EP1051633A1 - Verification de systemes catv - Google Patents

Verification de systemes catv

Info

Publication number
EP1051633A1
EP1051633A1 EP99903329A EP99903329A EP1051633A1 EP 1051633 A1 EP1051633 A1 EP 1051633A1 EP 99903329 A EP99903329 A EP 99903329A EP 99903329 A EP99903329 A EP 99903329A EP 1051633 A1 EP1051633 A1 EP 1051633A1
Authority
EP
European Patent Office
Prior art keywords
data
circuit
ofthe
reflections
raised cosine
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99903329A
Other languages
German (de)
English (en)
Other versions
EP1051633A4 (fr
Inventor
Daniel E. Rittman
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Trilithic Inc
Original Assignee
Trilithic Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Trilithic Inc filed Critical Trilithic Inc
Publication of EP1051633A1 publication Critical patent/EP1051633A1/fr
Publication of EP1051633A4 publication Critical patent/EP1051633A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N17/00—Diagnosis, testing or measuring for television systems or their details
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/46—Monitoring; Testing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L12/00—Data switching networks
    • H04L12/28—Data switching networks characterised by path configuration, e.g. LAN [Local Area Networks] or WAN [Wide Area Networks]
    • H04L12/2801—Broadband local area networks
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L43/00—Arrangements for monitoring or testing data switching networks
    • H04L43/50—Testing arrangements

