Pulse or digital communications – Testing
Reexamination Certificate
2000-09-29
2004-09-28
Phu, Phuong (Department: 2631)
Pulse or digital communications
Testing
C375S260000, C714S714000
Reexamination Certificate
active
06798830
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to generating test signals for verifying communications devices, and more particularly to creating multi-tone test signals for missing tone tests of different types of Digital Subscriber Line (xDSL) devices to allow easy and flexible testing of such devices.
BACKGROUND OF THE INVENTION
Generally, xDSL devices, and in particular ADSL devices, convey signals over a bandwidth that ranges from 25 kHz to 1.2 MHz. This bandwidth is divided into different bands or channels, each of which is typically 4.3125 kHz wide. For example, an ADSL signal includes approximately 250 different bands, each 4.3125 kHz wide. To convey data, an xDSL modem provides a different carrier frequency for each of its bands, and provides modulated data around each of the carriers.
One test for determining proper operation of an xDSL device, and in particular an ADSL device, is called a “missing tone” test. The missing tone test determines whether a component (e.g., a circuit such as an application-specific integrated circuit or ASIC) within the xDSL device (e.g., an ADSL modem) introduces distortion into the output of one or more of the channels or frequency bands processed by the device during device operation. To perform a missing tone test, an ATE (automatic test equipment) system generates a broad band test signal and applies it to the input of an xDSL device. In response, the device generates an output. The ATE system samples the output and performs a Fast Fourier Transform or FFT of the output signal. During the test, the ATE system systematically removes individual tones (i.e., bands or channels) from the input waveform (i.e., the test signal), and evaluates the resulting FFTs after an xDSL device processes such a test signal. If the removed tones do not appear in the FFTs (i.e., the FFTs have only low level power components at the missing tone locations) then the test passes. In other words, if the broad band test signal that includes one or more missing bands is applied to the xDSL device and the device produces an output signal that contains little or no output at the location of the missing bands, then the test operator may consider the xDSL device as having passed the test (i.e., is functioning properly) since the device introduced little or no distortion into the original broadband test signal. However, if large components or distortion appear at the missing tone locations (e.g., removed bands or channels), the test operator can consider the xDSL device as having failed the missing tone test since a significant component at a missing tone location indicates that circuitry within the xDSL device has introduced distortion into its output at the missing tone locations.
“Crest Factor” (also known as “PAR” or Peak-to-Average Ratio) is a particularly important characteristic of xDSL signals. Crest factor is defined as the maximum amplitude of a test signal, divided by the average (i.e., Route Mean Squared value) of the signal, or:
Crest Factor=Peak Signal/Average Signal.
For performing missing tone tests, manufacturers desire to specify a particular crest factor of the test signal. Then, they are able to claim that their part has a particular distortion or lack thereof (measured using missing tone tests) at a particular crest factor.
SUMMARY OF THE INVENTION
It is often exceedingly difficult, using conventional techniques, to maintain a desired crest factor of a test signal as tones (i.e., carrier signal frequencies) are systematically removed for the purpose of performing missing tone tests.
Frequently, in conventional testing techniques, the removal of a tone signal from a test signal significantly changes the crest factor of the resulting test signal. This is because the test signal is typically a composite or summation of sine or cosine waves with coinciding peaks that form a peak value in the test signal over a short duration of the overall period of the test signal. Accordingly, removal of one tone signal can significantly change the peak value and the average value of the resulting composite test signal. For example, removal of a tone signal (e.g., a carrier frequency) from a composite signal formed by ten tone signals (therefore leaving only nine remaining tone signals) may reduce the peak value by approximately ten percent and may also therefore affect the average value of the test signal.
To maintain a constant crest factor of a test signal, test developers using conventional techniques customarily perform the trial and error task of manually and repetitively adjusting the phases of the remaining tones in the test signal, and re-measuring the resulting crest factor of that test signal. This manual and repetitive process can consume significant time. As a test developer removes each tone from the broad band test signal for an xDSL device, in conventional systems, the developer must “re-balance” the phases of remaining tones to re-achieve the desired crest factor of the test signal.
The present invention significantly overcomes these and other problems associated with conventional signal generation and device testing techniques used for testing xDSL or other types of data communication devices.
More specifically, according to embodiments of the invention, a method is provided for generating a test signal. Preferably, the test signal is used as a test signal for xDSL devices under test. The method comprises the steps of selecting a set of frequencies (e.g., a range or ranges of tones between a start and stop frequency) for the test signal and selecting frequency sub-groups from the set of frequencies. Next, the method generates a respective sub-group composite signal for each frequency sub-group selected from the set of frequencies and then time shifts each respective sub-group composite signal in relation to other sub-group composite signals. Finally, the method generates the test signal by summing each respective time shifted sub-group composite signal to produce the test signal.
In this manner, sub-group composite signals each include a respective peak which when combined with other sub-group composite signals are spread out (e.g., time-shifted or delayed) across the test signal. During missing tone tests, if a tone frequency is removed from a sub-group of frequencies, the sub-group composite signal corresponding to that frequency may be affected, but the other sub-group composite signals will remain largely unaffected by the missing tone frequency. Since the other sub-group composite signals contain peaks at the desired value (as explained below), the test signal as a whole is less affected by the missing tone. Accordingly, removal of a signal from one of the sub-groups has a small effect on the average value and generally little or no effect on the peak value of the resulting test signal.
In another embodiment of the invention, the step of selecting a set of frequencies for the test signal includes the steps of determining at least one start frequency and at least one stop frequency defining-the set of N frequencies to be included in the test signal. The method embodiments can determine start and stop frequencies, for example, by obtaining such frequencies from a test developer (e.g., a person) that controls the signal generator configured to carry out the method embodiments of the invention. The method also includes the step of determining any intermediate frequencies to be included in the test signal between the at least one start frequency and the at least one stop frequency that occur at frequency intervals equal to a desired tone spacing of frequencies for the test signal. Such intermediate frequencies include all frequencies between the start and stop frequency, or, may include a discontinuous range or frequencies. The range may be discontinuous due to a test developer selecting missing tone frequencies to be omitted from the set of frequencies used for the test signal. In this manner, a selection of missing tone frequencies that the test developer provides between the start and stop frequencies defines the remaini
Chapin & Huang LLC
Chapin, Esq. Barry W.
Phu Phuong
Teradyne, Inc.
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