U.S. patent application number 11/376626 was filed with the patent office on 2006-09-28 for reflectometer with echo canceller.
Invention is credited to Amod V. Dandawate, Chunming Han, Xiao-Feng Qi.
Application Number | 20060217937 11/376626 |
Document ID | / |
Family ID | 34423274 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060217937 |
Kind Code |
A1 |
Han; Chunming ; et
al. |
September 28, 2006 |
Reflectometer with echo canceller
Abstract
Various embodiments are directed to a zero-echo canceling system
adapted to at least partially cancel a received zero-echo by
calculating an error signal as a difference between the zero-echo
and a zero-echo canceling signal.
Inventors: |
Han; Chunming; (Manalapan,
NJ) ; Dandawate; Amod V.; (Basking Ridge, NJ)
; Qi; Xiao-Feng; (Freehold, NJ) |
Correspondence
Address: |
KACVINSKY LLC
4500 BROOKTREE ROAD
SUITE 102
WEXFORD
PA
15090
US
|
Family ID: |
34423274 |
Appl. No.: |
11/376626 |
Filed: |
March 14, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10686340 |
Oct 14, 2003 |
7013226 |
|
|
11376626 |
Mar 14, 2006 |
|
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Current U.S.
Class: |
702/189 |
Current CPC
Class: |
G01M 11/3145
20130101 |
Class at
Publication: |
702/189 |
International
Class: |
G06F 15/00 20060101
G06F015/00 |
Claims
1. A method of reducing a zero-echo comprising: combining a
zero-echo canceling signal based on a transmitted pulse with a
received zero-echo to reduce the received zero-echo; and
calculating an error signal as a difference between the zero-echo
and the zero-echo canceling signal.
2. The method of claim 1, further comprising adjusting the
zero-echo canceling signal based on the error signal to decrease
the error signal and decrease the received zero-echo.
3. The method of claim 2, wherein adjusting comprises adjusting a
value of a voltage bridge or a variable resistor.
4. The method of claim 1, wherein the zero-echo has been
sufficiently cancelled or attenuated based on the combining to
allow a full-path reflection to be detected.
5. The method of claim 1, wherein the zero-echo comprises a short
path reflection of the transmitted pulse.
6. An apparatus comprising: a zero-echo canceller to combine a
zero-echo canceling signal based on a transmitted pulse with a
received zero-echo to reduce the received zero-echo and to
calculate an error signal as a difference between the zero-echo and
the zero-echo canceling signal.
7. The apparatus of claim 6, said zero-echo canceller to adjust the
zero-echo canceling signal based on the error signal to decrease
the error signal and decrease the received zero-echo.
8. The apparatus of claim 7, further comprising at least one of a
voltage bridge and a variable resistor to adjust.
9. The apparatus of claim 6, said zero-echo canceller to generate
the zero-echo canceling signal.
10. The apparatus of claim 6, said zero-echo canceller coupled to a
combining circuit and a voltage divider, the combining circuit
adapted to combine a zero-echo received from the voltage divider
and a zero-echo canceling signal received from the zero-echo
canceller.
11. The apparatus of claim 10, the combining circuit comprising an
adder.
12. The apparatus of claim 10, the combining circuit comprising a
subtraction circuit.
13. The apparatus of claim 10, the zero-echo canceller comprising a
variable voltage divider.
14. The apparatus of claim 13, the variable voltage divider
comprising a variable resistor.
15. A system comprising: a transceiver coupled to a DSL line to be
tested; a zero-echo canceller coupled to said transceiver, said
zero-echo canceller to combine a zero-echo canceling signal based
on a transmitted pulse with a received zero-echo to reduce the
received zero-echo and to calculate an error signal as a difference
between the zero-echo and the zero-echo canceling signal.
16. The system of claim 15, said zero-echo canceller to adjust the
zero-echo canceling signal based on the error signal to decrease
the error signal and decrease the received zero-echo.