Definitions

  • This invention relates to the detection of impedance mismatches in circuits. It is disclosed in the context of a system for detecting impedance mismatches in forward and return CATV signal paths, but it is believed to be useful in other applications as well.
  • a method for determining the transit time to a feature in a digital communication circuit comprises generating a quantity of data that is at least quasi-random, transmitting the quantity of at least -2- quasi-random data along the circuit from a transmitting end ofthe circuit, recovering reflections from the circuit adjacent the transmitting end ofthe circuit, correlating the _ reflections with the quantity of data to generate a correlation result, identifying a reflection peak in the result, and correlating the reflections with the data to generate a correlation result, for identifying a reflection peak in the result, and for determining a time delay to the reflection peak.
  • the method comprises a method for determining the location of an impedance mismatch in the circuit, the step of recovering reflections from the circuit comprising the step of recovering reflections from impedance mismatches in the circuit.
  • the method further comprises the step of multiplying the propagation velocity ofthe data through the circuit by the time delay to the reflection peak to determine the round trip distance to the impedance mismatch.
  • the steps are repeated, and average time delays over the number of repetitions are developed.
  • the step of developing average time delays over the number of repetitions comprises summing the time delays determined by the repetitions, and dividing the sum ofthe time delays by the number of repetitions.
  • the method further comprises the step of passing the data through a digital root raised cosine filter.
  • the step of passing the data through a digital root raised cosine filter comprises the step of passing the data through a digital root raised cosine filter with an excess bandwidth factor of 20%.
  • an apparatus for determining the transit time to a feature in a digital communication circuit comprises a first device for generating data that is at least quasi-random, a second device for coupling the first device to the circuit to transmit the at least quasi-random data along the circuit from a transmitting end ofthe circuit, a third device for recovering reflections from the circuit, the third device coupled to the circuit adjacent the -3- transmitting end ofthe circuit, and a fourth device for correlating the reflections with the data to generate a correlation result, for identifying a reflection peak in the result, ⁇ and for determining a time delay to the reflection peak.
  • the apparatus comprises an apparatus for determining the location of an impedance mismatch in the circuit, the third device recovering reflections from impedance mismatches in the circuit, and the fourth device comprising a fourth device for multiplying the propagation velocity ofthe data through the circuit by the time delay to determine the round trip distance to the impedance mismatch.
  • the fourth device comprises a fourth device for correlating multiple reflections with multiple strings of data, and for developing average time delays over the number of repetitions.
  • the fourth device comprises a fourth device for correlating multiple reflections with multiple strings of data to generate multiple correlation results, for identifying multiple reflection peaks in the multiple correlation results, for multiplying the propagation velocity ofthe data through the circuit by multiple time delays to the multiple reflection peaks, for summing the multiple time delays, and for dividing the sum ofthe time delays by the number of time delays.
  • the second device comprises a digital root raised cosine filter.
  • the second device comprises a digital root raised cosine filter with an excess bandwidth factor of 20%.
  • Fig. 1 illustrates the autocorrelation of a quasi-random sequence of data having eight possible values (-7, -5, -3, -1, 1, 3, 5 and 7) useful in understanding the present invention
  • Fig. 2 illustrates a filter characteristic useful in understanding the present invention
  • Fig. 3 illustrates the characteristic ofthe autocorrelation illustrated in Fig. 1 filtered by a filter having the characteristic illustrated in Fig. 2;
  • Fig. 4 illustrates plots ofthe product of a single autocorrelation and the averaging of two autocorrelations on the same graph to illustrate an aspect ofthe present invention;
  • Fig. 5 illustrates a partly block and partly schematic diagram of a circuit for testing the present invention
  • Fig. 6 illustrates a plot of power spectral density versus frequency of a test signal according to the present invention
  • Fig. 7 illustrates a plot of power spectral density versus frequency of a processed test signal according to the present invention
  • Fig. 8 illustrates a plot of amplitude versus time offset of averaged correlations of a processed test signal according to the present invention
  • Fig. 9 illustrates a plot of amplitude versus time offset of averaged correlations of a processed test signal according to the present invention.
  • Fig. 10 illustrates plots of amplitude versus time offset ofthe imaginary parts ofthe autocorrelations of two test signals according to the present invention as well as the average of these two plots.
  • receiver implementations of 64 QAM will include adaptive equalization designed to mitigate the effects of reflections, among - other imperfections.
  • adaptive equalizers have limits on their effectiveness. For example, if reflections are more than a few symbol times away, the equalizer may not be capable of compensating for them. Additionally, even if reflections are within the time range ofthe equalizer, if the magnitude of a reflection is too large, the equalizer will not converge, that is, it will not adapt in such a way as to mitigate the effects of whatever channel impairment(s) it is attempting to mitigate.
  • Typical values for reflection delay and magnitude equalization limits may be in the range of 5 symbol times and -10 dB, respectively.
  • Equipment has long been in use to measure reflections.
  • Such equipment includes, for example, TDRs, including OTDRs, and frequency sweep systems.
  • TDRs including OTDRs, and frequency sweep systems.
  • Each of these methods of measuring reflections requires the injection of a known signal into the cable system and observation ofthe cable system for the effects of reflections.
  • This method has the disadvantages that it requires a reference signal generation instrument, and either interfering with whatever signals are already on the system or requiring unused frequency spectrum in the system.
  • the method according to the present invention uses the 64 QAM data carrier itself to detect reflections without the need to converge an adaptive equalizer or demodulate data.
  • the invention makes use ofthe presence on the 64 QAM data carrier of random or at least quasi- random data.
