The present invention relates generally to analysis of periodic signals and, more particularly, to a robust and accurate technique for measuring waveforms of periodic signals, for example, in connection with testing and evaluating electronic circuits.
Measuring periodic signals is an important part of a variety of technical fields including, for example, test instruments such as oscilloscopes and programmable test fixtures. In these technical fields, measuring periodic signals typically includes automatically determining various parameters of the received signal. Among others, these parameters include voltage measurements (e.g., high and low values, amplitude, minimum and maximum values, overshoot values), time measurements (e.g., period, pulse widths), and transient measurements (e.g., rise-times and fall-times).
Various parameters of a periodic waveform are defined in terms of high and low values (or magnitudes) of a power-related parameter, such as voltage or current. Oscilloscopes and computer-based signal-measurement systems typically analyze and characterize the rise and fall times of a periodic waveform in terms of high and low voltage values. The high-low values (H and L) are also useful in performing pulse width and duty cycle measurements. For realistic signals with finite transition slopes, pulse widths and duty cycles are typically only meaningful when considered relative to a reference level. Often, H and L are treated as the 100% and 0% levels, respectively, and the 50% level is often used to generate a reference level for time measurements. The high and low value measurements, therefore, are considered significant in defining the voltage measurements of the waveform. For pulse-shaped waveforms, the high and low values (a.k a., the xe2x80x9ctopxe2x80x9d and xe2x80x9cbasexe2x80x9d values) correspond to the values at the xe2x80x98plateauxe2x80x99 regions, or the steady-state values before the next transition.
There are two commonly used methods for defining the voltage measurements of periodic waveforms, and they are respectively used for pulse-shaped waveforms and non-pulse-shaped waveforms. The method used for pulse-shaped waveforms is called the histogram method. The histogram method attempts to discern the ringing and spikes by creating the amplitude histogram of the waveform and search for dominant magnitude populations. The min-max method, used for non-pulse-shaped waveforms, involves finding the minimum and maximum values of the waveform to determine L and H, respectively. The min-max method is suitable for many types of signals but when used for pulse-shaped waveforms, the method is sensitive to waveform ringing and spikes; consequently, the method falls short of accurately measuring parameters such as rise times, fall times and overshoots. In practice, many applications have need to characterize both pulse-shaped waveforms and non-pulse-shaped waveforms, and the user often has to switch manually between these two high-low value defining methods to obtain meaningful results.
For pulse-shaped waveforms, the high and low values form the cornerstone in voltage measurements, because they are used in the definition of many other parameters. For example, positive overshoot corresponds as follows
O+=(Maxxe2x88x92H)/(Hxe2x88x92L)xc3x97100%.
In rise time and fall time (tr and tf) measurements, H and L are treated as 100% and 0% level respectively. The 10%-to-90% tr is defined as the time for the waveform to traverse from the 10% to the 90% level. The results of the high and low voltage measurement are therefore not only important in their own rights, but provide a reference for time and transient measurement results as well.
Conventional algorithms, which are often based on IEC standards, are relatively simple and require relatively moderate processing power or memory. Some oscilloscopes allow the choice of either the min-max method or the histogram method in determining H and L, while others uses only the histogram method.
The histogram method attempts to determine H and L, while ignoring ringing and spikes. Precise implementations of this method vary, but a common aspect of the method involves the creation of an amplitude histogram of the waveform and search for dominant magnitude populations. According to one approach, a histogram is made with one bin for each digitizing level. The histogram is split into two sections at the halfway point between the minimum and maximum (also called the mid-point PM). The level with the most points in the upper histogram is the high value and the level with most points in the lower histogram is the low value. If PM gives the largest peak value within the upper or lower histogram, PM is returned as H or L accordingly. If two or more histogram bins have the same maximum values, the bin that is farthest from M is chosen. It has been reported that this algorithm may not work well for two-level waveforms with greater than about 100% overshoot.
