The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "A digital oscilloscope is a tool utilized by engineers to view signals in electronic circuitry. As circuits and signals get ever faster, it is beneficial to have digital oscilloscopes capable of digitizing, displaying and analyzing these faster signals. The capability of a digital oscilloscope to digitize fast signals may be measured by its bandwidth and sample rate. The sample rate is the number of samples points taken of a waveform in a given amount of time and is inversely proportional to the sample period--the time between samples. If a sinusoidal frequency sweep is performed from DC up to higher frequencies, the bandwidth is the frequency at which the signal displayed on the digital oscilloscope screen is approximately 30% smaller than the input sine wave.
"Since one of the uses of the digital oscilloscope is to design and analyze new electronic devices, high end digital oscilloscopes generally operate at speeds much higher than the present state of the art in electronics. These speeds may be achieved through the use of ever-faster sampling chips or the use of alternate methodologies to provide the desired bandwidth.
"One such method involves triggering repeatedly on a periodic event. If an event is periodically repeating data obtained from multiple trigger events can be assembled together to provide a good view of the waveform. More particularly, the scope may repeatedly trigger on an event and acquire only a few points of the waveform (sometimes only one point of the waveform) on each trigger event. Scopes having this functionality are sometimes called 'sampling scopes.' After repeated triggers, the points are reassembled according to the sampling algorithm to create a higher 'effective' sample rate version of the waveform. Furthermore, the repeated trigger events permit averaging, which can be utilized to increase the signal-to-noise ratio (SNR) and therefore enable further bandwidth increases. However, such a sampling scope presupposes a repetitive input signal so that the representation of the waveform can be generated over many triggers.
"This technique may be unsuitable where the signal that is to be analyzed is not repetitive. For instance, the user of the oscilloscope may want to capture a non-repetitive event such as the cause of some failure in an electronic system. The trigger event may happen repeatedly but the signal around the trigger event may be different. Therefore, it is desirable to achieve a high bandwidth and sample rate with only a single trigger event. Such digital oscilloscopes are sometimes called real-time scopes, and acquisitions taken utilizing only a single trigger event are called single-shot acquisitions.
"In real-time digital oscilloscope design the required sample rate of the sampling system is a function of the bandwidth of the analog signal to be acquired. In order to accurately represent the signal the sample rate of the sampling system should be at least twice that of the highest frequency being digitized. This is often called the 'Nyquist rate.'
"One method for improving sample rate is time interleaving. This method utilizes multiple digitizing elements that sample the same waveform at different points in time such that the waveform resulting from combining the waveforms acquired on these multiple digitizers forms a high sample rate acquisition. For example, in a system having a two analog-to-digital converters, or ADCs, the first ADC samples the signal, then the second ADC samples the signal, then the first and so on. The digital output of the ADCs may then be multiplexed or otherwise combined to yield a composite digital corresponding to the analog input signal. Use of interleaving accordingly eases the speed requirements of each of the individual ADCs.
"Use of interleaving in digital oscilloscopes may accordingly provide the significant advantage of increasing the effective bandwidth of the oscilloscope. With a given set of ADCs, a substantially higher sample rate may be achieved with the use of interleaving. Increasing the sample rate correspondingly increases the maximum frequency that may be sampled by the system, which is commonly called the 'bandwidth' of the oscilloscope. The term bandwidth actually refers to a frequency range rather than an upper limit. The lower end of the range is generally understood to be mound 0 Hz for an oscilloscope, so the nominal bandwidth of an oscilloscope generally corresponds to the maximum frequency that can be sampled by the system. Thus, a two-fold increase in sample rate can provide around a two-fold increase in oscilloscope bandwidth.
"Where interleaving is employed the timing relationship, gain, and offset of each digitizing element is usually matched. When digitizers are mismatched in these characteristics the accuracy of the digitized waveform is compromised.
"One symptom of mismatched digitizers is error signal generation. A specific type of error signal is an artifact signal created by errors in the interleaving process. One common artifact signal is a spurious tone. When multiple digitizers work in an interleaved configuration to digitize a waveform and a single tone is applied to the system, multiple tones result. The frequency location of the spurious tones is determined by the input frequency and the number of digitizers employed. The magnitude and phase of the spurious tones is determined by the input frequency magnitude and phase, as well as the response characteristics of the individual digitizers, including the response characteristics of the various signal paths leading to each digitizing element. These spurious tones serve to degrade the quality of the digitizing system, as measured with the aforementioned specifications.
"These and other design issues impose practical restrictions on the degree or order of interleaving used in digital oscilloscopes. Further improvement of bandwidth in digital oscilloscopes has generally been accomplished by design and use of faster front-end amplifiers and ADCs. The performance of the amplifiers and samplers, however, is generally limited by the state of the art in integrated circuit fabrication."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "A method for improving bandwidth of an oscilloscope involves, in preferred embodiments, use of frequency up-conversion and down-conversion techniques to achieve a system bandwidth that significantly exceeds the bandwidth attainable through interleaving alone. In an illustrative embodiment the technique involves separating an input signal into a high frequency content and a low frequency content, down-converting the high frequency content in the analog domain so that it may be processed by the oscilloscope's analog front end, digitizing the low frequency content and the down-converted high frequency content, and forming a digital representation of the received analog signal from the digitized low frequency content and high frequency content. The digital representation is formed in various implementations by up-converting the content to its original frequency band and then recombining the high frequency content with the low frequency content. In preferred implementations this is achieved by passing the high frequency content through a high pass filter, mixing that content with a sinusoidal waveform to generate higher and lower frequency images, substantially eliminating the higher frequency image with a low pass filter, digitizing and upsampling the lower frequency image, mixing the digitized and upsampled content with a periodic waveform having substantially the same frequency as the sinusoidal waveform to generate higher and lower frequency images, and then combining the higher frequency image with the low frequency content to form a digital representation of the original input waveform. In this manner preferred embodiments achieve the significant advantage that bandwidth may be enhanced beyond the limits associated with interleaving and the state of the art in amplifier and ADC design.