  • the present invention utilizes the autocorrelation properties of random data.
  • the correlation of two N length sequences x[n] and y[n] of data is defined as
  • Correlation is a measure ofthe similarity of two sequences. Autocorrelation detects periodic similarities in a single sequence. Since random data should have no periodicity, autocorrelation of random or at least quasi-random data should be nearly zero at all offsets ofthe auto correlated signal except zero offset.
  • Fig. 1 illustrates the autocorrelation of a quasi-random sequence. Each point in the sequence can assume one of 2 3 , or 8, possible values. Eight possible data values were chosen for this plot because each ofthe I and Q subchannels of a 64 QAM data stream is made up of symbols with eight possible values. As Fig. 1 illustrates, the autocorrelation exhibits the expected single peak at zero offset and values close to zero everywhere else, indicating that the function is aperiodic.
  • a digital root raised cosine filter with an excess bandwidth factor of 20% is emerging as the standard transmit data filter.
  • the characteristic of a digital root raised cosine filter with an excess bandwidth factor of 20% is illustrated in Fig. 2. While such a filter constrains the bandwidth ofthe transmitted energy to a 6 MHZ channel (which permits multiple channels to be transmitted on a single CATV conductor), it also has the effect of broadening the autocorrelation peak illustrated in Fig. 1.
  • Fig. 3 illustrates an enlarged view ofthe autocorrelation illustrated in Fig. 1 filtered by the filter whose characteristic is illustrated in Fig. 2. As illustrated in Fig. 3, the main lobe ofthe autocorrelation ofthe filtered data occupies approximately four samples on either side ofthe central peak.
  • Fig, 4 illustrates two plots. Both illustrate autocorrelation of a signal with a -lOdB reflection 150 m away (300 m round trip).
  • the broken line plot illustrates the results of a single autocorrelation.
  • the solid line plot illustrates the average often autocorrelations.
  • RJm] (R x [m] + R Q [ ⁇ I]) (cos ⁇ + jsin ⁇ ). (8) 0
  • an arbitrary waveform generator 20 such as, for example, a Stanford Research Systems model DS345 AWG, having an output sample rate of 40 MHZ was programmed to output 16,100 samples of 64 QAM data repeatedly. This corresponds to about 2000 symbols of 64 QAM data at 30 Mbps.
  • the data was modulated on a 4 MHZ carrier.
  • This signal was then placed on a reflection-generating cable system 22, and a digital oscilloscope 24 such as, for example, a Gage model CS1012 oscilloscope sampling at 20 Msamples/se ⁇ , was used to monitor the reflections.
  • the test circuit included 50 ⁇ -75 ⁇ impedance matching transformers 26, 28, two-way 75 ⁇ splitters 30, 32, 34, and a length 36, for example, 150.1 m, of test cable, such as, for example, Belden 9231 or 9266 video cable, a terminal length of, for example, 6.9 m of Channel Master RG-59 video cable terminated by a 2 dB pad.
  • test cable such as, for example, Belden 9231 or 9266 video cable
  • the data recovered by the digital oscilloscope 24 was then transferred to the Matlab program running on a PC 40 for analysis according to the present invention.
  • Several experiments using different lengths 36 of test cable and terminations were run to determine the accuracy and resolution ofthe reflection delay and amplitude measurement.
  • Fig. 6 illustrates a typical power spectrum of collected test data, including the transmitted signal and the reflection.
  • the signal width is less than 6 MHZ and is centered at 4 MHZ.
  • the power spectrum extends from DC to the 10 MHZ Nyquist limit.
  • the first step is to recover the I and Q data. This is accomplished by complex downconversion to a center frequency, ideally 0 Hz, but in any event much less than the symbol rate. Frequencies in the range of 50 KHz are acceptable. As previously demonstrated, phase offset does not adversely affect the measurement either.
  • the resulting signal is low pass filtered to remove the component at twice the original carrier frequency. The result of this filtering is illustrated in Fig. 7. Since the signal is now complex valued, all frequencies from DC to the sampling frequency must be illustrated to illustrate the signal's spectral density.
  • Fig. 8 From Fig. 8, one significant reflection at approximately 1.5 ⁇ sec. is immediately recognizable. To identify others, the scale of Fig. 8 is expanded in Fig. 9. Fig. 9 illustrates three plots, two in broken lines and one in solid lines.
  • the broken line plot with the lowest amplitude sidelobes was generated using the test apparatus illustrated in Fig. 5 in which the length of test cable included 150.1 m of Belden 9231 video cable, 6.9 m of Channel Master RG-59 video cable and a 2 dB pad terminating the RG-59 cable.
  • This plot illustrates a fairly well-defined reflection at 1.566 ⁇ sec., corresponding to termination ofthe Channel Master RG-59 cable, and an amplitude about 3.6 dB lower than the other broken line plot, the plot generated from the same lengths of cable with the 2 dB terminal pad removed.
  • the solid line plot was generated using the test apparatus illustrated in Fig. 5 with the 150.1 m length of 9231 cable only.
  • the propagation velocity in the 9231 cable is 2 x 10 8 m/sec.
  • the 1.566 ⁇ sec. offset thus corresponds to a round trip distance of 313.2 m compared with the 314 m (150. lm + 6.9 m) actual distance.
  • the 9231 cable itself generated a reflection sidelobe peak at 1.497 ⁇ sec., corresponding to a distance of 299.4 m versus an actual round trip distance of 300.2 m.
  • the presence ofthe 2 dB pad termination is also evident in the difference in the amplitudes ofthe sidelobes ofthe two broken line plots.
  • the width ofthe main lobe in the illustrated embodiment will obscure any reflections within a couple of hundred nsec, corresponding to a distance of about 30 m or so.
  • the main lobe component is entirely in the real component ofthe autocorrelation response.
  • the imaginary component is simply 2R TQ [m], which is zero when there is no reflection.
  • Fig. 10 illustrates this effect.
  • a significant -11- response can be seen quite close to zero offset in the imaginary component.
  • the absolute magnitude ofthe response is dependent upon the phase angle ofthe carrier to the reflection. It is likely that a scheme repeating this test at multiple carrier frequencies could extrapolate an absolute amplitude, since changing the carrier frequency will also change the phase angle to the reflection.
  • modulation is typically quadrature phase shift keying or 16 QAM.
  • the delay resolution ofthe measurement will not be sensitive to the modulation format. It is, however, proportional to the modulation bandwidth. The wider the bandwidth, the finer the resolution and the closer to the source reflections can be detected.
  • the data must still be randomized or at least quasi-randomized. Again, however, this is generally a standard feature of digital communications systems.

