Cross-talk measurement apparatus with near-end compensation

A pulse-based cable crosstalk measurement instrument provides near-end cross talk (NEXT) information for characterizing the performance of local area network (LAN) cable systems. Accuracy of the cross-talk measurement is enhanced by measuring and mathematically removing the cross-talk effects of the near-end connector. Accuracy of the cross-talk measurement is further enhanced by conducting the cross-talk measurement with pulses of differing pulse widths selected to have increased pulse energy in frequency ranges of interest. A composite cross-talk response using the individual cross-talk responses of the individual pulse widths concatenated together is created and then compared with an industry-standard pass fail limit to provide a pass-fail decision on a LAN cable test system under test.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to pending U.S. patent application Ser. No. 
08/220,068 filed Mar. 30, 1994, entitled "Cable Crosstalk Measurement 
System." 
BACKGROUND OF THE INVENTION 
This invention relates generally to electrical cable testing and 
troubleshooting, and in particular to measuring the cross-talk 
characteristics of local area network cables. 
Local area networks (LAN's) now connect a vast number of personal 
computers, workstations, printers, and file servers in the modern office. 
A LAN system is typically implemented by physically connecting all of 
these devices with copper-conductor twisted-wire pair ("twisted-pair") LAN 
cables, the most common being an unshielded twisted-pair (type "UTP") LAN 
cable which is 8-wire cable which is configured as 4 twisted-pairs. Each 
of the four twisted-pairs function as a transmission line which conveys a 
data signal through the LAN cable. Each end of the LAN cable is terminated 
in an industry-standard connector, the most common being the type "RJ-45" 
connector. In a typical installation, LAN cables may be routed through 
walls, floors, and ceilings of the building. LAN cable systems require 
constant maintenance, upgrades, and troubleshooting because LAN cables and 
connectors are subject to breakage, offices and equipment must be moved, 
and new equipment is added. 
The tasks of installing, replacing, or re-routing cables typically fall on 
a professional cable installer or in-house network specialist. During the 
installation phase, each cable is routed through the building and a 
connector is attached to each end of the new cable. Each wire in the cable 
must be connected to its proper respective electrical connection on both 
ends of the cable in order for the LAN connection to function. A LAN cable 
system that has been improperly installed, or has faulty cables or 
connectors, can result in data transmission errors. Therefore, the LAN 
cable system must be tested to verify proper operation and performance. 
The relative speed of data communication over LAN cable systems has been 
steadily increasing. 100 megabits per second is an increasingly common 
data rate. Copper wire LAN cable systems, closely related to traditional 
multiconductor telephone cable systems commonly found in commercial 
buildings, have been pushed to accommodate these higher data rates. Copper 
wire LAN cable systems have the advantage over their optical fiber 
counterparts of being substantially less expensive and more versatile. 
However, the increasing demands for network speed and associated bandwidth 
have been accompanied by increased burdens on the network specialist to 
maintain network reliability in the face of more esoteric problems 
encountered at higher data rates. 
It is no longer sufficient to merely obtain proper electrical connections 
through a particular network link. More subtle problems can surface that 
may cumulatively destroy network performance at higher data rates. For 
this reason, LAN cables are now classified into different performance 
levels based on their ability to handle high speed data traffic. The 
network specialist must now be careful to select the appropriate minimum 
level. For example, the accidental inclusion of telephone-grade cable, 
which is physically similar to higher performance LAN cables but with 
unacceptable bandwidth and cross-talk characteristics, into a portion of 
the network may result in a non-functional network connection. 
Furthermore, the total number of LAN cable connectors, which have been 
found to contribute to significant performance degradations to a LAN cable 
system, must be kept to a minimum in a given signal path lest the 
cumulative network performance degradation become too severe. Critical 
network parameters for the network specialist to know include network 
bandwidth (measured in terms of signal attenuation versus frequency) and 
near-end cross-talk (commonly referred to as "NEXT") between twisted pairs 
in the cable, which also varies as a function of frequency. The terms NEXT 
and cross-talk will be used interchangeably because the cross-talk 
measurements in this context occur at the near-end of the LAN cable. 
Cross-talk is a measure of the level of isolation between any two 
twisted-pairs within a LAN cable system. Maintaining a specified minimum 
level of cross-talk isolation is important in preventing interference 
between adjacent twisted pairs in order to maintain network reliability. 
The industry working group Telecommunications Industry Association (TIA) 
have promulgated a standard for cross-talk which specifies a minimum level 
of cross-talk isolation over a frequency range of 1 to 100 megahertz. The 
cross-talk standard is essentially a pass-fail limit fine. LAN cable 
networks with cross-talk occurring above the limit line at any frequency 
is considered as failing. In maintaining a network that complies with the 
TIA standard, the network specialist can be reasonably assured of full 
network performance with no significant error contributions from 
cross-talk between twisted pairs. 
Test instrument manufacturers are striving to build test instruments to 
assist the network specialist in fully testing LAN cable systems 
generally, including the cross-talk performance. In order for a LAN cable 
system to pass the TIA specified limit, the crosstalk performance of the 
LAN cable network must exceed the specified limit at all frequencies 
within the specified range. However, the cross-talk test as specified by 
the TIA imposes several significant burdens on test instrument designers. 
First, the cross-talk measurement must be done over an entire range of 
frequencies, thereby requiring a series of measurements which adequately 
cover the frequency range while maintaining an acceptable total test time 
and accuracy for the user. Second, the cross-talk measurement includes the 
performance of all connectors in the system in the mated condition only. 
The corollary of this rule is that a connector at the near-end of the LAN 
cable that is mated with the test instrument becomes part of the test 
instrument connector and must not be included in the measurement of the 
LAN cable network. The cross-talk measurement must therefore exclude or 
minimize the contribution of the near-end connector in order to obtain a 
cross-talk measurement in accordance with the TIA specification. Because 
the near-end connector is typically the largest contributor to the 
cross-talk as seen by the test instrument, steps must be taken to minimize 
its effects or the test instrument will become somewhat "blinded" by the 
effect of the near-end connector overwhelming the response of the rest of 
the LAN cable system resulting in poor measurement accuracy. Thus, the 
effects of the near-end connector must be minimized to enhance measurement 
accuracy and decrease the chances of incurring a false indication of pass 
or fail for cross-talk measurements close to the specification limit. 
Prior art LAN cable test instruments that measure cross-talk performance 
have not fully addressed the requirements of the TIA specification. Low 
cost LAN cable test instruments typically evaluate cross-talk performance 
at only one frequency, for example 10 megahertz, and thereby provide a 
quick, albeit incomplete, indication of the cross-talk performance of the 
LAN cable system. Such an instrument operates by injecting a high 
frequency signal into a selected twisted-wire pair and monitoring the 
relative level received on another twisted-wire pair with a receiver and 
level detector. In this way, gross wiring errors in the LAN cable system, 
such as crossed-pair wiring errors, can be detected. 
