN_t through UN^ within the serial arrangement of M upper inductors, inductor UL_X having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M, M lower inductors connected in a series arrangement and identified in their serial arrangement as LLj through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LN_X through LN_M within the serial arrangement of M lower inductors, inductor LL_X having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M, and

M-1 capacitors identified as through C_M , each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1; wherein values of the M upper inductors, the M lower inductors, and the M-1 capacitors in each synthetic delay line are selected to effectively change a length and delay of the branch to which the synthetic delay line is coupled, and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

12. A system as recited in claim 11, further including a capacitor C_M coupled between UN_M and LN_M, a value of C_M selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

13. A system as recited in claim 12, further including a capacitor C₀ coupled between UN₀ and LN₀, a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

14. A system as recited in claim 12, the values of the M upper inductors, the M lower inductors, and capacitors Cj through C_M selected to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

15. A system as recited in claim 12, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 100 feet of length to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted- pair communication link.

16. A system as recited in claim 12, the values of the M upper inductors, the M lower inductors, and capacitors Cj through C_M selected to move the quarter- wave short caused by the branch to a frequency of between about 1J to 1.4 MHz.

17. A system as recited in claim 12, wherein M=3, each of the M upper inductors and M lower inductors has a value of about 4.7 microhenries, C₁ has a value of about 330 picofarads, C₂ has a value of about 470 picofarads, and C₃ has a value of about 680 picofarads.

18. A system as recited in claim 12, wherein UN_M and LN_M of each synthetic delay line are couplable to a device capable of communicating over the twisted-pair communication link.

19. A system as recited in claim 12, wherein UN_M and LN_M of each synthetic delay line are couplable to a telephone jack.

20. A method for reducing effects of quarter- wave shorts caused by branched wiring on communications over a communication link wherein, for one or more branches on the communication link, the method comprising the steps of: coupling a delay element at a distal end of the branch; and selecting each delay element to effectively add length and delay to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

21. A method for reducing effects of quarter- wave shorts caused by branched wiring on communications over a twisted-pair communication link wherein, for one or more branches on the communication link, the method comprising the steps of: creating a synthetic delay line at a distal end of the branch, comprising the steps of connecting M upper inductors in a series arrangement, the M upper inductors identified in their serial arrangement as ULj through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UN_j through UN_M__! within the serial arrangement of M upper inductors, inductor UL_X having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M, connecting M lower inductors in a series arrangement, the M lower inductors identified in their serial arrangement as LLj through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LN_X through LN_M._t within the serial arrangement of M lower inductors, inductor LL_X having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M, and connecting M-1 capacitors identified as through C_M.j to the M-1 upper nodes and the M-1 lower nodes, each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1, and coupling UN₀ and LN₀ to the distal end of the branch; and selecting the values of the M upper inductors, the M lower inductors, and capacitors C₁ through C_M._j for the synthetic delay line to effectively change a length and delay of the branch, and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

22. A method as recited in claim 21, the step of creating a synthetic delay line further including the steps of: coupling a capacitor C_M between UN_M and LN_M; and selecting a value of C_M in consideration of the values of the M upper inductors, the M lower inductors, and capacitors C₁ through C_M__! to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

23. A method as recited in claim 22, the step of creating a synthetic delay line further including the steps of: coupling a capacitor C₀ between UN₀ and LN₀; and selecting a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

24. A method as recited in claim 22, the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

25. A method as recited in claim 22, the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively add about 100 feet of length to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

26. A method as recited in claim 22, the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the.M lower inductors, and capacitors through C_M to move the quarter- wave short caused by the branch to a frequency of between about 1J to 1.4 MHz.

27. A method as recited in claim 22, the step of creating a synthetic delay line further including the step of setting M=3, selecting each of the M upper inductors and M lower inductors to have a value of about 4.7 microhenries, selecting C to have a value of about 330 picofarads, selecting C₂ to have a value of about 470 picofarads, and selecting C₃ to have a value of about 680 picofarads.

28. A method as recited in claim 22, wherein for one or more synthetic delay lines, the method further includes the step of coupling UN_M and LN_M to a device capable of communicating over the twisted-pair communication link.

