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 that can digitize, display and analyze 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 sample 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 around 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.
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