More elaborate LAN cable test instruments attempt to provide cross-talk 
measurements over the specified frequency range of 1 to 100 megahertz. As 
with the low-cost LAN cable test instrument, the measurement is 
essentially analog, with a receiver and level detector tracking the signal 
frequency of a test signal source and measuring the relative level of the 
received test signal. The test signal source may generate a series of test 
frequencies either in the form of a continuous swept sine or as a 
collection of predetermined discrete frequencies generated in a sequence. 
Each cross-talk measurement is then compared to the specification limit 
for that frequency to determine whether it passes or fails. Failure of any 
cross-talk measurement results in a conclusion that the LAN cable network 
cross-talk performance is faulty: This measurement technique tends to be 
relatively slow because adequate settling time must be allowed between 
each measurement and there is necessarily a tradeoff between total 
measurement time and the number of frequencies tested. 
Furthermore, such instruments which measure amplitude but not phase cannot 
remove, or control, for the effects of the near-end connector. The 
solution to this nearend connector cross-talk problem has been to 
substitute a different type of connector at the instrument end, typically 
an industry-standard DB-15 type connector and mating plug, which is 
optimized to provide only a nominal amount of cross-talk back into the 
instrument. The DB-15 connector is connected to a high quality LAN cable 
which functions as a patch cable to the LAN cable system being tested. 
A disadvantage exists when the user patch cable is considered part of the 
LAN cable system. Such patch cables invariably include industry-standard, 
high cross-talk RJ-45 connectors at each end. Thus, because a DB-15 
connector does not directly mate with an RJ-45 connector, an unavoidably 
high cross-talk DB-15 to RJ-45 adapter is required and becomes part of the 
test instrument per the TIA specification. Consequently, under the common 
situation in which the network specialist wishes to include the user patch 
cable as part of the LAN cable system in order to fully test the LAN cable 
system, measurement accuracy is degraded by the uncertainty contributed by 
the RJ-45 connector. Another disadvantage of this technique is that the 
cross-talk effects of the DB-15 connector are reduced but cannot be 
completely controlled for nor eliminated from the measurement, which 
adversely affects the measurement accuracy of the instrument. The accuracy 
of the cross-talk measurement is thus highly dependent on the quality of 
the patch cable and connector, which thereby creates an unknown error 
contribution. 
Therefore, it would be desirable to provide a LAN cable test instrument 
which rapidly measures the cross-talk response of a LAN cable system over 
a desired frequency range of 0.1 to 100 megahertz and which automatically 
compensates for and electronically removes the effects of the near-end 
network connector to achieve improved cross-talk measurement accuracy and 
to eliminate the need for a special instrument connector, patch cable, and 
adapters. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a pulse-based cross-talk LAN 
cable test instrument provides a measurement of the cross-talk 
characteristic of a LAN cable system as a function of frequency in order 
to evaluate its relative performance. 
A cable test instrument applies a test signal in the form of narrow pulses 
to a selected transmission line of a LAN cable while the cross-talk 
response induced in another transmission line in the same LAN cable is 
measured and stored as a time record in digital memory. The instrument is 
connected to the near-end of the LAN cable while a remote unit provides 
proper termination at the far-end to prevent unwanted reflections of the 
test signal. In an unshielded twisted-pair (type "UTP") LAN cable, each 
transmission line consists of a twisted-wire pair. The time record is 
built over a series of measurements using a sequential sampling technique 
to improve the effective time resolution of the measurement. The LAN cable 
test instrument analyzes the cross-talk response by performing a discrete 
Fourier transform on the time record to provide cross-talk versus 
frequency information. The measured cross-talk may then be compared with a 
specification limit line to reach a pass-fail decision which is 
communicated to the instrument user. 
Enhanced accuracy of the cross-talk measurement employing a pulse-based 
measurement technique is obtained in two ways. First, a selected set of 
pulses, with differing pulse durations that correspond with optimal signal 
energy at desired frequency ranges, are chosen such that a composite 
frequency response can be assembled using measurements based on each of 
the pulses of the set. Conducting measurements based on pulses of 
differing width takes advantage of a well-known property called 
bandwidth-time invariance in which narrower pulses spread their energy 
over a wider frequency spectrum but at a correspondingly lower amplitude. 
Conversely, wider pulses can be employed to provide extra energy for 
measurements at lower frequency ranges for improved measurement accuracy. 
Second, the cross-talk effects of the near-end connector coupled directly 
to the test instrument are measured separately from the rest of the LAN 
cable system and mathematically subtracted from the cross-talk 
measurement. This technique takes advantage of the fact that, although the 
cross-talk characteristic of a given connector is not known, its physical 
location and corresponding location in the time record is known, allowing 
the effects of the near-end connector to be separately measured. During 
the course of a cross-talk measurement, the effects of the near-end 
network connector coupled to the test instrument are mathematically 
subtracted from the composite measurement that includes both the near-end 
connector and the remainder of the LAN cable system, leaving only the 
response of the LAN cable system. 
One object of the present invention is to provide a test instrument capable 
of measuring cross-talk between transmission lines of a LAN cable system 
as a function of frequency with enhanced measurement accuracy. 
Another object of the present invention is to provide a test instrument 
capable of measuring cross-talk between transmission lines of a LAN cable 
system as a function of frequency with enhanced measurement accuracy by 
employing stimulus pulses of selected pulse widths. 
An additional object of the present invention is to provide a test 
instrument capable of measuring cross-talk between a pair of transmission 
lines of a LAN cable system as a function of frequency with enhanced 
measurement accuracy by electronically removing undesirable cross-talk 
effects at a selected distance along the pair of transmission lines. 
Another object of the present invention is to provide a LAN cable test 
instrument capable of measuring near-end cross-talk as a function of 
frequency by employing stimulus pulses of selected widths and 
electronically removing the cross-talk effects of the near-end network 
connector. 
