(12) UK Patent Application (19) GB (11) 2 190 186 (13 )A
`
`(43) Application published 11 Nov 1987
`
`(21) Application No 8611405
`
`(22) Date of filing 9 May 1986
`
`(51) INT CL•
`G01N 21/17
`
`(71) Applicant
`Dr Jeremy Kenneth Arthur Everard,
`Electronic Engineering Department, King's College,
`London, Strand WC2R 2LS
`
`...
`
`(72) Inventor
`Dr Jeremy ·Kenneth Arthur Everard
`
`(74) Agent and/or Address for Service
`C. Haslar,
`Patent Dept, NRDC, 101 Newington Causeway, London
`SE1 6BU
`
`(52) Domestic classification (Edition I):
`G1A A6 A7 A9 C13 C1 C5 C6 ca C9 CD DM G10 G16 G17
`G18 G7 P10 P12 P16 P17 P18 P3 P6 P9 R7 S12 S4 T14
`T20 T22 T23 T24 T5
`H4D 265 72X 749 751 759 775 776 777 779 783 L SPS
`
`(56) Documents cited
`None
`
`(58) Field of search
`G1A
`H4D
`Selected US specifications from IPC sub-classes G01 N
`G01S
`
`(54) Greatly enhanced spatial
`detection of optical backscatter for
`sensor applications
`
`(57) A pseudo random bit sequence
`is amplitude modulated onto a light
`source (other types of modulation
`are discussed in the specification)
`and this modulated beam is
`transmitted down an optical fibre or
`other material and the detected
`backscatted signal is multiplied with
`a digitally delayed reference version
`of the transmitted sequence. By
`varying the delay between the
`transmitted and the reference
`sequence, spatial information can be
`recovered with improved signal to
`noise ratios compared to
`conventional Optical Time Domain
`Reflectometry. This enables
`improvements in the signal to noise
`ratio of the backscatter allowing
`reduced signal averaging times or
`reduced peak transmitted power.
`The measurement of the amplitude,
`frequency, phase and polarisation of
`the scatter can be used to
`characterise the properties of the
`material and also, to measure any
`external parameter on which the
`backscatter is dependent using
`video, and coherent detection.
`Enhanced fibre loss measurement
`techniques and enhanced fibre
`discontinuity measurement
`techniques are described. Distributed
`temperature sensors are described
`
`Continued overleaf ...
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`The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.
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`The claims were filed later than the filing date within the period prescribed by Rule 25(1) of the Patents Rules 1982.
`
`HALLIBURTON, Exh. 1004, p. 0001
`
`

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`HALLIBURTON, Exh. 1004, p. 0002
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`

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`GB2190186A
`
`SPECIFICATION
`
`Greatly enhanced spatial detection of optical backscatter for sensor applications
`
`5 The measurement of the spatially distributed backscatter of light from an optical fibre is often a
`problem due to the. fact that direct detection and signal averaging techniques are often required.
`The use of high power lasers can reduce the amount of signal averaging, however such lasers
`are usually bulky and very expensive. The measurement of such backscatter can be used to
`characterise the losses in the fibre (due to the Rayleigh losses) or in the measurement of
`10 distributed temperature sensors (Raman and Brillouin interactions).
`The measurement of the amplitude, spectra, phase and polarisation of the backscatter can also
`be used to characterise the properties of the material and the influence of any external para(cid:173)
`meters which influence the properties of the materials producing the backscatter. The measure(cid:173)
`ment of backscatter can therefore be used to measure any external measurand which influences
`15 the amplitude, phase, polarisation and frequency of the backscatter.
`At present to measure the spatial properties of the backscatter, Optical Time domain reflecto(cid:173)
`metry techniques, herafter defined as OTDR, are used in which a pulse is launched into the fibre
`and a photodetector, amplifier and sampling gate combination are used to measure the backscat(cid:173)
`ter.
`The time delay between the transmitted pulse and the sampling gate being fired, defines the
`slot in the fibre over which the backscatter is measured.
`The pulse width and sampling aperture define the spatial resolution. The signal is then aver(cid:173)
`aged to improve the signal to noise ratio. However the maximum sampling rate is fixed by the
`length of the fibre to ensure results free from ambiguity. In other words the backscatter from
`25 only one pulse should be sampled. The maximum repetition rate is therefore:
`
`20
`
`repmax= 1 /2cL
`
`45
`
`where c is the speed of light in the fibre
`30 and L is the length of the fibre.