17. The system of claim 16, further comprising at least one of a
voltage bridge and a variable resistor to adjust.
18. The system of claim 15, said zero-echo canceller to generate
the zero-echo canceling signal.
19. The system of claim 15, said zero-echo canceller coupled to a
combining circuit and a voltage divider, the combining circuit
adapted to combine a zero-echo received from the voltage divider
and a zero-echo canceling signal received from the zero-echo
canceller.
20. The system of claim 15, the zero-echo canceller comprising a
variable voltage divider.
Description
RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/686,340 filed on Oct. 14, 2003 which issued
as U.S. Pat. No. 7,013,226 on Mar. 14, 2006.
BACKGROUND INFORMATION
[0002] Time domain reflectometry (TDR) may include an analysis of a
conductor or other line (e.g., wire, twisted pair, coaxial cable,
fiber optic line) by sending a pulsed signal into the line and then
examining the reflection of that transmitted pulse. TDR may be
useful in a variety of different applications. For example, TDR may
be used to measure the length of a line or loop based on the delay
of the reflection, or to measure the quality of the line, such as
by examining the amplitude or other qualities of the reflection.
TDR may be used, for example, to qualify or test a line for the
suitability of Digital Subscriber Line (DSL) or cable
communications, etc.
[0003] When TDR is performed, unwanted signal echoes can sometimes
be a problem. For example, in many cases, the same port may be used
both to transmit the pulse onto the line and to receive the
reflected pulse from the line. In some cases, due to a reflection
from the line, one of the paths that the transmitted pulse may
travel is from the transmitter directly to the local receiver. This
may create an echo (or unwanted reflection) that may be, in some
cases, received almost immediately (e.g., 1-10 microseconds) after
the transmission of the pulse, due to the very short path. This
echo or unwanted reflection typically does not characterize the
line, and therefore, may not be helpful in the TDR analysis of the
line or loop. This echo or unwanted reflection which may begin near
(e.g., just after) the time of pulse transmission, or near time
zero, may be referred to herein as zero-echo (e.g., the echo
substantially near time zero).
[0004] Due to the short path, a zero-echo may often have a
relatively large amplitude and may even saturate the receiver. As a
result, the zero-echo may create a blind time interval during which
the reflectometer may not be able to measure any other reflections.
Depending on the configuration, many other types of unwanted echoes
or unwanted reflections may occur as well.
[0005] FIG. 1 is a diagram illustrating a conventional
reflectometer. The reflectometer may send a pulse from pulse
transmitter 105 to the loop (or line) 115 and measures the
reflection with the local receiver 110. The receiver may receive
the pulse that travels the path ABCBD, which is the desired
reflection to be received (e.g., to analyze the loop or line). The
receiver may also receive a pulse that travels the path ABD, which
may be referred to as zero-echo. If a reflected signal
substantially arrives at the receiver before the zero-echo has
dissipated or died out, the two reflections may merge and typically
cannot be separated. This may create a blind time interval during
which a conventional reflectometer may not be able to measure
reflections. Therefore, a need may exist for an improved
reflectometer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a diagram illustrating a conventional
reflectometer.
[0007] FIG. 2 is a diagram illustrating a reflectometer according
to an example embodiment.
[0008] FIG. 3 is a flow chart illustrating operation of a
reflectometer according to an example embodiment.
[0009] FIG. 4 is a timing diagram illustrating pulse reflections
both with and without use of a zero-echo canceling system according
to an example embodiment.
[0010] FIG. 5 is a diagram illustrating a reflectometer according
to another example embodiment.
[0011] FIG. 6 is a block diagram illustrating a modem according to
an example embodiment.
DETAILED DESCRIPTION
[0012] In the detailed description, numerous specific details are
set forth in order to provide a thorough understanding of the
embodiments of the invention. It will be understood by those
skilled in the art, however, that embodiments of the invention may
be practiced without these specific details. In other instances,
well-known methods, procedures and techniques have not been
described in detail so as not to obscure the foregoing
embodiments.