Other features, attainments, and advantages will become apparent to those 
skilled in the art upon a reading of the following description when taken 
in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is an illustration of a LAN cable test instrument and a remote unit 
coupled to the near-end and the far-end respectively of a typical LAN 
cable system. The terms near-end and far-end are adopted by convention to 
mean the respective ends of LAN cable connection, with the near-end being 
that with the test instrument 10 and the far-end being the opposite end. A 
test instrument 10 is coupled via a near-end connector 12 and a patch 
cable 14 to a connector 16. The test instrument 10 is typically applied in 
a user area 18 in which the desired peripherals such as computer 
workstations and printers are located in a typical office environment. The 
connector 16 is coupled to a LAN cable 20 located in a cable run 22. The 
cable run 22 is the route of the LAN cable 20 through the physical 
facility, and may run under floors, through walls, above ceilings, and 
other spaces within the facility. The length of the LAN cable 20 is 
typically limited to 100 meters and may have multiple LAN cable connectors 
within its span, as required to gain extensions or repair breaks. The LAN 
cable 20 is coupled to a connector 24 which appears at a 
telecommunications closet 26 which typically comprises a centralized 
receiving point for multiple user areas 18 and contains LAN network 
distribution equipment including, for example, "hubs", "routers", and 
"bridges", which are well known in the data communications field to 
facilitate network communication functions. A patch cable 28 coupled to a 
far-end connector 30 complete the LAN connection. To facilitate the 
testing of the connection through the LAN cable, a remote unit 32 is 
coupled to the far-end connector 30 to provide a proper termination of the 
test signals generated by the test instrument 10. 
FIG. 2 is a schematic diagram of a typical UTP, eight-conductor, 
copper-wire LAN cable 40 that is tested by the present invention. Wire 
pairs 42a-d are twisted together within the LAN cable 40 in such a way as 
to maximize the signal isolation or cross-talk between any two wire-pairs. 
On either end of the LAN cable 40 are LAN cable connectors 44 and 44' 
which mate with other LAN cable connectors to form the desired electrical 
connections according to industry convention. Wire pairs 42a-d are coupled 
to a predetermined set of connections within the connectors 44 and 44' to 
form pairs 1-2, 3-6, 4-5, and 7-8 respectively which function as the four 
separate, independent transmission lines. The connectors 44 and 44' are 
called the type RJ-45 connector which is adopted from the telephone 
industry. Multiple LAN cables 40 are coupled together by mating the 
connectors 44 and 44' to form completed connections. The LAN cable 40 and 
the LAN cable connectors 44 and 44' thus comprise the basic building block 
of a LAN cable system with may be comprised of any number of such building 
blocks. 
The key in achieving a desired level of cross-talk performance is in 
maintaining a balanced capacitance between any two twisted pairs in the 
LAN cable 40 so that signal voltages in one twisted pair do not induce 
voltages in another twisted pair. This capacitance balance has become 
increasingly critical as the frequency of the signal voltage has increased 
due to higher data rates sent through the LAN cable 40. The LAN cable 40 
is available commercially under assorted data grades which define its 
level of performance over desired frequency range. This level of 
performance is a direct function of the physical consistency of the 
twisting of the wire pairs 42a-d along any given span of the LAN cable. 
Because of continual improvements in the manufacturing of the LAN cable 40 
thereby increasing its relative data grade, the mated connectors 44 and 
44' are often the primary contributors to the overall capacitance 
imbalance between any two wire pairs. The number of such mated connections 
in a LAN cable system often becomes a matter of concern in maintaining 
network performance at higher data rates. 
FIG. 3 is a schematic diagram that defines a LAN cable system 50 for the 
purposes of a cross-talk measurement standard. Because of the 
uncertainties faced by the network specialist regarding the data grades of 
all the LAN cables forming the segments between the near-end and the 
far-end of a LAN cable connection, the relative lengths of each segment, 
and number of LAN cable connectors between the near-end and the far-end, 
measuring the level of cross-talk through the entire network of 
connections therefore becomes critical. 
At the same time, a need for a standardized method of measuring cross-talk 
required a universal definition of what comprises a LAN cable system 10. 
The Telecommunications Industry Association (TIA) arrived at a definition 
that a LAN cable system includes all mated connections between the 
near-end and the far-end excluding the near-end and far-end connectors 
themselves. The near-end connector 12 is mated with a workstation 52 and 
thus becomes part of the workstation and not part of the LAN cable system 
50. Similarly, the far-end connector 30 is mated with LAN equipment 54, 
which may comprise a hub, bridge, router, or patch panel, and therefore 
becomes part of the LAN equipment 54. The LAN cable system 50 comprises 
the patch cable 14 (shown in FIG. 1), the LAN cable connector 16, the LAN 
cable 20, the LAN cable connector 24, and the patch cable 28. Excluded are 
the contributions to cross talk of the near-end connector 12 and the 
far-end connector 30 because they are mated with the workstation 52 and 
the LAN equipment 54 respectively, which are not pan of the LAN cable 
system 50. Under this definition, the test instrument 10 (shown in FIG. 1) 
which mates with the near-end connector 12 must therefore attempt to 
exclude the contribution of the near-end connector 12 and the far-end 
connector 30 in order to obtain a cross-talk measurement that fully 
conforms to the industry definition. 
FIG. 4 is a diagram illustrating a LAN cable test instrument and remote 
unit testing the LAN cable system according to the prior art. The 
instrument 10 (shown in FIG. 1) is coupled via the connector 12 to the LAN 
cable system 50 via an special patch cable 15 which replaces the user 
patch cable 14 of FIG. 3 for the cross-talk measurement. 
The instrument 10 is coupled to the near-end connector 12 which in turn is 
coupled to the patch cable 15. In order to conform as closely as possible 
to the TIA definition of a LAN cable system, the contributions to 
cross-talk of the near-end connector 12 must be minimized. Because the 
measurement is analog, the cross-talk contribution of the near-end 
connector 12 is minimized mechanically by employing a special connector 
such as a type DB-15 that has a better capacitance balance than the 
industry standard RJ-45 45 connector. Employing a special connector for 
the near-end connector 12 requires the special patch cable 15 which is 
unique to the test instrument 10. The patch cable 15 is coupled to the LAN 
cable connector 16 and is measured as part of the LAN cable system 50. The 
contribution to the cross-talk measurement of the near-end connector 12, 
while assumed to be low, cannot be controlled for under this arrangement. 
Furthermore, the user patch cable 14 which normally connects to the 
workstation 52 is not tested because it has been replaced with the 
instrument patch cable 15. 
Cross-talk measurements are performed by injecting a test signal into a 
twisted-pair of the LAN cable system 50 (shown in FIG. 3) and measuring 
the relative signal level induced in another twisted-pair as measured at 
the near-end by the instrument 10. In the prior art, the test signal is an 
analog signal of known level and the test signal is monitored by an analog 
receiver within the instrument 10 which functions as a level detector. For 
example, a 10 megahertz cross-talk measurement may be performed by 
injecting a sine wave with a frequency of 10 megahertz into twisted pair 
1-2 and measuring the induced signal level in twisted pairs 3-6, 4-5, or 
7-8. The ratio of the induced signal level to the injected signal level 
may be expressed in decibels (dB) to express the level of cross-talk 
isolation between any two twisted pairs. 
Because a cross-talk measurement typically does not vary smoothly as a 
function of frequency, the cross-talk measurement must be performed over 
multiple frequencies and compared against a specification limit. It is 
desirable to check the cross-talk performance at a substantial number of 
frequencies within the range of interest to avoid missing a failing 
cross-talk measurement which may fall between other measurement 
frequencies. 