29. A method as recited in claim 22, wherein for one or more synthetic delay lines, the method further includes the step of coupling UN_M and LN_M to a telephone jack.

30. A device for reducing effects of quarter- wave shorts caused by branched wiring on communications over a communication link, comprising: a synthetic delay line couplable to a distal end of a branch on the communication link; a pulse generator couplable to the distal end of the branch on the communication link for generating and communicating pulses over the communication link; a time-domain reflectometer couplable to the distal end of the branch on the communication link for evaluating a reflected signal received over the communication link, the reflected signal resulting from the generated pulses, and for generating control signals for adjusting the synthetic delay line; an adjustment control circuit coupled to the time-domain reflectometer for receiving the control signals, and coupled to the synthetic delay line for adjusting the synthetic delay line in accordance with the control signals to effectively change a length and delay of the branch to which the synthetic delay line may be coupled and move a quarter-wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

31. A device for reducing effects of quarter-wave shorts caused by branched wiring on communications over a twisted-pair communication link, comprising: a synthetic delay line comprising

M upper inductors connected in a series arrangement and identified in their serial arrangement as UL_t through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UNj through UN_M.j within the serial arrangement of M upper inductors, inductor ULj having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M,

M lower inductors connected in a series arrangement and identified in their serial arrangement as LLj through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LNj through LN_M__! within the serial arrangement of M lower inductors, inductor LLj having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M, and

M-1 capacitors identified as C_x through C_M._1; each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1, wherein UN₀ and LN₀ are couplable to a distal end of a branch on the twistedpair communication link; a pulse generator coupled to UN_M and LN_M for generating and communicating pulses over the twisted-pair communication link; a time-domain reflectometer coupled to UN_M and LN_M for evaluating a reflected signal received over the communication link, the reflected signal resulting from the generated pulses, and for generating control signals for adjusting the synthetic delay line; and an adjustment control circuit coupled to the time-domain reflectometer for receiving the control signals, and coupled to the synthetic delay line for adjusting the synthetic delay line by adding or removing inductors from the synthetic delay line in accordance with the control signals to effectively change a length and delay of the branch to which the synthetic delay line may be coupled and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

32. A device as recited in claim 31, further including a capacitor C_M coupled between UN_M and LN_M, a value of C_M selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M._j to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

33. A device as recited in claim 32, further including a capacitor C₀ coupled between UN₀ and LN₀, a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors C_x through C_M to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

34. A device as recited in claim 32, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

35. A device as recited in claim 32, the values of the M upper inductors, the M lower inductors, and capacitors C₁ through C_M selected to effectively add about 100 feet of length to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

36. A device as recited in claim 32, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to move the quarter- wave short caused by the branch to a frequency of between about 1J to 1.4 MHz.

37. A device as recited in claim 32, wherein M=3, each of the M upper inductors and M lower inductors has a value of about 4.7 microhenries, has a value of about 330 picofarads, C₂ has a value of about 470 picofarads, and C₃ has a value of about 680 picofarads.

38. A device as recited in claim 31, wherein: if the time-domain reflectometer detects dips in the reflected signal occurring prior to a window around a time t-- corresponding to quarter- wave short that does not interfere with communications over the twisted-pair communication link, the time- domain reflectometer generates add control signals that cause the adjustment control circuit to attempt to add length and delay to the synthetic delay line; if the time-domain reflectometer detects dips in the reflected signal occurring after the window, the time-domain reflectometer generates subtract control signals that cause the adjustment control circuit to attempt to subtract length and delay to the synthetic delay line; and if the time-domain reflectometer detects dips in the reflected signal occurring within the window, the time-domain reflectometer causes the adjustment control circuit to make no adjustment to the synthetic delay line.

39. A device as recited in claim 38, further including:

M upper switches identified as USi through US_M, each upper switch US_X corresponding to an upper inductor UL_X and connected in parallel across UL_X, where X varies from 1 to M; and M lower switches identified as LSj through LS_M, each lower switch LS_X corresponding to a lower inductor LLx and connected in parallel across LL_X, where X varies from 1 to M; wherein the adjustment control circuit attempts to add length and delay to the synthetic delay line by opening one or more pairs of switches (US_X, LS_X); and wherein the adjustment control circuit attempts to subtract length and delay from the synthetic delay line by closing one or more pairs of switches (US_X, LS_X).