FIG. 2 shows the histogram obtained for an example waveform sn in FIG. 1, where n is the sample number. The x-values for the two dominant peak clusters in the histogram determine the low and high values. FIG. 3 illustrates an expanded view of the region of FIG. 2 that corresponds to the peak cluster for the high value. Two peaks are roughly equal in magnitude, namely at 19.5 mV and 22 mV. These peaks correspond to the two flat regions in the waveform. The desired high value should be just before the next transition takes place, i.e., at 22 mV. This method used in determining the next transition could have easily picked up 19.5 mV instead of 22 mV.
For further information pertaining to the above and related approaches, reference may be made to the following documents: U.S. Pat. No.: 5,495,168, entitled, xe2x80x9cMethod of Signal Analysis Employing Histograms to Establish Stable, Scaled Displays In Oscilloscopes,xe2x80x9d European Patent Document EP0518116, entitled, xe2x80x9cMethod for Measuring the Peak Value of an Alternating Voltage;xe2x80x9d IEC 60469 Ed. 2.0b: 1987, xe2x80x9cPulse Techniques and Apparatus. Part 1: Pulse Terms and Definitions;xe2x80x9d IEC 60469 Ed. 2.0b: 1987, xe2x80x9cPulse Techniques and Apparatus. Part 1: Pulse Measurement and Analysis;xe2x80x9d and TDS684A, TDS744A and TDS784A, xe2x80x9cDigitizing Oscilloscopes User Manual 070-8991-02;xe2x80x9d and LeCroy Digital Oscilloscopes Operator""s Manual, 9350/54 Series.
Accordingly, there is a need for an improved approach to high-low magnitude measurement. Such an improved approach would be advantageous if it were more robust, had improved sensitivity when compared with known histogram methods, and possessed the added advantage of automatically adapting the high-low definitions to the signal at hand, without having to explicitly specifying whether or not the waveform is pulse-shaped.
The present invention is directed to analyzing the high and low values of a periodic waveform. Various example embodiments of the present invention are directed to systems and methods involving the use of a weighted histogram for analyzing high and low values in pulse-shaped waveforms as well as non-pulse-shaped waveforms. Traditionally, the histogram method is the method of choice for analyzing high and low values in pulse-shaped waveforms, whereas another method (the min-max method) is typically used for non-pulse-shaped waveforms. The present invention is advantageous in providing an accurate determination of the high and low values in both pulse-shaped and non-pulse-shaped waveforms and alleviating the need for a user to switch manually between the two approaches to obtain meaningful results. The present invention is exemplified through a number of implementations and applications, some of which are summarized below.
According to one aspect of the present invention, a system for analyzing a waveform of a signal having at least one period employs a processor adapted to generate a waveform histogram of the signal with weighted data samples, and to determine from the waveform histogram the high and low values in the waveform. Another aspect of the present invention is directed to a method involving the same approach.
In more specific applications, weighting the sampled data includes weighting sampled data with a weight W(n), where W(n) is a function of a plurality of subweights, W1(n), W2(n) and W3(n), with n corresponding to a sample count and each of W1(n), W2(n) and W3(n), respectively, representing a different weighting amount.
In an even more specific embodiment, the above-identified subweights are assigned as follows: subweight W1(n) corresponds to an amount that increases with the distance from a sampled-data region representing an immediately adjacent high slope; subweight W2(n) corresponds to an amount that decreases with additional samples; and subweight W3(n) corresponds to an amount that increases with an increased magnitude of the sampled data relative to a waveform median value.
Using the weighted histogram approach, embodiments of the present invention are more robust and have improved sensitivity when compared with the conventional histogram method. Embodiments of the present invention have the added advantage of automatically adapting the high-low definitions to the signal at hand, without explicitly specifying whether or not the waveform is pulse-shaped.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. For example, in other embodiments, various aspects of these embodiments are combined. The figures and detailed description which follow more particularly exemplify these embodiments.