Performing more cross-talk measurements with increasingly fine frequency 
spacing comes at the expense of increased total measurement time and 
frequency resolution is thus traded off against the probability of missing 
a failing cross-talk measurement. In the prior art, adding measurement 
frequencies directly increases total measurement time because each 
measurement requires a fixed amount of time to complete. In the present 
invention, the use of the fast Fourier transform to achieve a frequency 
representation of a digitized time record has resulted in substantial 
gains in measurement speed for a given frequency resolution as compared 
with the prior art. Cross-talk measurements at frequency increments of 150 
kilohertz over the frequency range of 1 to 100 megahertz are generally 
considered adequate to accurately measure the worst case margin to the 
specification limit line, based on empirical findings. 
FIG. 5 is a diagram illustrating a LAN cable test instrument and remote 
unit testing the LAN cable system including a user patch cable and a 
special patch cable according to the prior art. The instrument 10 is 
coupled to the connector 12 and instrument patch cable 15 as in FIG. 4. 
The user patch cable 14 couples the instrument patch 15 to the connector 
16. This situation would occur when the network specialist desires to test 
the entire LAN cable system 50 including the user patch cable 14. The 
disadvantage of this technique is that the connector 16 is now part of the 
test instrument 10 and its cross-talk contributions become an error source 
to the overall cross-talk measurement. Thus, the cross-talk measurement is 
degraded from its actual level because of the undesired inclusion of the 
connector 16 in the cross-talk response. Connector 16 is necessarily of 
type R J-45 in most cases to properly mate with the corresponding 
connector in the LAN cable system 50 and thus contributes a significant 
amount of cross-talk. 
FIG. 6 is a diagram illustrating a LAN cable instrument and remote unit 
testing the LAN cable system according to the present invention. The 
instrument 10 (shown in FIG. 1) is coupled to the near-end connector 12 
which in turn is coupled to the patch cable 14 similar to the diagram of 
FIG. 4 but with the difference that the instrument 10 of the present 
invention may connect to the near-end connector 12 in the form of the 
industry-standard RJ-45 connector. 
The LAN cable system, including the same patch cable 14 that couples the 
workstation 52 (shown in FIG. 3) to the LAN cable connector 16 may be 
tested simply by removing the near-end connector 12 from the workstation 
52 and coupling it to the instrument 10 without having to substitute a 
special patch cable. In this way, the cross-talk measurement of the LAN 
cable system 50 by the instrument 10 more closely matches the cross-talk 
level actually encountered by the workstation 52 under normal operation. 
The remote unit 32 is coupled to the far-end connector and provides proper 
termination of the LAN cable system as in FIG. 4. 
Using a series of narrow pulses as the test signal, the instrument 10 of 
the present invention electronically measures the cross-talk of the 
near-end connector 12 and mathematically removes its cross-talk 
contribution from the cross-talk measurement of the LAN cable system 50. 
The instrument 10 thus does not depend on the relative cross-talk 
performance of any particular near-end connector 12. The user patch cable 
14 may be used if its length is at least 2 meters so that the cross-talk 
effects of the connector 12 may be measured and removed. The measurement 
is also in accordance with the TIA "channel" definition, which 
specifically includes the user patch 14 but excludes the connector 12. The 
method of measuring and mathematically removing the cross-talk 
contribution of the near-end connector 12 is explained more fully below. 
FIG. 7 is a simplified block diagram of the test instrument 10 (shown in 
FIG. 1) according to the present invention. The near-end connector 12 is 
coupled to the wire pairs 42a-d (shown in FIG. 2) contained with the LAN 
cable system 52. The wire pairs 42a-d are further coupled to a switch 
matrix 70 which selectively couples one of the wire pairs 42a-d to an 
output of a pulse generator 72 and selectively couples another of the wire 
pairs 42a-d to an input of a sample-and-hold circuit (S/H) 74 which 
captures a voltage level present at the input upon receipt of a signal at 
a control input. Each wire pair is essentially a balanced transmission 
line. The output of the pulse generator 72 and the input of the S/H 74 are 
unbalanced or referenced to ground in the preferred embodiment of the 
present invention, requiting the addition of transformers (not shown) for 
each wire pair to convert from a balanced to an unbalanced transmission 
line. The wire pairs 42a-d are drawn as single lines because each pair 
constitutes a single transmission line. The pulse generator 72 sends a 
pulse on receipt of a signal at a control input. An output of the 
sample-and-hold circuit is coupled to an input of an analog-to-digital 
converter (ADC) 76 which digitizes the voltage level received from the S/H 
74 on receipt of a signal at a control input. An output of the ADC 76 is 
coupled to an acquisition memory 78 which stores the digital measurements 
as a digital time record on receipt of a signal at a control input. An 
acquisition time controller 80 is coupled to the control input of the S/H 
74, ADC 76, and pulse generator 72 to facilitate a repetitive digital 
sampling process that achieves a high equivalent sampling rate with a 
minimum of timing errors by the coordinated generation of the control 
signals at the proper times to the respective control inputs of the S/H 
74, ADC 76, and pulse generator 72. The S/H 74, ADC 76, acquisition time 
controller 80 and acquisition memory 78 together comprise a digitizer 79 
which digitizes the received pulse responses using repetitive sequential 
sampling to obtain a higher equivalent sampling rate than is readily 
obtainable with real-time sampling techniques. In the preferred embodiment 
of the present invention, the equivalent sampling rate of the digitizer 79 
is 500 megasamples per second, or conversely, a time resolution of 2 
nanoseconds per point. The measurement process is then one of assembling a 
4,096 point time record sample by sample with 2 nanosecond resolution in 
the acquisition memory 78 at an actual sample rate of approximately 4 
megahertz. A 4,096 point discrete Fourier transform is then calculated to 
obtain the frequency domain representation of the time record. 
A microprocessor 82 controls the overall measurement process and is coupled 
to a control input of the switch matrix 70 to select the respective wire 
pairs to measure and to a control input of the acquisition time controller 
80 to control the acquisition process. The microprocessor 82 is further 
coupled to a display 86, a keypad/knob 88, an instrument memory 90, and a 
digital signal processor (DSP) 92 via an instrument bus 84. 
Instrumentation bus 84 contains parallel data and address lines to 
facilitate communications between the devices in a manner well known in 
the electronics field. A time record collected in the acquisition memory 
78 is transferred to the instrument memory 90 for storage or further 
digital manipulation such as discrete Fourier transform. DSP 92, a 
special-purpose signal processing circuit, may be used in lieu of the 
microprocessor 82 to convert a time record into a frequency domain 
representation using a discrete Fourier transform function. The DSP 92 is 
a commercially available signal processor integrated circuit that is 
typically faster at performing fast Fourier transforms than a general 
purpose microprocessor at equivalent clock speeds. The keypad/knob 88 and 
display 86 comprise the user interface of the instrument 10. The memory 90 
is used to store digital time records, frequency domain representations, 
and instrument calibration data and may be comprised of a single 
integrated circuit or multiple integrated circuits using technologies well 
known in the electronics field. 