40. A device as recited in claim 39, wherein: the pulse generator generates and communicates multiple pulses over the twisted-pair communication link; the time-domain reflectometer performs multiple evaluations of the reflected signal and periodically generates control signals; for each add control signal received, the adjustment control circuit opens another pair of switches (US_X, LS_X) until all pairs of switches are open, after which add control signals will have no effect; and for each subtract control signal received, the adjustment control circuit closes another pair of switches (US_X, LS_X) until all pairs of switches are closed, after which subtract control signals will have no effect.

41. A device as recited in claim 32, wherein UN_M and LN_M are couplable to a device capable of communicating over the twisted-pair communication link.

42. A device as recited in claim 32, wherein UN_M and LN_M are couplable to a telephone jack.

43. A system for reducing effects of quarter- wave shorts caused by branched wiring on communications over a communication link comprising at least one synthetic delay circuit, each synthetic delay circuit comprising: a synthetic delay line couplable to a distal end of a different branch on the communication link; a pulse generator coupled to the distal end of the branch on the communication link to which the synthetic delay line is coupled for generating and communicating pulses over the communication link; a time-domain reflectometer coupled to the distal end of the branch on the communication link to which the synthetic delay line is coupled for evaluating reflected pulses received over the communication link, the reflected pulses resulting from the generated pulses, and for generating control signals for adjusting the synthetic delay line; and an adjustment control circuit coupled to the time-domain reflectometer for receiving the control signals, and coupled to the synthetic delay line for adjusting the synthetic delay line in accordance with the control signals; wherein each synthetic delay line is adjusted to effectively change a length and delay of the branch to which the synthetic delay line is coupled and move a quarter- wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

44. A system for reducing effects of quarter- wave shorts caused by branched wiring on communications over a communication link comprising at least one synthetic delay circuit, each synthetic delay circuit comprising: a synthetic delay line comprising

M upper inductors connected in a series arrangement and identified in their serial arrangement as ULj through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UNj through UN_M._X within the serial arrangement of M upper inductors, inductor ULj having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M,

M lower inductors connected in a series arrangement and identified in their serial arrangement as LLi through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LNj through LN_M__! within the serial arrangement of M lower inductors, inductor LLj having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M, and

M-1 capacitors identified as Cj through C_M._l5 each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1, wherein UN₀ and LN₀ are couplable to a distal end of a different branch on the twisted-pair communication link; a pulse generator coupled to UN_M and LN_M for generating and communicating pulses over the communication link; a time-domain reflectometer coupled to UN_M and LN_M for evaluating reflected pulses received over the communication link, the reflected pulses resulting from the generated pulses, and for generating control signals for adjusting the synthetic delay line; and an adjustment control circuit coupled to the time-domain reflectometer for receiving the control signals, and coupled to the synthetic delay line for adjusting the synthetic delay line in accordance with the control signals; wherein each synthetic delay line is adjusted to effectively change a length and delay of the branch to which the synthetic delay line is coupled and move a quarter-wave short caused by the branch to a frequency that^'does not interfere with communications over the communication link.

45. A system as recited in claim 44, further including a capacitor C_M coupled between UN_M and LN_M, a value of C_M selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors C_x through C_M__! to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

46. A system as recited in claim 45, further including a capacitor C₀ coupled between UN₀ and LN₀, a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively change the length and delay of the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

47. A system as recited in claim 45, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

48. A system as recited in claim 45, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to effectively add about 100 feet of length to the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted- pair communication link.

49. A system as recited in claim 45, the values of the M upper inductors, the M lower inductors, and capacitors through C_M selected to move the quarter- wave short caused by the branch to a frequency of between about 1J to 1.4 MHz.

