Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-04-10
2004-11-09
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06816242
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of digital signal processing, and more particularly to the use of Gaussian pulses to perform Time Domain Reflectometry to locate and characterize faults in a Device Under Test.
DESCRIPTION OF THE RELATED ART
In the field of digital signal processing there is often a need to detect a signal pulse in a transmission medium, such as an electrical cable or optical fiber, or an electronic or optical device. For example, in electronic or optical systems, when a signal encounters a change in impedance in the transmission medium, a partial reflectance of the signal occurs, decreasing the amplitude of the propagated signal. These impedance changes can substantially degrade the performance of a device or transmission medium, and so a great deal of effort has been spent detecting and analyzing such interconnect discontinuities.
One technique which has been used to great effect in this area is Time Domain Reflectometry (TDR). TDR involves measuring reflection in an unknown device or medium in comparison to reflection due to a standard impedance. In other words, TDR compares reflected energy to incident energy on a single-line transmission system. Known incident stimulus is applied to the standard impedance and intentionally propagated toward the unknown device or medium. Reflections from the unknown device or medium then propagate back to the source, and amplitude and time of the reflected signal(s) is compared to the incident stimulus. The reflected signal magnitude (and possibly the waveform) is a function of the incident signal magnitude and the nature of the impedance change. The time elapsed between the detection of the incident and the reflected signal is a function of the overall distance traveled and the velocity of propagation of the signal. Thus, by detecting and timing the reflected pulse with respect to the original pulse, the distance to an interconnect discontinuity may be determined. In this way, for example, a flaw, such as a kink, may be located precisely in the device or medium.
Fast sampling systems with TDR capability can be very useful for studying interconnect discontinuities. In a typical TDR system, a step generator, such as a switched current source, is used to generate a step pulse, which is propagated through a T-connector to a digitizer and the device under test (DUT) respectively. The reflected signal (from the DUT) is received by the digitizer and the superpositioned waveforms analyzed to determine the elapsed time, and to compare amplitude and waveform of the pulses.
However, there are a number of drawbacks to this approach. The use of a step function or signal is problematic because the steep rise of the leading edge of the step entails many high frequency signal components. In other words, the step signal is a wide bandwidth pulse. Transmission media and electronic and optical components behave differently for different frequencies, leading to such distorting effects as dispersion and dissipation of the pulse energy. The effect of dispersion is particularly troublesome for step pulses in TDR applications because the leading edge of the step is used to determine the timing of the pulse. The dispersion of high frequency components in the edge can smooth the edge to such a degree that precise timing of the reflected pulse may be difficult or even impossible.
Current systems developed to perform TDR with step pulses involve extremely high speed sampling and tend to be very expensive, costing on the order of one hundred thousand dollars each. The use of modulated Gaussian pulses could avoid many of the problems associated with step pulses in that Gaussian pulses are extremely smooth, have low bandwidth, and are particularly suitable for closed form analytic operations and representations. However, there are currently no known systems or methods to perform TDR with Gaussian pulses, due to difficulties in accurately detecting, timing, and characterizing Gaussian pulses in a noisy medium.
Therefore, systems and methods are desired to detect and characterize modulated Gaussian pulses in a noisy medium.
SUMMARY OF THE INVENTION
A system and method for detecting and characterizing Gaussian pulses is presented. A system and method for performing Time Domain Reflectometry (TDR) using Gaussian pulses is also described. The system may include a computer, comprising a CPU and a memory, wherein the memory is operable to store one or more software programs for performing TDR, and wherein the CPU is operable to execute the software programs. The system may also include an arbitrary waveform generator (AWG) coupled to the computer, and a digitizer coupled to the computer and the AWG. The system may be coupled to a Device or Medium, hereafter referred to as a Device Under Test (DUT). It should be noted that the DUT may be any kind of device, including a stand alone device, a PC board, an instrument, an electric or optical circuit, or an electronic or optical transmission medium, among others.
In the preferred embodiment, the computer system comprises a PCI eXtensions for Instrumentation (PXI) system which includes one or more PXI computer boards or cards plugged into a PXI backplane, such as a “PC on a card”, housed in a PXI chassis. In other words, the PXI cards may comprise the memory and CPU which are operable to respectively store and execute one or more computer software programs implementing the present invention. The PXI system may also include or couple to a display, such as a monitor, for displaying visual information, such as results, to a user, as well as an I/O interface for receiving input and sending output to external systems or components. In one embodiment, the display may be comprised in the PXI chassis. In another embodiment, the display may be external to the PXI chassis. In one embodiment, the I/O interface may also be comprised on a PXI card.
The AWG may be operable to generate a Gaussian pulse and transmit the Gaussian pulse to the digitizer and to the Device Under Test (DUT). The DUT may be further operable to reflect at least a portion of the transmitted Gaussian pulse in the form of one or more reflected pulses to the digitizer. A digital signal containing a modulated Gaussian pulse and signal noise may be received, such as from the digitizer, from some other external system, or from the memory medium of the computer system. The signal may also include one or more reflected pulses. For example, in the TDR system described above, the AWG may generate a Gaussian pulse and transmit the Gaussian pulse to the digitizer and the Device Under Test (DUT). The DUT may reflect at least a portion of the transmitted Gaussian pulse to the digitizer, comprising the one or more reflected Gaussian pulses. The digitizer may receive and digitize the signal comprising the transmitted Gaussian pulse and the one or more reflected Gaussian pulses, and store the digitized signal in the memory.
An estimation of N Gaussian pulse parameters may be determined for a Gaussian pulse comprised in the signal, where N is greater than or equal to one. The Gaussian pulse parameters may include &agr;
p
(the inverse of the Gaussian pulse variance), t
p
(the Gaussian pulse time shift), and &ohgr;
p
(the Gaussian pulse carrier frequency or base frequency). The pulse variance is related to the width of the Gaussian pulse. The pulse time shift is a measure of the time interval from some arbitrary origin to the peak of the Gaussian pulse. The pulse carrier frequency refers to the frequency of the modulation of the Gaussian pulse. In one embodiment, the estimation of Gaussian pulse parameters may be determined using any of various prior art techniques, such as the zoom-in approach, for example, which is well known in the art.
Once the estimation of the Gaussian pulse parameters is determined, a plurality of permutations of the estimated Gaussian Pulse parameters may be generated by adding or subtracting a small delta amount to or from each parameter value, producing a plurality of parameter sets representing a corres
Qian Shie
Zhang Nanxiong
Hood Jeffrey C.
Meyertons Hood Kivlin Kowert & Goetzel P.C.
National Instruments Corporation
Nguyen Tu T.
Williams Mark S.
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