FIG. 8, comprised of FIG. 8A, 8B, and 8C, is a flow chart of the overall 
measurement process employed by the LAN cable test instrument according to 
the present invention. FIG. 8A covers the process of measuring the 5, 16, 
and 64 nanosecond calibration pulse responses by connecting the receive 
and transmit channels with a substantially lossless, 0 decibel reference 
connection in order to normalize subsequent measurements against that 
direct measurement. FIG. 8B covers the process of measuring the 
pulse-response of the LAN cable system 50 (shown in FIG. 3) using 5 
nanosecond, 16 nanosecond, and 64 nanosecond pulse widths, removing the 
cross-talk effects of the near-end connector 12, and calculating the 
frequency domain cross-talk function for each pulse response. FIG. 8C 
covers the process of assembling the composite cross-talk response of the 
LAN cable system 50 from the three cross-talk functions, comparing the 
composite response against a specification limit, and providing 
measurement results to the user. 
Arriving at a composite cross-talk response requires using a substantial 
amount of data storage and mathematical manipulation. Two primary types of 
data records are employed in the present invention: time domain records 
and frequency domain records. Frequency domain records are related to 
their equivalent time domain records via the Fourier transform and the 
data are in the form of complex numbers with real and imaginary 
components. Time domain records contain only real data with no imaginary 
components. By convention, the time domain records are assigned names in 
lower case letters and the equivalent frequency domain records are 
assigned the same names but in upper case. 
The following table summarizes all of the variables relevant to the 
measurement process explained in FIG. 8A-C. Each of the variables 
represents a series of data stored as an array data structure with 
individual data points normally accessible by use of indices in a manner 
well known in the computer field. 
norm64--the time record of the 64 nanosecond pulse response when the 
transmit and receive channels are connected together where "norm" refers 
to normalization dam. 
NORM64--the frequency domain representation of norm64 which represents the 
0dB reference level for normalizing cross-talk measurements using 64 
nanosecond pulses where "NORM" refers to normalization data. 
norml16--the time record of the 16 nanosecond pulse response when the 
transmit and receive channels are connected together where "norm" refers 
to normalization dam. 
NORM16--the frequency domain representation of norm16 which represents the 
0dB reference level for normalizing cross-talk measurements using 16 
nanosecond pulses where "NORM" refers to normalization data. 
norm5--the time record of the 5 nanosecond pulse response when the transmit 
and receive channels are connected together where "norm" refers to 
normalization data 
NORMS--the frequency domain representation of norm5 which represents the 0 
dB reference level for normalizing cross-talk measurements using 5 
nanosecond pulses where "NORM" refers to normalization data. 
cut5--the time record of the 5 nanosecond pulse response of the LAN cable 
system with the transmit channel coupled to a transmission line and the 
receive channel coupled to another transmission where "cut" refers to 
cable under test. 
CUT5--the frequency domain representation of cut5 where "CUT" refers to 
cable under test. 
cut16--the time record of the 16 nanosecond pulse response of the LAN cable 
system with the transmit channel coupled to a transmission line and the 
receive channel coupled to another transmission line where "cut" refers to 
cable under test. 
CUT16--the frequency domain representation of cut16 where "CUT" refers to 
cable under test. 
cut64--the time record of the 64 nanosecond pulse response of the LAN cable 
system with the transmit channel coupled to a transmission line and the 
receive channel coupled to another transmission. 
CUT64--the frequency domain representation of cut16 where "CUT" refers to 
cable under test. 
necc--the time record of the near-end connector pulse response as 
constructed from data contained in cut5 where "necc" refers to near-end 
connector cross-talk. 
NECC--the frequency domain representation of necc where "NECC" refers to 
near-end connector cross-talk. 
NEXT5--the frequency domain representation of the near-end cross-talk 
response of the LAN cable system using the 5 nanosecond pulse response of 
the LAN cable system which is calculated by dividing CUT5 by NORM5 
(normalization) and subtracting NECC (near-end compensation). 
NEXT16--the frequency domain representation of the near-end cross-talk 
response of the LAN cable system using the 16 nanosecond pulse response of 
the LAN cable system which is calculated by dividing CUT5 by NORM5 
(normalization) and subtracting NECC (near-end compensation). 
NEXT64--the frequency domain representation of the near-end cross-talk 
response of the LAN cable system using the 64 nanosecond pulse response of 
the LAN cable system which is calculated by dividing CUT5 by NORM5 
(normalization) and subtracting NECC (near-end compensation). 
COMP.sub.-- NEXT--the frequency domain representation of the near-end 
cross-talk response of the LAN cable system using selected portions of 
NEXT5, 
NEXT16, and NEXT64 concatenated together. 
Referring now to FIG. 8A, the measurement process begins with a process 100 
labeled START wherein the test instrument 10 (shown in FIG. 1) may be 
first started and initialized. Process 102 labeled INSERT 0 dB CALIBRATION 
ARTIFACT is an instrument calibration process in which 5, 16, and 64 
nanosecond width pulses generated by the pulse generator 72 over the 
TRANSMIT line (shown in FIG. 7) are coupled via a short connection 104 to 
the RECEIVE line and the S/H 74. The connector 12 has a substantially 
lossless, 0 decibel reference connection which comprises an electrical 
short 104 between a selected combination of wire pairs chosen to 
correspond with the setting of the switch matrix 70. The connector 12 is 
in turn coupled to the instrument 10 for calibration purposes. 
Process 106 labeled MEASURE PULSE RESPONSE RECORDS--5 NS, 16 NS, 64 NS 
PULSE WIDTHS is a measurement of the pulses generated by the pulse 
generator 72 (shown in FIG. 7) so that subsequent pulse response 
measurements may be normalized against that direct measurement. The pulse 
response is measured by the test instrument 10 and time records containing 
the 4,096 point measured pulse response records named norm5, norm 16, and 
norm64 are created and stored in the memory 90. The names norm5, norm16, 
and norm64 are a symbolic representation of the amplitude data stored as a 
time record commonly stored in the form of an array data structure with 
individual data points within the array accessible via an index. 
In process 108, labeled CALCULATE 0 dB NORMALIZATION DATA, frequency 
representations of norm5, norm16, and norm64 are calculated using a fast 
Fourier transform in a matter well known in the electronics field using 
the DSP 92 (shown in FIG. 6). In the preferred embodiment of the present 
invention, the chosen length of norm5, norm16, and norm64 is 4096 points, 
which is a power of 2 to facilitate a fast Fourier transform (FFr), an 
efficient implementation of the discrete Fourier transform algorithm 
employed by the DSP 92. 