Landscapes

  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Health & Medical Sciences (AREA)
  • Biomedical Technology (AREA)
  • General Health & Medical Sciences (AREA)
  • Multimedia (AREA)
  • Measurement Of Resistance Or Impedance (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

Abstract

L'invention concerne un procédé et un appareil permettant de déterminer l'emplacement d'une inadaptation d'impédances dans un circuit (22) de communication numérique, lequel appareil génère (20) au moins des données quasi-aléatoires, transmet ces données par le circuit (22) à partir d'une extrémité émettrice du circuit (22), récupère (24) les échos imputables aux ruptures d'impédances dans le circuit (22) adjacent à l'extrémité émettrice du circuit (22), met en corrélation lesdits échos et les données de façon à générer un résultat de corrélation, identifie un pic de réflexion dans le résultat, et multiplie la vitesse de propagation des données dans le circuit (22) par le rapport retard/pic de réflexion.
EP99903329A 1998-01-23 1999-01-22 Verification de systemes catv Withdrawn EP1051633A4 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US7240998P 1998-01-23 1998-01-23
US72409P 1998-01-23
PCT/US1999/001432 WO1999038023A1 (fr) 1998-01-23 1999-01-22 Verification de systemes catv

Publications (2)

Publication Number Publication Date
EP1051633A1 true EP1051633A1 (fr) 2000-11-15
EP1051633A4 EP1051633A4 (fr) 2001-08-22

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP99903329A Withdrawn EP1051633A4 (fr) 1998-01-23 1999-01-22 Verification de systemes catv

Country Status (4)

Country Link
EP (1) EP1051633A4 (fr)
AU (1) AU2338099A (fr)
CA (1) CA2315759A1 (fr)
WO (1) WO1999038023A1 (fr)

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2002025299A1 (fr) * 2000-09-23 2002-03-28 Nokia Corporation Circuit electronique et procede permettant de tester une ligne
US7298396B2 (en) 2003-08-25 2007-11-20 Spx Corporation Apparatus and method for monitoring transmission systems using embedded test signals
US7034545B2 (en) 2003-12-30 2006-04-25 Spx Corporation Apparatus and method for monitoring transmission systems using off-frequency signals

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4041381A (en) * 1974-10-09 1977-08-09 Lim Ching Hwa Methods and equipment for testing reflection points of transmission lines
WO1992011710A1 (fr) * 1990-12-20 1992-07-09 British Telecommunications Public Limited Company Systeme de telecommunication optique
JPH04225130A (ja) * 1990-12-27 1992-08-14 Anritsu Corp 光伝送装置
WO1993007687A1 (fr) * 1991-10-04 1993-04-15 British Telecommunications Public Limited Company Reseau avec systeme de surveillance de lignes
WO1996005665A1 (fr) * 1994-08-17 1996-02-22 Northern Telecom Limited Localisation d'erreurs dans des systemes de communication optiques

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE4220410C1 (de) * 1992-06-19 1993-11-25 Siemens Ag Verfahren zum Bestimmen eines Fehlers auf einer elektrischen Übertragungsleitung
US5369366A (en) * 1993-02-12 1994-11-29 Cable Repair Systems Corporation Method of finding faults in a branched electrical distribution circuit

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4041381A (en) * 1974-10-09 1977-08-09 Lim Ching Hwa Methods and equipment for testing reflection points of transmission lines
WO1992011710A1 (fr) * 1990-12-20 1992-07-09 British Telecommunications Public Limited Company Systeme de telecommunication optique
JPH04225130A (ja) * 1990-12-27 1992-08-14 Anritsu Corp 光伝送装置
WO1993007687A1 (fr) * 1991-10-04 1993-04-15 British Telecommunications Public Limited Company Reseau avec systeme de surveillance de lignes
WO1996005665A1 (fr) * 1994-08-17 1996-02-22 Northern Telecom Limited Localisation d'erreurs dans des systemes de communication optiques

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 016, no. 577 (P-1460), 16 December 1992 (1992-12-16) -& JP 04 225130 A (ANRITSU CORP), 14 August 1992 (1992-08-14) *
See also references of WO9938023A1 *

Also Published As

Publication number Publication date
WO1999038023A1 (fr) 1999-07-29
EP1051633A4 (fr) 2001-08-22
CA2315759A1 (fr) 1999-07-29
AU2338099A (en) 1999-08-09

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