`NB this assumes that multiple reflections from the fibre ends are negligible.
`Signal averaging will improve the s/n ratio by a ratio of the square root of the repetition rate.
`This is because the noise current in the photodetector or the ensuing load amplifier combination
`is usually proportional to the root of the bandwidth whereas the signal current is proportional to
`35 the optical signal power. Every time the signal is sampled and averaged the rms value of the
`noise current reduces by the square root of 2 and the signal stays the same. If the signal is
`integrated the signal component doubles and the noise increases by the square root of 2.
`The only ways to improve the signal to noise ratio for a given length of fibre and given
`resolution using OTDR is to increase the peak power in each pulse or to use a bank of narrow
`40 band harmonic filters tailored to pass the signal and reduce the noise. However a very large
`number of filters would be required. This would also limit the speed of response, however this
`technique should not be ignored as a satisfactory implementation ay be possible in future.
`FMCW can be used, however spectral analysis is required in the receiver after the detector. Also
`the light source needs to have a narrow spectral width for good spatial resolution.
`A more satisfactory way of improving the s/n ratio is to increase the average transmitted and
`hence received power in the time interval without causing ambiguity thus allowing more effective
`time integration.
`In this invention a pseudo random bit sequence (hereafter defined as PRBS) is amplitude
`modulated onto a light source (other types of modulation are discussed in other parts of this
`50 specification) and this modulated beam is transmitted down an optical fibre or any material and
`the detected backscatted signal is multiplied with a digitally delayed version of the transmitted
`sequence, herafter the transmitted sequence will be called the the reference PRBS.
`The Pseudo random bit sequence referred to in this patent means a pseudo random sequence
`of bits which appear to have a noise like spectra where the bit sequence is repeated after a
`55 specific number of bits and hence time interval. The sequences may consist of binary (on and
`off pulses) Fig. 1. or multiple level pulses (for example -1,0, + 1 Fig. 2.). The pseudo random
`sequence can also be multilevel with levels from two to infinity. The pseudo random sequence is
`also designed to have specific autocorrelation properties. The number of bits in the pseudo
`random sequence before the sequence repeats and the time taken before the sequence starts to
`60 repeat (hereafter called the sequence repeat time) can be varied according to the specifications
`of the sensor system.
`Spatial information is obtained by multiplying the detected backscattered signal with a delayed
`version of the pseudo random bit sequence, the delay being implemented digitally. By varying
`the delay the backscatter from different points can be measured. The delay can also be varied
`65 using analogue techniques (for example a delay line).
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`HALLIBURTON, Exh. 1004, p. 0011
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`In the Pseudo random bit sequences the sequence repeat time is equivalent to the repmax of
`the OTDR case and the bit length is equivalent to the sampling aperture. The bit length also
`defines the spatial resolution.
`This allows the average power transmitted and received to be increased by having many bits
`5 in a sequence allowing more effective use of signal averaging.
`The extra signal power occupies the full noise bandwidth. This technique can be thought of in
`terms of sampling in that the ambiguity due to the increase in sampling rate is removed by
`arranging for the unwanted sampled terms to average out to be approximately zero by designing
`the pseudo random bit sequences to have specific autocorrelation properties.
`Pseudo random sequences can be designed with a varierty of properties. By using this
`technique with a maximally flat pseudo random bit sequence a specific autocorrelation function
`can be produced where the peak occurs when the delay between the transmitted and received
`sequence is zero. If this peak is normalised to 1 then the correlation between the two pseudo
`random sequences when the delay is not zero is constant at -1 /n where n is the number of
`15 bits before the sequence repeats. This assumes that the integration is performed over one
`complete sequence and that the peak of the autocorrelation function was normalised to 1. Fig.
`3.
`Pseudo random sequences with different autocorrelation functions can also be used.
`The sampling rate is now therefore increased to the bit rate which is now 1 /(bit width in
`20 time). For a given bandwidth photodiode (hence resolution) the bit rate can be increased until
`inter symbol interference becomes a problem which allows a s/n improvement over conventional
`OTDR of approximately between:
`
`SQUARE ROOT OF ((Bit width in time)/(sampling rate of conventional OTDR)).