[0013] Embodiments of the present invention may include apparatuses
for performing the operations herein. This apparatus may be
specially constructed for the desired purposes, or it may comprise,
in part or in whole, a general purpose computing device selectively
activated or reconfigured by a program stored in the device. Such a
program may be stored on a storage medium, such as, but is not
limited to, any type of disk including floppy disks, optical disks,
CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random
access memories (RAMs), electrically programmable read-only
memories (EPROMs), electrically erasable and programmable read only
memories (EEPROMs), flash memory, magnetic or optical cards, or any
other type of media suitable for storing electronic instructions,
and capable of being coupled to a system bus for a computing
device.
[0014] In the following description and claims, the terms coupled
and connected, along with their derivatives, may be used. In
particular embodiments, connected may be used to indicate that two
or more elements are in direct physical or electrical contact with
each other. Coupled may mean that two or more elements are in
direct physical or electrical contact. However, coupled may also
mean that two or more elements may not be in direct contact with
each other, but yet may still cooperate or interact with each
other.
[0015] It is worthy to note that any reference in the specification
to "one embodiment" or "an embodiment" means in this context that a
particular feature, structure, or characteristic described in
connection with the embodiment may be included in at least one
embodiment of the invention. The appearances of the phrase "in one
embodiment" or "an embodiment" in various places in the
specification do not necessarily refer to the same embodiment, but
may be referring to different embodiments.
[0016] It should be understood that embodiments of the present
invention may be used in a variety of applications. Although the
present invention is not limited in this respect, the circuits
disclosed herein may be used in many apparatuses such as in
transmitters and receivers of a communication system,
modulator/demodulators (Modems), DSL modems, cable modems,
reflectometers, and other systems or devices, such as computers,
gateways, bridges, hubs, and a wide variety of test equipment, such
as reflectometers and the like, although the scope of the invention
is not limited in this respect.
[0017] Referring to the Figures in which like numerals indicate
like elements, FIG. 2 is a diagram illustrating a reflectometer
according to an example embodiment. The reflectometer 200 of FIG. 2
may include a pulse transmitter 105 to transmit pulses and a
receiver 220 to receive and measure one or more pulse reflections.
Reflectometer 200 may advantageously also include a zero-echo
canceling system 202 to detect and at least partially cancel or
decrease a zero-echo or other reflections. Zero-echo canceling
system 202 may include, for example, a voltage divider 205, a
zero-echo canceller 210 and a subtraction circuit 215, although the
invention is not limited thereto. The zero-echo canceling system
202 will now be briefly described.
[0018] Referring to FIG. 2, voltage divider 205 may be coupled
between an output of pulse transmitter 105 and loop (or line) 115.
A voltage divider may be a circuit, for example, to provide a
voltage that is different from a voltage at an input. A voltage
divider, such as voltage divider 205, may for example include one
or more resistors to cause a voltage drop across the resistor(s)
when current flows through the resistor(s) based on Ohm's law,
although the invention is not limited thereto. In this example
embodiment, the voltage output by voltage divider 205 at point B
may be less than the voltage input to divider 205 at point A.
[0019] In another embodiment, voltage divider 205 may have an
impedance or resistance that substantially matches the
characteristic impedance of loop 115, so as to decrease the number
of reflections across the loop. For example, if the characteristic
impedance of loop 115 is 100 Ohms, then a resistor of 100 ohms may
advantageously be used for voltage divider 205.