NORM5=FFT (norm5) 
NORM16=FFT (norm16) 
NORM64=FFT (norm64) 
NORMS, NORM16, and NORM64, in upper case, are the frequency domain 
representations of the time records norm5, norm16, and norm64 which 
constitute the normalization data that represent the calibration data of 
the instrument 10. The data contained in NORMS, NORM16, and NORM64 are 
complex values as a function of frequency. A set of normalization data 
NORMS, NORM16, and NORM64 would be acquired and stored in the memory 90 
for every combination of twisted pairs in the preferred embodiment of the 
present invention. 
In process 110 labeled SAVE 0 dB NORMALIZATION DATA IN MEMORY, the 
normalization data are stored in the memory 90 (stored in FIG. 7). The DSP 
92 produces data files containing complex numbers which comprise real and 
imaginary components which are stored as NORM5, NORM16, and NORM64 in the 
memory 90 as array data structures. The processes 102, 106, and 108 for 
obtaining normalization data are typically performed as a factory 
calibration of the instrument 10 and the NORMS, NORM16, and NORM64 data 
are stored in memory 90 on a semi-permanent basis until the next 
instrument calibration. A typical period between factory calibrations is 
12 months in the preferred embodiment of the present invention. 
Referring now to FIG. 8B, the measurement process continues with a process 
112 labeled MEASURE LAN CABLE SYSTEM UNDER TEST--5 NS, 16 NS, 64 NS. The 
same selected wire pairs 40a and 40d, corresponding to the wire pairs 
chosen in the processes 108 and 110 (shown in FIG. 8A) for the 0 dB 
calibration measurement, are chosen for measurement in the process 112. In 
the process 112, the LAN cable system 50 is measured by repetitively 
sampling a stream of pulses of a selected pulse width into the wire pair 
40a. The response of each pulse at a selected time interval is then 
measured and stored at the appropriate location in the time record in the 
acquisition memory 78 (shown in FIG. 6). Completed pulse-response time 
records of the LAN cable system 50 at pulse widths of 5, 16 and 64 
nanoseconds are then stored in memory 90 as cut5, cut16, and cut64 as 
array data structures. 
In process 114, labeled CONSTRUCT TIME RECORD OF NEAR END CONNECTOR, the 
cross-talk response of the connector 12 is constructed from the pulse 
response data contained in cut5 to form an undesired-response time record. 
The 5 nanosecond pulse response data was chosen because it provides the 
highest resolution which is necessary to separate the pulse response of 
the connector 12 from the pulse response of the adjacent connector 16 
(shown in FIG. 1). The cross-talk response of the connector 12 may be 
readily extracted from the data contained in cut5 because its location 
along the pair of transmission lines represented by the wire pairs 40a and 
40d is known and remains constant. This location is selected using the 
index of array of data stored in cut5 and the data copied to another data 
array called conn. Because there is some overlap between the pulse 
responses of the connectors 12 and 16, the overlapping portion is ignored, 
resulting in a truncated estimate of the undesired pulse response of the 
connector 12. In this way, the undesired-response time record conn is 
constructed to contain only the pulse response of the connector 12. This 
process of constructing the pulse response record of the connector 12 is 
explained more fully below. 
In process 116, labeled CALCULATE FREQUENCY REPRESENTATION 0F TIME RECORDS, 
the frequency domain representation of the time records conn, cut5, cut16, 
and cut64, which have been obtained as explained above, are computed using 
the DSP 92 (shown in FIG. 7). The lengths of conn, cut5, cut16, and cut64 
are all 4,096 points, which is a power of 2, to facilitate the fast 
Fourier calculation. 
CONN=FFT (conn) 
CUT5=FFT (cut5) 
CUT16=FFT (cut16) 
CUT64=FFT (cut64) 
The results of each calculation are stored in memory 90 as CONN, CUT5, 
CUT16, and CUT64 respectively. The frequency resolution of the points in 
the data files CONN, CUTS, CUT16, and CUT64 is derived using the following 
formula: 
Frequency Resolution (Hertz)=(effective sample rate/length of time record) 
=500 megahertz/4,096 points 
=122.07 kilohertz per point 
In process 118 labeled CALCULATE NEAR-END CONNECTOR NEXT ESTIMATE, the 
normalized cross-talk response represented by NECC is calculated. 
EQU NECC=CONN/NORM5 
NORM5 is the normalization data for the 5 nanosecond pulse obtained in 
processes 102 and 106. CONN is the frequency representation of the 
cross-talk response of the connector 12 which was obtained from the 5 
nanosecond pulse response data contained in cut5. By normalizing the 
response CONN by the stimulus NORM5, the actual cross-talk function NECC 
of the connector 12 is derived. 
In process 120 labeled CALCULATE NEAR-END COMPENSATED NEXT--5 NS, 16 NS, 64 
NS, the cross-talk function of the LAN cable system 50 (shown in FIG. 3) 
is calculated for each of the pulse widths, including 5, 16, and 64 
nanoseconds with the cross-talk effects of the connector 12 removed. 
Because cross-talk is generally recognized as a ratio of response over 
stimulus, it is appropriate to obtain the magnitude of the ratio and 
express it in terms of decibels (dB). 
##EQU1## 
Referring now to FIG. 8C, in a process 124 labeled ASSEMBLE COMPOSITE NEXT 
RESPONSE, a composite cross-talk response of the LAN cable system 50 
(shown in FIG. 3) is assembled from portions of NEXT5, NEXT16, and NEXT64. 
Three pulse widths were employed to maximize the signal energy at various 
frequency ranges in a manner described more fully below. The composite 
cross-talk response, COMP.sub.-- NEXT is assembled using NEXT64 from 0.1 
to 10 megahertz, NEXT16 from 10 to 40 megahertz, and NEXT5(m) from 40 
megahertz to 150 megahertz. In this way, COMP.sub.-- NEXT maintains an 
optimal signal to noise ratio over the entire frequency of interest, 0.1 
to 100 megahertz. 
In process 126 labeled COME COMPOSITE NEXT RESPONSE TO SPECIFICATION 
LIMIT, COMP.sub.-- NEXT is compared with a specification limit line 
supplied by the Telecommunications Industry Association as the accepted 
pass-fail limit. A pass-fail decision is then made responsive to the 
results of the comparison. If any of the points within COMP.sub.-- NEXT is 
above its corresponding specification limit, the LAN cable system 50 under 
test is considered as a "fail". Otherwise, the LAN cable system 50 passes 
its cross-talk test. 