`
`25
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`and
`
`(The bit width in time)/(sampling rate of conventional OTDR)
`
`30
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`The exact improvement in the signal to noise ratio is dependent on the correlation between
`the bandwidth limited photo-detector noise and the bandwidth limited pseudo random bit se-
`quence as well as the detector integration and averaging times.
`The integration time for optimum performance should be one or N times (where N is a positive
`integer) the sequence repeat time where the sequence repeat time is defined as the time after
`35 which the bit sequence starts to repeat. There is usually no advantage in integrating fro more
`than one sequence repeat time.
`For the same peak transmitted power, the average power transmitted and received using
`pseudo random bit sequences can now be increased to approximately half the peak power
`transmitted in the OTDR case. In conventional OTDR the average power transmitted was:
`
`@
`
`(Peak power) . (repetition rate/bit length)
`
`The invention can be used in a number of ways:
`
`50
`
`45 SECTION A
`Video detection (In this case video detection means that the backscattered signal is directly
`incident on the Photodiode) where the signal out of the photo-diode is multiplied by an identical
`suitably delayed pseudo random bit sequence, the delay being produced using digital techniques.
`The system is shown in Fig. 4.
`A digital pseudo random generator is built using digital circuits ( 1 ). The output of ( 1) is
`amplitude modulated onto a laser (2). The light out of the laser (2) is coupled into an optical
`fibre (3) via a beam splitter (4) and a lens (5). The backscattered signal from the fibre is
`collimated by the lens (5) and deflected by the beam splitter (4) via a lens (7) onto the
`photodetector (6). The electrical output of the photo-detector (6) is amplified in an amplifier (8)
`55 and then multiplied in a multiplier (9) with a time delayed version of the original pseudo random
`sequence (1) using digital circuits (10, 11) where (10) is a delay circuit and (11) is another PRBS
`generator. The digital circuits (1, 10, 11) could be combined in one circuit and the delayed PRBS
`sequence can be produced in a number of ways. A typical circuit for the pseudo random
`generators ( 1, 10, 11) is shown in Fig. 5 where this circuit scans through the delay increasing the
`60 delay by one bit every time the pseudo random sequence starts to repeat. This is achieved
`using two identical PRBS generators fed by clock pulses where the clock pulses applied to one
`of the PRBS generators drops one pulse every time the pseudo random bit sequence starts to
`repeat. Another technique would be to detect the end of the sequence in each generator by
`looking at the states of the shift registers used to generate the pseudo random bit sequences
`65 and setting the delay using logic controlled either by a computer or by hardwired counters and
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`HALLIBURTON, Exh. 1004, p. 0012
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`GB2190186A
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`logic controlled by switches.
`The output from the the multiplier (9) is amplified ( 12) and then integrated or averaged ( 13)
`over N times the sequence repeat time where N is a positive integer. There is usually no
`advantage in integrating over more than one complete sequence repeat time. The information is
`5 then displayed ( 14) or processed and stored. The signal can be averaged over different times
`however the dynamic range would be reduced. By varying the delay between the pseudo
`random generators ( 1, 11) and observing the amplitude of the signal out of the integrator a
`picture showing the variation of backscatter with distance can be built up. The dynamic range is
`improved by using longer sequences, in other words a larger number of bits before the se-
`10 quence repeats.
`As the optical backscatter from the fibre is distributed all along the fibre then each bit not on
`the peak of the correlation will be multiplied by -1 /n. All the unwanted bits of backscatter will
`be summed together causing a large component of signal which could be similar in value to the
`wanted signal. The number of bits in the pseudo random bit sequence before it repeats in
`15 conjunction with the sequence repeat time sets the spatial resolution. The large unwanted
`component can be reduced by arranging for the pseudo random sequence repeat time to be
`considerably longer than the round trip time of the fibre. Another method would be chop the
`signal and subtract the value when there is no correlation at any point in the fibre from the value
`when the correlation peak is at the wanted position. The amplitude of the backscatter when
`20 there is no correlation in the fibre could be produced by adjusting the delay between the two
`PRBS generators to be less than the round trip propagation time of the laser beam between the
`laser and the input of the fibre or by arranging for the delay to be longer than the round trip
`propagation time of the fibre.
`This system can be used as it stands to measure the Rayleigh backscatter and the loss and
`25 discontinuities along a fibre.