[0020] Referring to FIG. 2 again, a zero-echo canceller 210 may be
coupled via line 207 to an output of pulse transmitter 105 and
coupled via line 213 to receiver 220. According to an example
embodiment, zero-echo canceller 210 may generate a zero-echo
canceling signal onto line 203 to at least partially cancel or
attenuate a zero-echo. The zero-echo canceling signal output on
line 203 may be generated based on a transmitted pulse received via
line 207. According to an example embodiment, zero-echo canceller
210 may comprise a variable voltage divider (e.g., including one or
more variable resistors or potentiometers) to output a variable
portion of the transmitted pulse onto line 203 as a zero-echo
canceling signal. In an example embodiment, the zero-echo canceling
signal may comprise a signal that has substantially the same shape
of the transmitted pulse, but the amplitude of the zero-echo
canceling signal may be adjusted, although the invention is not
limited thereto.
[0021] A subtraction circuit 215 may be coupled via line 209 to an
output of voltage divider 205 (e.g., at point B) and is coupled to
an output of zero-echo canceller 210 via line 203, and outputs a
signal via line 211 to receiver 220. In an example embodiment,
subtraction circuit 215 may subtract the zero-echo canceling signal
received via line 203 from the zero-echo received via line 209, to
produce an output via line 211 that is coupled to receiver 220.
[0022] In operation of FIG. 2, pulse transmitter 105 transmits a
pulse that is passed through voltage divider 205 before being sent
to the loop 115 at point B. The transmitted pulse is also input via
line 207 to zero-echo canceller 210. According to an example
embodiment, zero-echo canceller 210 may output via line 203 a
variable portion of the transmitted pulse to subtraction circuit
215. This output on line 203 from zero-echo canceller 210 may be
described as a zero-echo canceling signal since it may be used to
at least partially cancel or decrease the zero-echo.
[0023] A zero-echo (short path reflection) of the transmitted pulse
may be passed via line 209 to subtraction circuit 215, due to
reflection at point B, for example. In order for the desired signal
of interest being received to have substantially no presence at
point A, the pulse transmitter 105 may be designed to have a
substantially low output impedance (e.g., around 1 Ohm output
impedance), although the invention is not limited thereto. This low
output impedance of pulse transmitter 105 may allow any return echo
(reflection) to be substantially eliminated at point A due to the
large voltage drop across voltage divider 205 (e.g., around 100
ohms).
[0024] Subtraction circuit 215 may, for example, calculate the
difference between the zero-echo (short path reflection provided
via path ABD) received via line 209 and the zero-echo canceling
signal on line 203 (which may be a variable portion of the
transmitted pulse), and outputs this difference via line 211 to
receiver 220 as an error signal. Receiver 220 may then control or
adjust the zero-echo canceller 210 (e.g., via a canceller control
signal on line 213) to more closely match the zero-echo canceling
signal to the zero-echo. In this manner, the zero-echo canceling
system 202 may operate to decrease the error signal input on line
211 and better attenuate or cancel the zero-echo before it is
received by receiver 220.
[0025] This process (e.g., generating a pulse, generating a
zero-echo canceling signal based on the transmitted pulse,
calculating the difference between the two signals or error signal,
and adjusting the zero-echo canceller to decrease the error signal)
may be repeated, so that the zero-echo canceller 210 may be tuned
or adjusted to decrease or substantially cancel the zero-echo
(short path reflection) as received by the receiver 220. By
substantially canceling or at least decreasing the zero-echo
received at the receiver 220 using the adjustable zero-echo
canceling signal, the blind time interval for receiver 220 may be
substantially eliminated or at least reduced. By decreasing or
attenuating the zero-echo at the receiver 220 using this technique,
this may allow receiver 220 to more effectively detect and measure
the desired (full path) reflection after it traverses path ABCBD.
The zero-echo canceller 202 may sufficiently cancel or at least
partially attenuate the zero-echo and allow the full reflection
(e.g., echo received via path ABCBD) to be detected and measured by
receiver 220, even for short loops where the full reflection may
partially overlap with the zero-echo.
[0026] According to an example embodiment, cancellation or
reduction of the zero-echo may be assisted or simplified due to the
occurrence or timing of the zero-echo near or just after the pulse
transmission (near time zero). The close proximity in time of the
zero-echo to the pulse transmission (e.g., 1-2 microseconds after
the pulse transmission) may allow a zero-echo canceling signal to
be generated based on the transmitted pulse and then used to at
least partially cancel or offset the zero-echo before it arrives at
the receiver 220.