In process 128 labeled COMMUNICATE PASS/FAIL INFORMATION TO USER, the 
results of the pass/fail decision of the process 126 are communicated to 
the user via the display 86 (shown in FIG. 7) of the instrument 10. A 
graphical display of the cross-talk response of the LAN cable system 50 
using the data contained in COMP.sub.-- NEXT may also be shown as well as 
a simple pass or fail indication. 
In process 130 labeled END, the measurement process ends. Under instrument 
control, the measurement process may automatically return to the START 
process 100 (shown in FIG. 8A) or an interim measurement process to repeat 
the measurement continuously. 
Referring now to FIG. 9, there is shown a graph illustrating a 5 nanosecond 
pulse response of a typical LAN cable system 50 (shown in FIG. 3) stored 
as a time record in the memory 90 (shown in FIG. 7) in the test instrument 
10. This graph represents the typical contents of the data files cut5, 
cut16, and cut64 which may be obtained as described above. 
In FIG. 9, the lower trace illustrates the contents of cut5 obtained by 
measuring the LAN cable system 50 comprising a 2 meter patch cable, a 67 
meter cable, a first one meter patch cable, a 33 meter cable, a second one 
meter patch cable, a 17 meter cable, followed by a third one meter patch 
cable, and a termination (not shown) provided by the remote unit 32 (shown 
in FIG. 1). It will be noted that for purposes of clarity, the graph has 
been drawn showing only the absolute magnitude of the pulse response data 
and with a weighting factor that enhances the amplitude of the pulse 
response as the distance increases to compensate for losses in the cable. 
The peaks of the graph correspond to the LAN cable connectors in the LAN 
cable system 50 that couple the various links described. Peak 140 
corresponds to the near-end connector 12. Peak 142 corresponds to the LAN 
cable connector 16 (shown in FIG. 3) at the other end of the patch cable 
14. Peaks 144 and 146 correspond to LAN cable connectors at either end of 
the first 1 meter patch cable. Peaks 148 and 150 correspond to LAN cable 
connectors at either end of the second 1 meter patch cable. Peaks 152 and 
154 correspond to LAN cable connectors at either end of the third 1 meter 
patch cable. 
The peak 140 corresponding to the cross-talk of the near-end connector 12 
is substantially higher than the other peaks in this instance. As 
explained above, the cross-talk response of the LAN cable system 50 
excludes the response of the near-end connector 12 according to the TIA 
specification and the present invention provides for its removal. If only 
the cross-talk response of the LAN cable system 50 using 5 nanosecond 
width pulses were desired, it would be a simple matter to "zero out" the 
portion of the cut5 response related to the peak 140 in the time record 
and thus obtain a valid cross-talk response according to the industry 
definition. However, 16 nanosecond and 64 nanosecond width pulse 
measurements are also employed for improved signal to noise ratios at 
desired frequency ranges, thus requiring normalization and subtraction in 
the frequency domain as is done in the process 120 (shown in FIG. 8B). 
Referring now to FIG. 10, a graph illustrating the composite cross-talk 
response of the LAN cable system 50 as compared with a predetermined NEXT 
pass-fail limit line. The vertical scale is the NEXT loss in decibels and 
the horizontal scale is frequency in megahertz. A more negative NEXT loss 
is more desirable. Continuing the example from FIG. 9, a representative 
LAN cable system 50 consists of a 2 meter patch cable, a 67 meter cable, a 
first one meter patch cable, a 33 meter cable, a second one meter patch 
cable, a 17 meter cable, followed by a third one meter patch cable, and a 
termination provided by the remote unit 32 (shown in FIG. 1). The 
cross-talk response shown in FIG. 10 is a composite of NEXT64 from 0.1 to 
10 megahertz, NEXT16 from 10 to 40 megahertz, and NEXT5 from 40 megahertz 
to 155 megahertz, as is performed in the process 124 (shown in FIG. 8C). 
A trace 160 corresponds with the pass-fail limit line. A trace 162 
corresponds with the composite cross-talk response of the LAN cable system 
50 as calculated. As shown, every point of the trace 162 is below the 
trace 160. The test instrument 10 would thus return a decision of "pass" 
according to this cross-talk response in the process 126 in FIG. 8C. 
Referring now to FIG. 11, there is shown a graph illustrating the actual 
pulse response of a typical near-end LAN cable connector 12 using points 
from a predetermined section of the time record and a truncated estimate 
of the actual pulse response as stored in the time record conn. The 
vertical axis represents amplitude while the horizontal axis represents 
memory locations in conn. The pulse response of the near-end connector 12 
is measured in the process 112, along with the LAN cable system 50 under 
test, and the results are stored in the data file cut5. Separating the 
pulse response of the near-end connector 12 from that of the LAN cable 
connector 16 at the other end of the patch cable is critical. If the two 
pulse responses cannot be segregated, the pulse response of the nearend 
connector 12 cannot be accurately measured and then mathematically 
subtracted from subsequent pulse response measurements. Trace 170 in the 
upper portion of the graph shows the actual pulse response of the near-end 
connector 12 (shown in FIG. 1) with 100 ohm termination resistors 
substituted for the LAN cable system 50 (shown in FIG. 3). Superimposed on 
the upper trace is the pulse response of a LAN cable connector 16 which 
must be separable in order to obtain an accurate cross-talk response 
measurement of the near-end connector 12. 
Ensuring this separation requires that three critical parameters be 
accounted for and traded off against each other. These parameters include: 
(a) the minimum length of the patch cable 14 (shown in FIG. 1), (b) the 
pulse width of the test signal, and (c) the length of the truncated 
estimate of the actual pulse response of the near-end connector 12. 
The length of the patch cable is the first critical parameter to be 
accounted for. A shorter patch cable decreases the separation between the 
two pulse responses. A design goal for the test instrument 10 was to have 
a patch cable 14 that did not require an excessive minimum length. In the 
present invention, the test instrument 10 must have a patch cable with a 
minimum length of 2 meters to ensure that the pulse responses of the 
connectors 12 and 16 are indeed separable. 
The 5 nanosecond pulse width, the narrowest pulse width available in the 
present invention, was chosen to measure the pulse response of the 
near-end connector 12 because it provides the highest resolution of the 
three available pulse widths in discerning individual pulse responses in a 
time record. A narrower pulse produces a narrower pulse response which is 
more easily separable. 
The length of the truncated estimate of the actual pulse response of the 
near-end connector 12 was chosen to include as much of the near-end 
connector pulse response as possible, stopping short of point where the 
pulse response of the connector 16 starts to overlap. The truncated 
estimate is shown as a trace 174 in the lower portion of the graph. In the 
preferred embodiment of the present invention, the truncated estimate in 
the data file conn(m) was chosen from m=120 to 148. All other values 
except m=149 in the data file are set to 0. 