50. A system as recited in claim 45, wherein M=3, each of the M upper inductors and M lower inductors has a value of about 4.7 microhenries, has a value of about 330 picofarads, C₂ has a value of about 470 picofarads, and C₃ has a value of about 680 picofarads.

51. A system as recited in claim 44, wherein: if the time-domain reflectometer detects dips in the reflected signal occurring prior to a window around a time t_n corresponding to quarter-wave short that does not interfere with communications over the twisted-pair communication link, the time- domain reflectometer generates add control signals that cause the adjustment control circuit to attempt to add length and delay to the synthetic delay line; if the time-domain reflectometer detects dips in the reflected signal occurring after the window, the time-domain reflectometer generates subtract control signals that cause the adjustment control circuit to attempt to subtract length and delay from the synthetic delay line; and if the time-domain reflectometer detects dips in the reflected signal occurring within the window, the time-domain reflectometer causes the adjustment control circuit to make no adjustment to the synthetic delay line.

52. A system as recited in claim 51, further including:

M upper switches identified as US_X through US_M, each upper switch US_X corresponding to an upper inductor UL_X and connected in parallel across UL_X, where X varies from 1 to M; and M lower switches identified as LSj through LS_M, each lower switch LS_X corresponding to a lower inductor LL_X and connected in parallel across LL_X, where X varies from 1 to M; wherein the adjustment control circuit attempts to add length and delay to the synthetic delay line by opening one or more pairs of switches (US_X, LS_X); and wherein the adjustment control circuit attempts to subtract length and delay from the synthetic delay line by closing one or more pairs of switches (US_X, LS_X).

53. A system as recited in claim 52, wherein: the pulse generator generates and communicates multiple pulses over the twisted-pair communication link; the time-domain reflectometer performs multiple evaluations of the reflected signal and periodically generates control signals; for each add control signal received, the adjustment control circuit opens another pair of switches (US_X, LS_X) until all pairs of switches are open, after which add control signals will have no effect; and for each subtract control signal received, the adjustment control circuit closes another pair of switches (US_X, LS_X) until all pairs of switches are closed, after which subtract control signals will have no effect.

54. A system as recited in claim 45, wherein UN_M and LN_M of each synthetic delay line are couplable to a device capable of communicating over the twisted-pair communication link.

55. A system as recited in claim 45, wherein UN_M and LN_M of each synthetic delay line are couplable to a telephone jack.

56. A method for reducing effects of quarter-wave shorts caused by branched wiring on communications over a communication link wherein, for one or more branches on the communication link, the method comprises the steps of: coupling a synthetic delay line at a distal end of the branch; generating and communicating pulses from the distal end of the branch over the communication link; evaluating reflected pulses received over the communication link at the distal end of the branch, the reflected pulses resulting from the generated pulses, and generating control signals for adjusting the synthetic delay line; and adjusting the synthetic delay line in accordance with the control signals; wherein each synthetic delay line is adjusted to effectively change a length and delay of the branch to which the synthetic delay line is coupled and move a quarter- wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

57. A method for reducing effects of quarter-wave shorts caused by branched wiring on communications over a communication link wherein, for one or more branches on the communication link, the method comprises the steps of: creating a synthetic delay line at a distal end of the branch, comprising the steps of connecting M upper inductors in a series arrangement, the M upper inductors identified in their serial arrangement as UL[ through UL_M, each connection between the M upper inductors defining M-1 upper nodes identified in order as UN_X through UN_M__j within the serial arrangement of M upper inductors, inductor UL_j having an unconnected terminal UN₀ and inductor UL_M having an unconnected terminal UN_M, connecting M lower inductors in a series arrangement, the M lower inductors identified in their serial arrangement as LL_t through LL_M, each connection between the M lower inductors defining M-1 lower nodes identified in order as LN_t through LN_M.j within the serial arrangement of M lower inductors, inductor LL_t having an unconnected terminal LN₀ and inductor LL_M having an unconnected terminal LN_M, and connecting M-1 capacitors identified as C_x through C_M to the M-1 upper nodes and the M-1 lower nodes, each capacitor C_κ connected between an upper node UN_K and a lower node LN_K, where K varies from 1 to M-1, and coupling UN₀ and LN₀ to the distal end of the branch; generating and communicating pulses from the distal end of the branch over the communication link; evaluating reflected pulses received over the communication link at the distal end of the branch, the reflected pulses resulting from the generated pulses, and generating control signals for adjusting the synthetic delay line; and adjusting the synthetic delay line in accordance with the control signals; wherein each synthetic delay line is adjusted to effectively change a length and delay of the branch to which the synthetic delay line is coupled and move a quarter-wave short caused by the branch to a frequency that does not interfere with communications over the communication link.