[0027] Subtraction circuit 215 may be used to then subtract the
echo canceling signal from the zero-echo, although the invention is
not limited thereto. Alternatively, a circuit (e.g., zero-echo
canceller) may be used to invert the amplitude of the received
pulse or portion thereof and generate a zero-echo canceling signal
that has a substantially equal but opposite amplitude as compared
to the zero-echo. This zero-echo canceling signal may then be
summed or added with the output from voltage divider 205 (e.g.,
using a summing circuit rather than a subtraction circuit),
although the invention is not limited thereto. Therefore, there
terms "adding" or "summing" or "combining" may encompass both
adding and subtracting, since it may depend on the polarity or sign
of the amplitude of the zero-echo canceling signal. Subtraction
circuit 215 may be more generally a combining circuit for combining
the zero-echo with the zero-echo canceling signal.
[0028] Zero-echo canceling system 202 may also include a
synchronizer 225 to synchronize clocks for both the pulse
transmitter 105 and the receiver 220. Synchronizer 225 may allow
pulse transmitter 105 and receiver 220 to be synchronized for time
zero, which is the time the pulse is transmitted from transmitter
105. This may allow the receiver to more accurately measure the
delay for the echo, and therefore, more accurately measure the
length of the loop BC.
[0029] Receiver 220 may also control the shape of the transmitted
pulse output from transmitter 105. A wider pulse may be needed for
longer loops, so the echo is not attenuated too much. However, more
accurate measurements for the loop can sometimes be obtained using
a transmitted pulse that is narrower. Thus, it may be desirable to
use a narrower pulse for shorter loops to improve timing accuracy,
and to use a wider pulse for longer loops. After one or more pulse
transmissions and measurements at receiver 220 of the echo, the
pulse width may be adjusted based on the amplitude or other
characteristics of the echo received at receiver 220, for example,
although the invention is not limited thereto.
[0030] FIG. 3 is a flow chart illustrating operation of a
reflectometer according to an example embodiment. At 305, a pulse
is transmitted. For example, the pulse may be transmitted to a loop
115 or line to be tested. The (full) reflection for the full path
ABCBD may be the reflection of interest since it may characterize
the loop or be used to analyze the loop.
[0031] At 310, a zero-echo canceling signal may be generated based
on the transmitted pulse (this may be a signal that may be
estimated to assist in canceling or reducing the zero-echo). At
315, a zero-echo (or short path reflection) may be received or
detected, for example, based on the short path reflection from the
transmitter to the receiver.
[0032] At 320 in FIG. 3, the zero-echo canceling signal may be
added to (or combined with) the zero-echo to at least partially
cancel or reduce the received zero-echo (e.g., before it is
received by the receiver). This may be performed using an adder
circuit or a subtraction circuit, although the invention is not
limited thereto. According to an example embodiment, by at least
partially canceling or reducing the zero-echo, the blind time
interval for the receiver 220 may be reduced, although the
invention is not limited thereto.
[0033] At 325, an error signal may be calculated, for example, as
the difference between the zero-echo and the zero-echo canceling
signal. At 330, the zero-echo canceling signal is adjusted to
decrease the error signal. This adjustment may include adjusting
the amplitude of the zero-echo canceling signal. This feedback may
allow the zero-echo canceling to be improved or tuned over
time.