In formulating the truncated estimate in the data file conn, an offset 
value is inserted at point 149 to force the overall average of the data 
record to zero. 
##EQU2## 
conn(149)=offset conn(1:119)=0 
conn(150:4096)=0 
The cross-talk response of the near-end connector is based on capacitive 
coupling, which necessarily has no d.c. value in its pulse response. Thus, 
forcing the average value to equal ensures that will there be no 
significant d.c. (direct current) value that occurs in the frequency 
representation of the truncated time record, contained in the data file 
CONN, when the fast Fourier transform is performed. 
EQU CONN=FFT (conn) 
Referring now to FIG. 12, there is shown a graph illustrating a frequency 
domain representation of residual cross-talk response comparing the 
effects of the near-end connector on the cross-talk response of the LAN 
cable system with and without compensation for the near-end connector 12 
(as shown in FIG. 1) according to the present invention. The vertical axis 
is cross-talk loss in decibels (dB) and the horizontal axis is frequency 
in megahertz. Residual cross-talk response is measured with the near-end 
connector 12 terminated with 100 ohm resistors at each of the wire pairs 
42a-d so the pulse response of the near-end connector 12 is the only one 
present in the time record. 
The upper trace 160 is the specification limit of FIG. 10. 
A trace 190 shows the residual cross-talk response with no compensation for 
comparison purposes. In other words, in the process 120 of FIG. 8B, the 
cross-talk calculation does not subtract the effect of the near-end 
connector 12 which appears as NECC. Thus, a theoretically perfect LAN 
cable system 50, with no pulse response, is only 12 dB below the 
specification limit as shown by the difference between the trace 160 and 
190. This relatively small difference, when coupled with a LAN cable 
system 50 which has a cross-talk response substantially close to the 
specification limit, results in a high probability that a false pass or 
fail indication will be given to the user. 
A trace 192 shows the residual cross-talk response with compensation 
according to the present invention. Now, a theoretically perfect LAN cable 
system 50, with no cross-talk pulse response, would be measured over 30 dB 
below the specification limit, as shown by the difference between the 
trace 160 and the trace 190. Adding compensation for the near-end 
connector 12 thus provides a substantial improvement in measurement 
accuracy and reduction in errors in returning a false pass or fail 
indication to the user. 
Referring now to FIG. 13, there is shown a graph illustrating the 
relationship between pulse width in the time domain and the distribution 
of pulse energy in the frequency domain. The left portion of the graph is 
the time domain representation of a 64 nanosecond width square pulse 200, 
a 16 nanosecond width square pulse 202, and a 5 nanosecond width square 
pulse 204 which together comprise a set of pulse widths. The vertical axis 
is relative amplitude and the horizontal axis is pulse width in 
nanoseconds. The pulses 200, 202, and 204 have the same relative amplitude 
in the graph. Each of the pulses 200, 202, and 204 have an equivalent 
frequency domain representation as illustrated on the right portion of the 
graph. Trace 206 is the frequency domain representation of the 64 
nanosecond pulse 200. The trace 206 shows substantial drops in relative 
energy at 15.625 megahertz intervals which would make measurements near 
those frequencies unusable. Similarly, trace 208 is the frequency domain 
representation of the 16 nanosecond pulse 202 which shows substantial 
drops in relative energy at 62.5 megahertz intervals. Trace 210 is the 
frequency domain representation of the 5 nanosecond pulse 204 which has 
its first substantial drop in relative energy at 200 megahertz, which is 
outside the frequency range of interest in the present invention. 
Referring now to FIG. 14, there is shown a graph illustrating the process 
of using differing pulse widths to obtain increased pulse energy at 
selected frequency ranges to obtain improved measurement accuracy 
according to the present invention. While the traces 206, 208, and 210 
(shown in FIG. 13) each have substantial drops in energy at various 
frequencies, a well-known property called bandwidth-time invariance can be 
used to take advantage of selected frequency ranges where the relative 
pulse energy is higher. According to bandwidth-time invariance, narrower 
pulses spread their energy over a wider frequency spectrum but at a 
correspondingly lower amplitude. Conversely, wider pulses can be employed 
to provide extra energy for measurements at lower frequency ranges for 
improved measurement accuracy. 
In the graph, the traces 206, 208, and 210 have been concatenated to form a 
composite frequency response as shown by the heavy line. This composite 
response maintains a relative high amount of energy at the low end of the 
frequency of 0.1 to 10 megahertz where such energy is most needed. At 0.1 
megahertz, for example, the relative pulse energy of the 64 nanosecond 
pulse is 22 dB higher than that of the 5 nanosecond pulse. Between 10 
megahertz and 40 megahertz, the 16 nanosecond pulse energy is employed to 
provide additional pulse energy above that of the 5 nanosecond pulse while 
avoiding the energy "dropouts" of the 6 nanosecond pulse. Above 40 
megahertz, adequate pulse energy is maintained at all other frequencies in 
the range of interest below 150 megahertz using the 5 nanosecond pulse. 
The particular combination of pulse widths chosen in the preferred 
embodiment of the present invention was chosen to provide an optimal 
amount of measurement accuracy over the frequency range of interest 
between 0.1 megahertz to 100 megahertz. Measurement accuracy is most 
critical for measurements that are in close proximity to the specification 
limit line. The specification limit line, which appears graphically as the 
trace 160 in FIG. 10, varies as a function of frequency. 
##EQU3## 
EQU f=frequency in megahertz 
At low frequencies, for example 1 megahertz, the limit line is -57 dB, 
which means that pulse response signal returned for measurement is 
relatively small. The test instrument has a "noise floor" which is a 
minimum sensitivity level that is relatively constant over the frequency 
range. The signal to noise ratio directly governs measurement accuracy. 
Thus, in order to maintain adequate measurement accuracy at low 
frequencies where the signal to noise ratio begins to deteriorate, the 
present invention employs a wider pulse width to provide more signal 
energy. Conversely, at higher frequencies, narrower pulse widths are 
employed to obtain adequate bandwidth to cover the frequency range of 
interest. 
It will be obvious to those having ordinary skill in the art that many 
changes may be made in the details of the above described preferred 
embodiments of the invention without departing from the spirit of the 
invention in its broader aspects. For example, the present invention may 
be employed to remove undesired cross-talk responses from any number of 
selected locations along a pair of transmission lines, limited by the 
effects of the intervening transmission line to estimate the undesired 
pulse responses with sufficient accuracy to effect their removal and 
sufficient resolution to separate the undesired cross-talk responses from 
the desired responses. Furthermore, different pulse widths may be employed 
to obtain increased energy at other frequency ranges of interest. The 
frequency response of the different pulse widths may be concatenated in 
any combination desired to obtain a composite response shaped as needed. 
Therefore, the scope of the present invention should be determined by the 
following claims.