"Also disclosed herein is an artifact signal correction system that in preferred embodiments is used to compensate for error tones generated during interleaving. The artifact signal correction system may include a mixing component to generate a waveform corresponding to an artifact such as an error tone, whereupon that waveform may be combined with the input waveform to substantially eliminate the artifact. In one embodiment, an input waveform and a periodic digital waveform are fed into a mixer to generate a mixed waveform with substantially the same frequency content as the input waveform except that the frequency is reversed and the phase content is negative. The periodic digital waveform may be synchronized to the digitizing elements such that the waveform has a positive magnitude during portions of the waveform sampled by a first digitizing element and a negative magnitude during portions of the waveform sampled by a second digitizing element. The mixed waveform may then be input to a digital filter that converts the phase and amplitude of the error tone to substantially the same phase and amplitude of the corresponding tones in input waveform. The converted and mixed waveform may then be synchronized with the input waveform by applying a delay to the input waveform that accommodates the aforementioned mixing and converting operations. An inverted version of the mixed waveform may then be added to the input waveform so as to substantially reduce or eliminate the error tones.
"Phase differences between multiple frequency bands may be accommodated in preferred embodiments by providing a signal processing system that compensates for the relative phase difference so that the combination of the bands is constructive throughout a substantial portion of a band overlap or crossover region. In one embodiment, a signal combining system may include a comparator for determining a relative phase difference between the two signals within a predefined crossover region, a phase adjusting element for adjusting a phase of one of the two signals, and a combiner for combining the phase-adjusted signal with the other of the two signals. In another aspect, a method for adjusting a phase relationship between signals from multiple frequency bands that are being summed may include filtering a first of the signals by applying an integer samples delay, a fractional sample delay filter, and an allpass filter bank; and summing the filtered first signal with a second signal.
"The details of various additional features are set forth in the accompanying drawings and the description below. Other aspects and advantages will become apparent from the description, the drawings, and the claims.
BRIEF DESCRIPTION OF DRAWINGS
"For a more complete understanding of the preferred embodiments, reference is made to the following description and accompanying drawings, in which:
"FIG. 1 is a schematic representation of an oscilloscope;
"FIG. 2 is a block diagram representation of a two channel oscilloscope front end;
"FIG. 3 is a graphical representation of radio frequency (RF) power at each stage of a high frequency (HF) signal path;
"FIG. 4 is a graphical representation of an overall gain at each stage of the HF signal path;
"FIG. 5 is a graphical representation of a noise power at each stage of the HF signal path;
"FIG. 6 is a graphical representation of a signal-to-noise ratio (SNR) at each stage of the HF signal path;
"FIG. 7 is a graphical representation of an overall noise metric at each stage of the HF signal path;
"FIG. 8 is a block diagram representation of a signal processing configuration;
"FIG. 9 is a block diagram representation of a digital signal processing (DSP) system;
"FIG. 10 is a representation of a configuration menu;
"FIG. 11 is a block diagram representation of a calculation of the phase of a reference tone;
"FIG. 12 is a graphical representation of a low frequency (LF) low pass (LP) filter magnitude response;
"FIG. 13 is a graphical representation of a high frequency (HF) low image filter magnitude response;
"FIG. 14 is a graphical representation of a HF notch filter magnitude response;
"FIG. 15 is a graphical representation of the combination of the HF low image and the notch filter response;
"FIG. 16 is a graphical representation of the combination of the HF low image and notch filter response showing rejection at the reference tone frequency;
"FIG. 17 is a representation of a digital local oscillator (LO) tone generator;
"FIG. 18 is a graphical representation of the digitally mixed combination of the HF low image and the notch filter response;
"FIG. 19 is a graphical representation of the digitally mixed combination of the HF low image and the notch filter;
"FIG. 20 is a graphical representation of the HF high image filter magnitude response;
"FIG. 21 is a graphical representation of the overall HF digital filter response;
"FIG. 22 is a graphical representation of a LF and HF path digital filter response;
"FIG. 23 is a graphical representation of a LF and HF path digital filter response;
"FIG. 24 is a graphical representation of a LF and HF path digital filter response;
"FIG. 25 is a digital oscilloscope screen showing a horizontal settings menu;
"FIG. 26 is a digital oscilloscope screen fragment showing an internal acquisition configuration; and
"FIG. 27 is a digital oscilloscope screen fragment showing acquisition system settings.
"Like reference numbers and designations in the various drawings indicate like elements."
For additional information on this patent application, see: Pupalaikis, Peter J.; Graef, David C. High Bandwidth Oscilloscope. Filed
Keywords for this news article include: Electronics, Digital Filters, Signal Processing,
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