`By looking at the spectral properties of the backscatter using optical bandpass (or lowpass or
`highpass) filters placed between the beam splitter (4) and the lens (7) or detector (6) of the
`system shown in Fig. 4, the Brillouin and Raman lines and other spectra can be measured. The
`spectra and amplitude of these lines can be used to give direct measurement of temperature
`30 along the fibre. It can also be used to measure any external parameter on which the backscatter
`is dependent. The system is shown in Fig. 6. where the only change from Fig. 4. is the filter or
`filters or polarisation detectors or modifiers ( 15). The Raman and Brillouin backscatter are caused
`by acousto-optic interactions between the input optical wave and the phonon waves in the fibre.
`These interactions produce sidebands on either side of the optical beam, the amplitude and
`35 spectrum of which are frequency dependent.
`The spatial temperature distribution can be derived by measuring the amplitude of the Brillouin
`or Raman Stokes line (the lower frequency sideband) or by measuring the Brillouin or Raman
`antistokes line (the upper frequency sideband) or by measuring the ratio of the amplitudes of the
`Stokes to the Antistokes lines at the same offset frequency using the techniques described in
`40 section A and Fig. 6. By measuring the ratio of the stokes and antistokes line by either
`switching filters or using two optical paths the optical cross section can be removed from the
`equations. Calibration can be enhanced by putting parts of the fibre in known temperature
`regions. Enhanced temperature measurement can also be made by launching optical signals into
`both ends of the fibre and thus obtaining a measurement from either end of the fibre.
`A temperature sensor could also be made by measuring either the Stokes line or the antis-
`tokes line and taking a ratio of one of these lines to the Rayleigh backscatter to normalise out
`the losses in the fibre.
`Spatial Temperature sensors can also be built using the pseudo random modulated optical
`beam by using a fibre with absorbtion edges which move in frequency with temperature. The
`50 amplitude of the back scattered Rayleigh light would then be dependent on the temperature at
`specific points along the fibre. The signal can be further improved by launching two pseudo
`random modulated beams into the fibre each at a different optical frequency. The frequency of
`one beam should be arranged to be varying with temperature by being on the absorbtion edge.
`The other beam should be arranged to be away from the absorbtion edge to calibrate out
`55 varying losses in the fibre and pulse to pulse variation. In this invention absorbtion edge means
`an optical frequency where the absorbtion coefficient alpha in the fibre varies with small changes
`in optical frequency.
`The absorption edge could be either due to the materials within the fibre or could be
`absorbing materials placed in line with the fibre at specific intervals along the fibre.
`The coherent detection schemes described in Section B and C could also be used to measure
`the spatial temperature distribution described above.
`Having detected the backscatter using the system described in section A the amplitude,
`spectra, phase and polarisation of the signal can be measured and spatially resolved at different
`points along the fibre by using filters or polarisation detectors or modifiers or by using phase
`65 modulators in the path between the laser and the beam splitter or the path between the beam
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`HALLIBURTON, Exh. 1004, p. 0013
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`4
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`GB2190186A
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`4
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`splitter and the photo-detector.
`Therefore the use of the pseudo random sequence modulated signal can be used to obtain
`spatial information on the amplitude, spectra, phase and polarisation of the backscatter and
`hence any external measurand on which the amplitude, spectra, phase and polarisation of the
`5 backscatter are dependent. The invention is therefore useful for obtaining spatial information on
`any external measurand which affects amplitude, spectrum, phase and polaristion of the back-
`scatter.
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`SECTION B
`10 Coherent detection (In this case coherent detection means that the optical backscatter is
`incident on the photo-detector with an optical local oscillator. A constant local oscillator signal
`similar to the transmitted optical signal without any modulation on it is applied with the optical
`backscattered light from the fibre to the input of the photodetector.
`The system is shown in Fig. 7.
`A digital pseudo random generator is built using digital circuits (1). The output of (1) is
`amplitude modulated onto a laser (2). Other forms of modulation can be used as described
`elsewhere in this specification. The light out of the laser (2) is coupled into an optical fibre (3)
`via a beam splitter (4). The backscattered signal from the fibre is deflected by the beam splitter
`(4) via a lens (7) onto the photodetector (6). An optical local oscillator beam (15) is derived
`20 from the other reflection from the beam splitter (4) and by using a mirror ( 17), the optical local
`oscillator beam is arranged to be incident onto the photo-detector (6) via a lens (7) simultane-
`ously with the optical backscatter. The electrical output of the photo-detector (6) is then
`amplified, if necessary, in an amplifier (8). The output of the amplifier (8) or the photo-detector
`(6) is then power detected (16) or demodulated if other modulation schemes are used.
