High-speed digital technology brings many problems to the design engineers for verification and debugging while opening up new technologies and achieving wide-ranging innovations. Among these problems, the first and foremost are incidental or intermittent events, such as laser pulses or metastability. These events are difficult to identify and characterize, requiring test and measurement equipment to provide both high sample rates and superior data capture capabilities. This puts extremely high demands on the performance of the oscilloscope. In the past, we had to choose between resolution and capture length. The storage length of all oscilloscopes is limited; the higher the sample rate, the faster the instrument memory fills and the smaller the time window for data acquisition. Conversely, capturing data over long periods of time generally requires the expense of lowering the horizontal resolution (sampling rate).
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Because current oscilloscopes provide high sampling rates and high bandwidth, the key issue now is to optimize the quality of information captured by the oscilloscope, including how to capture multiple events at a sufficiently high horizontal resolution for efficient analysis; how to store only Display the necessary data to optimize the use of memory. Fortunately, Tektronix' advanced oscilloscopes using FastFrame segmented storage technology have improved storage efficiency and data acquisition quality, eliminating this paradox.
Take advantage of record length
Consider the single pulse shown in Figure 1. It was acquired on a Tektronix DPO7254 digital phosphor oscilloscope at a sampling rate of 20 GS/s in a 1000-point waveform. At this sample rate, most of the waveform details can be seen.
However, if you want to view multiple consecutive pulses, you must lengthen the time window of the acquisition. To accommodate the storage capacity provided by the instrument, the sample rate must be reduced or the length of the long window must be reduced. Of course, reducing the sample rate itself will reduce the horizontal resolution.
You can also increase the record length and pull down the acquisition time window without reducing the sample rate. However, this approach has its limitations. Despite advances in storage technology, high-speed acquisition memory is still a valuable resource, and it is difficult to discern how much storage capacity is sufficient. Even if someone thinks the record length is long, you may still not be able to capture the last, and perhaps the most critical, event.
As can be seen from Figure 2, the time window is extended by a factor of 10 to capture more consecutive pulses. The way to achieve this is to increase the grid of the grid lines displayed on the screen and increase the record length while keeping the sample rate constant. This larger acquisition brings some disadvantages: a large number of acquisitions increase the storage requirements of NVRAM and hard disk; a large number of acquisitions affect the I/O transfer rate (ie GPIB throughput); more record lengths increase the user The cost of the commitment.
Because the oscilloscope has to process more information, the inactivity time period or "dead time" between acquisitions increases, causing the update rate to drop. In view of these contradictions, the contradiction between the need for high sampling rates and the lack of storage length provided by each channel must be constantly balanced.
Figure 1: A single pulse captured at high resolution.
Figure 2: Multiple pulses captured at high resolution over long record lengths.
Segmented storage structure
To solve this problem, the industry has developed many strategies. One popular method is the "segmented storage" scheme. Instruments using this storage technology, such as Tektronix oscilloscopes using FastFrame segmented storage technology, allow the provided memory to be divided into a series of segments and then fill each segment with a trigger acquisition at the desired sample rate.
By carefully defining the trigger conditions, this technique can capture only the waveforms or segments of interest, and then each event captured is stored in its own numbered bucket. Individual segments or frames can be viewed individually in capture order, or multiple segments or frames can be layered to show similarity and comparison results.
This feature basically allows scanning through unwanted segments of the waveform so that the focus can be on the signal of interest. Figure 3 illustrates this approach. By using the FastFrame segmentation memory technology in the DPO7254 oscilloscope, it captures pulses at a sampling rate of 20 GS/s with the same small record length shown in Figure 1. The segmented storage contents are overlapped so that all the pulses are stacked on each other on the screen.
Figure 3: Multiple pulses can be captured at high resolution by using Tektronix FastFrame segmentation storage technology.
The advantages of this approach include: high waveform capture rate improves the ability to capture infrequent events; high sample rates are used to preserve waveform detail; there is no dead time between captured pulses, ensuring efficient use of record length memory; Visually compare waveform segments to determine if there is an "extension" exception event in the overlapping stack.
Frame and frame length compared to record length
Tektronix' oscilloscopes using FastFrame segmented storage technology allow the provided acquisition memory to be divided into frames (storage segments) consisting of hundreds of thousands of samples. This feature facilitates a burst trigger rate of 400,000 frames per second (number of acquisitions per second), which is equivalent to a maximum dead time of 2.5 microseconds, which is significantly faster than most other oscilloscopes.
When activated, FastFrame Segmentation Technology automatically calculates and selects the length of the record required to accommodate the number of frames you select and the number of points per frame (frame length). Based on the instrument memory provided, it calculates the product of the number of frames and the length of the frame, selects the most recent record length, and determines the number of frames that are suitable for the memory.
You can view each frame individually, or use the multi-function knob on the mouse, virtual keyboard, or instrument main console to select multiple frame numbers and scroll through the frames. When determining a particular frame of interest, you can use the instrument functions to calibrate, measure, amplify, and analyze waveforms in detail.
To quickly see the anomalous events that are prominent in the common shape of the waveform, multiple frames can be overlapped to show common and offset points. The "View Multiple Frames" option in FastFrame segmented storage technology uses color to overlap a selected number of frames, highlighting the frequency at which points overlap each other. The red dot indicates that the frequency of occurrence is high, and the blue dot indicates that the frequency of occurrence is low.
Average/envelope frame
FastFrame segmented storage technology supports the standard "Sample" acquisition mode and advanced mode, including "Peak Detect", "Hi-Res" and "WfmDB". The options in the "FastFrame. Setup" menu provide an extra frame for the "Envelope" or "Average" mode at the end of the recording.