58. A method as recited in claim 57, the step of creating a synthetic delay line further including the steps of: coupling a capacitor C_M between UN_M and LN_M; and selecting a value of C_M in consideration of the values of the M upper inductors, the M lower inductors, and capacitors through C_M_, to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

59. A method as recited in claim 58, the step of creating a synthetic delay line further including the steps of: coupling a capacitor C₀ between UN₀ and LN₀; and selecting a value of C₀ selected in consideration of the values of the M upper inductors, the M lower inductors, and capacitors C_x through C_M to effectively change the length and delay of the branch and move the quarter- wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

60. A method as recited in claim 58, the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the M lower inductors, and capacitors through C_M to effectively add about 180-200 nanoseconds of delay to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twistedpair communication link.

61. A method as recited in claim 58 , the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the M lower inductors, and capacitors C_t through C_M to effectively add about 100 feet of length to the branch and move the quarter-wave short caused by the branch to a frequency that does not interfere with communications over the twisted-pair communication link.

62. A method as recited in claim 58, the step of creating a synthetic delay line further including the step of selecting the values of the M upper inductors, the M lower inductors, and capacitors C_x through C_M to move the quarter-wave short caused by the branch to a frequency of between about 1J to 1.4 MHz.

63. A method as recited in claim 58, the step of creating a synthetic delay line further including the step of setting M=3, selecting each of the M upper inductors and M lower inductors to have a value of about 4.7 microhenries, selecting Cj to have a value of about 330 picofarads, selecting C₂ to have a value of about 470 picofarads, and selecting C₃ to have a value of about 680 picofarads.

64. A method as recited in claim 57, the steps of evaluating reflected pulses and generating control signals further including the steps of: generating an add control signal for adding length and delay to the synthetic delay line if dips in the reflected signal are detected as occurring prior to a window around a time t-- corresponding to quarter-wave short that does not interfere with communications over the twisted-pair communication link; generating a subtract control signal for subtracting length and delay from the synthetic delay line if dips in the reflected signal are detected as occurring after the window; and making no adjustment to the synthetic delay line if dips in the reflected signal are detected as occurring within the window.

65. A method as recited in claim 64: the step of creating a synthetic delay line further including the steps of connecting M upper switches identified as USj through US_M, each upper switch US_X corresponding to an upper inductor UL_X and connected in parallel across UL_X, where X varies from 1 to M, and connecting M lower switches identified as LS_X through LS_M, each lower switch LS_X corresponding to' a lower inductor LL_X and connected in parallel across LL_X, where X varies from 1 to M; and the step of adjusting the synthetic delay line in accordance with the control signals further including the steps of attempting to add length and delay to the synthetic delay line by opening one or more pairs of switches (US_X, LS_X), and attempting to subtract length and delay from the synthetic delay line by closing one or more pairs of switches (US_X, LS_X).

66. A method as recited in claim 65, further including the steps of: generating and communicating multiple pulses over the twisted-pair communication link; performing multiple evaluations of the reflected signal, and periodically generating control signals; opening another pair of switches (US_X, LS_X) for each add control signal received until all pairs of switches are open, after which add control signals will have no effect; and closing another pair of switches (US_X, LS_X) for each subtract control signal received until all pairs of switches are closed, after which subtract control signals will have no effect.

67. A method as recited in claim 58, wherein for one or more synthetic delay lines, the method further includes the step of coupling UN_M and LN_M to a device capable of communicating over the twisted-pair communication link.

68. A method as recited in claim 59, wherein for one or more synthetic delay lines, the method further includes the step of coupling UN_M and LN_M to a telephone jack.