`To obtain spatial information the demodulated signal is then multiplied in a multiplier (9) with a
`time delayed version of the original pseudo random sequence { 1) using digital circuits { 10, 11)
`where (10) is a delay circuit and (11) is another PRBS generator. The digital circuits (1,10,11)
`could be combined in one circuit and the delayed PRBS sequence can be produced in a number
`of ways. A typical circuit for the pseudo random generators (1, 10, 11) is shown in Fig. 5 where
`30 this circuit scans through the delay increasing the delay by one bit every time the pseudo
`random sequence starts to repeat. This is achieved using two identical PRBS generators fed by
`clock pulses where the clock pulses applied to one of the PRBS generators drops one pulse
`every time the pseudo random sequence starts to repeat. The output from the the multiplier (9)
`is amplified ( 12) and then integrated or averaged ( 13) over N times the sequence repeat time
`35 where N is a positive integer. There is usually no advantage in integrating over more than one
`complete sequence repeat time. The information is then displayed { 14) or processed and stored.
`The signal can be averaged over different times however the dynamic range would be reduced.
`By varying the delay between the pseudo random generators ( 1, 11) and observing the amplitude
`of the signal out of the integrator a picture showing the variation of backscatter with distance
`40 can be built up. The dynamic range is improved by using loner sequences, in order words a
`larger number of bits before the sequence repeats.
`As the optical backscatter from the fibre is distributed all along the fibre then each bit not on
`the peak of the correlation will be multiplied by -1 /n. All the unwanted bits of backscatter will
`be summed together causing a large component of signal which could be similar in value to the
`45 wanted signal. This problem can be overcome by arranging for the pseudo random sequence
`repeat time to be considerably longer {for example one hundred times) than the round trip time
`of the fibre. Another method would be chop the signal and subtract the value when there is no
`correlation at any point in the fibre from the value when the correlation peak is at the wanted
`position.
`The amplitude of the backscatter when there is no correlation in the fibre could be produced
`by adjusting the delay between the two PRBS generators to be less than the round trip
`propagation time of the laser beam between the laser and the input of the fibre or by arranging
`for the delay to be longer than the the round trip propagation time of the fibre.
`This technique can be used directly as it stands to measure the loss and Rayleigh backscatter
`55 and discontinuites along the fibre.
`To ensure that the phase of the optical local oscillator and detected signal are not in quadra(cid:173)
`ture {causing zero output) a phase wobulator can be incorporated in one of the optical paths
`between (2) and (4) or 2 detectors could be used. The phase wobulator consists of an optical
`device which modulates the phase by an angle around 90 degrees. If two detectors are used
`60 each detector has the backscattered signal combined with a local oscillators applied to it, where
`the phase of each local oscillator is in quadrature.
`To remove problems of DC offset in the detector and multiplier circuits the optical or elec(cid:173)
`tronic signal should be chopped at say 1 KHz, the chopping frequency preferably being above
`the flicker noise corner of the ensuing amplifier. The signals should be coupled into the multiplier
`65 in such a way that the voltage excursions are equally above and below DC. DC offsets larger
`
`50
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`HALLIBURTON, Exh. 1004, p. 0014
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`than the received signals would produce incorrect correlation functions. The phase wobulator can
`also· be used in place of the chopper.
`The technique described in section B can also be used to measure temperature by measuring
`the amplitude and spectra of the Brillouin backscatter from the fibre using coherent and hetero-
`5 dyne detection and performing the filtering at microwave frequencies Fig. 8. The backscatter
`would be detected coherently by mixing the backscattered signal with a part of the input beam
`on a photo-detector. The detected microwave signal would then be mixed with a microwave
`local oscillator to produce a signal around DC.
`The Brillouin backscatter consists of sidebands on either side of the carrier which are tempera-
`10 tu re dependent. The backscatter obeys the Bose Einstein thermal factor shown below where the
`Stokes line (lower frequency sideband) and the antistokes line (higher frequency sideband) give
`different results due to quantum effects.