This "Summary" frame draws a waveform graph using the envelope points (maximum and minimum values) or the average points of the selected number of frames.
For example, in the FastFrame segmented storage technology using the "Average" mode, when the number of frames is 10, the oscilloscope calculates the average of 10 frames and displays the average waveform in the last frame or "Summary" frame. If the FastFrame segmented storage technology uses the "Envelope" mode, the oscilloscope calculates the maximum and minimum values ​​of all waveforms in 10 frames, which are displayed as envelope waveforms in the last frame.
Time stamp of acquisition
The waveforms in each frame can only explain part of the situation. Important information is also embedded in the timing of each frame. Each trigger point has timing information called a time stamp. By analyzing the timestamp, you can determine when each event occurred and the relative time between events.
Time resolution
This technique captures trigger timing at very high resolution. Through the time interpolation method, the trigger timing is resolved into a portion where the sampling interval is very small. At high sample rates, this may be less than 1 ns. Although this resolution has little time-stamping for individual events, it provides a very powerful tool when measuring the time interval between multiple events.
FastFrame user interface
The first part of the control panel or the first "bar" provides basic options for definition and navigation acquisition when starting FastFrame segmentation storage technology. It controls the opening and closing of FastFrame segmentation storage technology, allowing selection of frame lengths and frames. It also provides options that can include "Summary" frames ("Envelope" or "Average").
Because Tektronix oscilloscopes offer very long record lengths, FastFrame segmented storage acquisitions can generate thousands of frames, and in some configurations, more than 1 million frames can be generated. The “Single Sequencing Mode Stop Condition†option in the first column menu allows you to stop the acquisition after the last frame fill, or to stop the acquisition by manually pressing the “Run/Stop†capture button on the oscilloscope console.
The second part of the FastFrame segmentation menu or the "View" bar is used to define and control how the frame is displayed. It controls: the input source, which will be the focus of the FastFrame segmentation storage; whether the waveform is viewed from the "Input" channel or the waveform from the "Math" channel, with or without the "ReferenceWaveform" channel; viewing multiple frames in an overlapping manner .
Possible input sources for FastFrame segmented storage technology are the data "Input" channel, the "Math" channel, and the oscilloscope's "Reference Waveform" channel. When choosing a primary source, focus on specific aspects of the potential problem with the device under test. The source channel can be a signal that is known to display an error, or it can be switched to another channel suspected of causing interference to change the focus. In both cases, the waveforms in the source channel frame and the waveforms of the other channels in the same time period are viewed.
The "Lock Frames Together" control determines whether or not to use the "Reference Waveform" channel for similar periods of time when viewing the "Input" channel and the "Math" channel. For example, the FastFrame segmentation storage technique always displays the frames of the "Input" channel together with the frames of the "Math" channel, because they are linked by their mathematical definition. However, the "Reference Waveform" may be completely different, so an engineer may or may not want to view the "Reference Waveform" along with the "Input" and "Math" channels in the same time period. FastFrame segmentation storage technology allows you to choose whether to link the "Input" and "Math" frames to the analog view of the "Reference Waveform".
The "View" column also controls the display of each frame and the overlapping view of multiple frames. If multiple frames are overlapping, you can overlap all the frames, or you can select the desired sequence of frames and only overlap the partial frames. You may want to select a reference frame from anywhere in all frames and then overlap a subset of the frames above. For example, if the selected frame of interest is frame number 12 (the total number of frames is 100), you can overlap and compare it to a sequence consisting of another 20 frames from frame number 13. The reference frame can be any frame, but is usually the first frame in the acquisition.
Finding anomalous events in a serial signal
Due to the increasing speed of serial communication protocols, designing and debugging complex systems is facing greater challenges. Small anomalies that interfere with communication circuits become more common, and finding and isolating is more difficult than ever.
The DPO7254's high sample rate and long record length allow it to capture large numbers of data points long enough to increase the probability of an undetectable anomaly in the identification signal. By using the "FastAcq" high-speed waveform acquisition mode and FastFrame segmentation storage technology, the instrument can quickly and intuitively provide waveform clues for further analysis.
With FastFrame segmentation technology, 1000 signal frames can be captured while maintaining high sample rates and appropriate time accuracy (time/division) and record length settings. You can scroll through the frames, but for 1000 frames, this process can be very time consuming and cumbersome. In order to speed up the comparison of frames, the overlapping pictures of all frames will be displayed by color coding. You can quickly see the frequent anomalies inside the waveform at a glance and identify areas for further analysis.
By using "Frame. Finder" on "Analyze", you can now focus on the sporadic (ie blue) frames, ignoring other frames from the layer and separating the layered frames. This will only leave one or a few frames for further investigation. You can specify any frame in the "Display Selected Frame" view field. Comparing selected frames to other frames is greatly simplified by using the mouse wheel or the instrument's multi-function knob. You can also use the "Frame. Delta Calculator" to easily access time stamp data on the screen, or you can save the entire time stamp table for offline analysis.
Summary of this article
In applications that require both high sample rates and long record lengths, adding more memory does not always solve the problem. For example, in an environment where a series of sporadic or intermittent events must be collected, the FastFrame segmented storage technology in Tektronix advanced oscilloscopes provides an ideal means of capturing only the necessary events. By selecting the acquisition memory and providing triggers and timestamps for each segment, FastFrame segmentation storage technology optimizes data collection and allows for more intelligent use of limited storage resources. (Tektronix Technology Corporation)
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