[0034] FIG. 4 is a timing diagram illustrating pulse reflections
both with and without use of a zero-echo canceling system according
to an example embodiment. The received pulse 405 may be generated
in the absence of a zero-echo canceling system 202 and is shown
with the dotted line. The received pulse 405 may include a full
path reflection (e.g., reflection of interest traveling path ABCBD)
of a pulse that is merged or overlaps with a zero-echo (short path
echo traveling path ABD). In this example, the loop to be tested is
short enough that the full path reflection is received quickly and
overlaps with the zero-echo, such that the two signals appear as
one received pulse 405. As a result of the merger or overlapping of
these two signals, the beginning and end of the full path
reflection cannot be identified, and creates a blind time interval
415 during which detection of a full path reflection of interest
may be difficult. In an example embodiment, a 1 microsecond pulse
transmitted onto a loop that is 300 feet or shorter may have a
return reflection that overlaps with a zero-echo. This description
is simply an example and the invention is not limited thereto.
[0035] In FIG. 4, the full path reflection 410 is shown in the
solid line and is received by receiver 220 when a zero-echo
canceling system (such as system 202, for example) may be used to
at least partially cancel or attenuate the zero-echo. In this case,
the full path reflection 410 is received substantially without the
presence of the zero-echo. Therefore, the use of a zero-echo
canceling system may be used to substantially eliminate or at least
decrease the blind time interval for the receiver, allowing even
relatively short loops to be tested using TDR.
[0036] FIG. 5 is a diagram illustrating a reflectometer according
to another example embodiment. Referring to FIG. 5, a pulse
generator 510 may be provided, such as a Stanford Research Systems
(SRS) DS345 generator. The generator 510 is coupled to a loop 115
via a buffer 505 and resistor R4. R4 may be comparable to voltage
divider 205 of FIG. 2. A zero-echo may be input to R6 based on the
transmission of a pulse from generator 510 and reflection at loop
115, for example. The output impedance of buffer 505 may be very
low so that any reflection received at R4 will not be substantially
transferred to R3.
[0037] A zero-echo canceling signal may be input to R3 based on the
generated pulse via buffer 505. Resistors R3 and R1 may be variable
resistors or potentiometers to allow a variable portion of the
transmitted pulse to be input to operational amplifier (op amp) 525
as a zero-echo canceling signal. The zero-echo is input to op amp
525 via resistor R6. Op amp 525 may output the difference between
the zero-echo (e.g., input via R6) and the zero-echo canceling
signal (e.g., input via R3) to output an error signal to a receiver
515. Receiver 515 may be, for example, a Tektronix TDS754D
Oscilloscope, and may include a computer 520. Computer 520 may be
coupled to oscilloscope via a bus, such as a general purpose
interface bus, or the like, although the invention is not limited
thereto. The resulting cancelled (or partially cancelled) zero-echo
may be provided as an error signal to computer 520. Computer 520
may then adjust the values of resistor R1 and/or R3, for example,
to adjust the zero-echo canceling signal in order to decrease the
zero-echo received by oscilloscope 515, although the invention is
not limited thereto. The generator 510 and the oscilloscope 515 may
be synchronized via a trigger synchronization signal.
[0038] FIG. 6 is a block diagram illustrating a modem
(modulator/demodulator) according to an example embodiment. Modem
616 may be a cable modem, a DSL modem, or other modem, for example,
although the invention is not limited thereto. Modem 616 may
include, for example, a transceiver 610 (including a transmitter
and a receiver) for transmitting and receiving signals, including
transmitting pulses and receiving reflections or echoes. A
processor 612 and a memory 614 are coupled to the transceiver. A
zero-echo canceling system, such as system 202, may be coupled to
transceiver 610 and processor 612. Processor 612 may control
various tasks performed by modem 616 including controlling the
operation of zero-echo canceling system 202. Modem 616 may operate
to transmit and receive data over a line, or may operate to test
the line by transmitting a pulse and receiving a full path
reflection. Modem 616 may advantageously at least partially cancel
or reduce a zero-echo to eliminate or at least reduce a blind time
interval as received by transceiver 610.
[0039] While certain features of the embodiments of the invention
have been illustrated as described herein, many modifications,
substitutions, changes and equivalents will now occur to those
skilled in the art. It is, therefore, to be understood that the
appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the embodiments of the
invention.
* * * * *