`
`l.=la1 (1 +NIT)
`Stokes
`
`15
`
`I •• = la1(N1T)
`Anti Stokes
`
`Where
`
`where NIT
`
`20
`
`exp
`
`hf)
`
`kT
`
`-1
`
`is the incident optical intensity
`k is Boltzmann's constant
`T is temperature (°K)
`h is Planck's constant
`f
`is the offset frequency
`a1 is the frequency dependent cross section
`
`For temperatures around room temperature and for an offset of around 30 GHz the Bose
`25 Einstein part of the relationship simplifiers to KT /hf as hf/KT is considerably lower than one.
`(hf /KT=7T). This also means that the Stokes and anti-stokes lines are similar in amplitude at
`room temperature.
`In the system described above the sum of the two lines will be detected as they will overlay
`on each other in the frequency domain. This is because a single mixer cannot distinguish
`30 between positive and negative frequencies. To separate the stokes and antistokes lines to
`enable temperature measurement using the ratio of the Stokes line to the anti-stokes line or to
`look at only one of the lines or to measure the ratio of the Stokes or anti-stokes lines to the
`Rayleigh backscatter lines, the lines need to be separated in the frequency domain using an
`optical frequency shifter (for example a bragg modulator). The frequency shifter can be used to
`35 insert a frequency shift in the reference optical beam to the mixer. This allows a controllable
`separation between the two lines as well as the Rayleigh line. The system is shown in Fig. 8.
`A digital pseudo random generator is built using digital circuits ( 1). The output of ( 1) is
`amplitude modulated onto a laser (2). Other forms of modulation are described elsewhere in this
`specification. The light out of the laser (2) is coupled into an optical fibre (3) via beam splitters
`40 (4) and (5) and a lens (6). These two beam splitters (4) and (5) are used to derive a frequency
`shifted optical local oscillator (for example using a Bragg modulator (7) to enable separation of
`the Brillouin Stokes and Anti-stokes lines. If it is not necessary to separate the Stokes and anti(cid:173)
`stokes lines the beam splitter (4) and frequency shifter (7) can be left out and the optical local
`oscillator can be derived using a mirror as shown in Fig. (7). The backscattered signal from the
`45 fibre is deflected by the beam splitter (5) via a lens (8) onto the photodetector (9). The
`frequency shifted optical local oscillator beam ( 10) is derived from the other reflection from the
`beam splitter (4) and this is arranged to be incident onto the photo-detector (9) via a lens (8)
`simultaneously with the optical backscatter. The electrical output of the photo-detector (9) is
`then amplified, if necessary or possible, in an amplifier ( 11). The output of the amplifier ( 11) or
`50 the photo-detector (9) is then applied to an RF mixer ( 12) with the output of an RF local
`oscillator ( 13) which is oscillating at a frequency similar to the frequency of the detected Stokes
`and Anti-stokes lines causing the Stokes and Antistokes lines to be downconverted to frequen-
`cies lower than a few Gigahertz where signal processing is easier.
`The two lines are then filtered and power detected ( 14) or demodulated if other modulation
`55 schemes are used as described elsewhere in this specification.
`If it is necessary to remove problems of DC offset in the detector and multiplier circuits the
`optical or electronic signal could be chopped at say 1 KHz, the chopping frequency preferably
`being above the flicker noise corner of the ensuing amplifier. The signals should be coupled into
`the multiplier in such a way that the voltage excursions are equally above and below DC. DC
`60 offsets larger than the received signals would produce incorrect correlation functions. The
`chopped output could be detected using a tuned amplifier tuned to the chopping frequency and
`a power detector or by using a Lock-in amplifier.
`To obtain spatial information about the spectral line required the demodulated signal is then
`multiplied in a multiplier ( 15) with a time delayed version of the original pseudo random se-
`65 quence ( 1) using digital circuits ( 16, 17) where ( 16) is a delay circuit and ( 17) is another PRBS
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`HALLIBURTON, Exh. 1004, p. 0015
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`

`

`...
`
`!'
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`GB2190186A
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`6
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`generator. The digital circuits (1, 16, 17) could be combined in one circuit and the delayed PRBS
`sequence can be produced in a number of ways. A typical circuit for the pseudo random
`generators ( 1, 16, 17) is shown in Fig. 5 where this circuit scans through the delay increasing the
`delay by one bit every time the sequence starts to repeat. This is achieved using two identical
`5 PRBS generators fed by clock pulses where the clock pulses applied to one of the PRBS
`generators drops one pulse every time the sequence starts to repeat. (The