An oscilloscope is a widely used electronic measuring instrument. It can transform the electrical signals that are invisible to the naked eye into visible images, so that people can study the changes of various electrical phenomena. The oscilloscope uses a narrow beam of high-speed electrons to create a tiny spot on a phosphor-coated screen (this is how traditional analog oscilloscopes work). Under the action of the signal under test, the electron beam is like the tip of a pen, which can plot the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe waveforms of various signal amplitudes over time, and can also be used to test various types of power, such as voltage, current, frequency, phase difference, amplitude, and so on.
The development of analog real-time oscilloscopes has not changed much in the basic structure. The following figure is a basic block diagram:
The analog real-time oscilloscope is simple in mechanism, and there is no process such as digitization and processing of signals. All of ART's signal conditioning, amplification, and display are done by analog devices, so from the signal entering the amplifier (or probe) to the last display on the CRT, it's almost real-time (the delay time is almost negligible).
However, analog oscilloscopes also have dead time, and signals that appear during dead time cannot be displayed on the screen. This dead time comes from the trigger system's "hold off" and the time to wait for the trigger. Therefore, analog oscilloscopes are not capable of capturing signals 100%. Different models of analog real-time oscilloscopes, the maximum waveform capture probability is about 30% to 70%, and the scanning speed can be as fast as 500,000 times/second. This is a very good indicator.
Let's look at the way the analog oscilloscope displays it - the CRT cathode ray tube. The electron beam is deflected by the deflecting plate, and then the fluorescent substance on the display screen is illuminated to form a waveform track. When the electron beam stops bombarding, the bright spots do not disappear immediately and a period of afterglow is retained. The afterglow time is 10μs-1ms for short afterglow, 1ms-0.1s for medium afterglow, 0.1s-1s for long afterglow, and greater than 1s for extremely long afterglow. The general oscilloscope is equipped with a medium afterglow oscilloscope, the high frequency oscilloscope uses a short afterglow, Under the effect of the afterglow effect, the brightness of each point on the waveform track is proportional to the number of times (frequency) of the bombardment. Therefore, the waveform displayed by the analog real-time oscilloscope not only has the information of time and amplitude, but also the information indicating the probability of occurrence of the signal in the brightness level, which is very beneficial for observation.
But on the other hand, this characteristic of the luminescent material also poses a problem: the brightness of the trajectory with too few bombardments will be low or even impossible to observe. Therefore, analog oscilloscopes are more suitable for repetitive signals (such as continuous sine waves) or signals with repetitive characteristics (such as analog video signals). The ability to observe a single signal (such as a single pulse or sporadic failure) is very limited.
To sum up, the analog real-time oscilloscope has the following main advantages: strong real-time performance, high probability of waveform capture, and intuitive display of three-dimensional (time, amplitude, and signal probability). The main disadvantages are: the inability to store data, limited analysis capabilities, insufficient ability to capture low-probability events, simple triggering, insufficient pre-trigger delays, and difficulty in bandwidth boosting (both front-end amplifiers and CRTs must be simultaneously enhanced). With the rise of digital motion and more and more single-signal measurement requirements, the shortcomings of analog oscilloscopes have gradually made it impossible to meet the test requirements. Therefore, since the 1980s, mainstream oscilloscope manufacturers have gradually turned to digital oscilloscopes. R&D and production.
Tektronix' 511 analog real-time oscilloscope marks the arrival of the era of commercial oscilloscopes. There are also some "oscilloscope" products before 511, but because it does not have a trigger system and calibration time base, vertical scale, can not provide a stable display waveform, nor can it be quantitative test, so it is only a qualitative observation tool. For the first time, the 511 added edge triggering to the "oscilloscope" test device to display stable waveforms, using a calibrated time base and vertical amplifier to provide quantitative test capability, greatly increasing applicability. In this way, a commercial oscilloscope was born.
The development of analog real-time oscilloscopes has not changed much in the basic structure. The following figure is a basic block diagram:
The analog real-time oscilloscope is simple in mechanism, and there is no process such as digitization and processing of signals. All of ART's signal conditioning, amplification, and display are done by analog devices, so from the signal entering the amplifier (or probe) to the last display on the CRT, it's almost real-time (the delay time is almost negligible).
However, analog oscilloscopes also have dead time, and signals that appear during dead time cannot be displayed on the screen. This dead time comes from the trigger system's "hold off" and the time to wait for the trigger. Therefore, analog oscilloscopes are not capable of capturing signals 100%. Different models of analog real-time oscilloscopes, the maximum waveform capture probability is about 30% to 70%, and the scanning speed can be as fast as 500,000 times/second. This is a very good indicator.
Let's look at the way the analog oscilloscope displays it - the CRT cathode ray tube. The electron beam is deflected by the deflecting plate, and then the fluorescent substance on the display screen is illuminated to form a waveform track. When the electron beam stops bombarding, the bright spots do not disappear immediately and a period of afterglow is retained. The afterglow time is 10μs—1ms for short afterglow, 1ms—0.1s for medium afterglow, 0.1s-1s for long afterglow, and greater than 1s for extremely long afterglow. The general oscilloscope is equipped with a medium afterglow oscilloscope, the high frequency oscilloscope uses a short afterglow, Under the effect of the afterglow effect, the brightness of each point on the waveform track is proportional to the number of times (frequency) of the bombardment. Therefore, the waveform displayed by the analog real-time oscilloscope not only has the information of time and amplitude, but also the information indicating the probability of occurrence of the signal in the brightness level, which is very beneficial for observation.
But on the other hand, this characteristic of the luminescent material also poses a problem: the brightness of the trajectory with too few bombardments will be low or even impossible to observe. Therefore, analog oscilloscopes are more suitable for repetitive signals (such as continuous sine waves) or signals with repetitive characteristics (such as analog video signals). The ability to observe a single signal (such as a single pulse or sporadic failure) is very limited.
To sum up, the analog real-time oscilloscope has the following main advantages: strong real-time performance, high probability of waveform capture, and intuitive display of three-dimensional (time, amplitude, and signal probability). The main disadvantages are: the inability to store data, limited analysis capabilities, insufficient ability to capture low-probability events, simple triggering, insufficient pre-trigger delays, and difficulty in bandwidth boosting (both front-end amplifiers and CRTs must be simultaneously enhanced). With the rise of digital motion and more and more single-signal measurement requirements, the shortcomings of analog oscilloscopes have gradually made it impossible to meet the test requirements. Therefore, since the 1980s, mainstream oscilloscope manufacturers have gradually turned to digital oscilloscopes. R&D and production.
The first generation of digital oscilloscopes, now called Digital Storage Oscilloscopes (DSOs), uses a serial working structure. The block diagram is as follows:
Digital storage oscilloscopes use ADC sampling, so the measured analog waveform can ultimately be stored in data format. Of course, digitized data can also be easily automated, spectral, mathematical, or other advanced analysis. Therefore, digital oscilloscopes are especially suitable for single-signal acquisition and analysis, which is a big breakthrough.
On the other hand, digital storage oscilloscopes are fully digitalized after the ADC, so the increase in bandwidth is limited only by the variable gain preamplifier bandwidth and the ADC's rate. With advances in technology, the Tektronix TDS6154C is now the industry's highest analog generation widest digital storage oscilloscope, reaching 12.5GHz (3dB). Broadband amplifiers are a core part of the design of ultra-high-bandwidth oscilloscope systems. Current mainstream designs use separate hardware amplifier design methods for each channel, so there is no limit to the performance of each channel. When the design bandwidth of each channel amplifier is insufficient, some oscilloscopes use the MIMO oscilloscope to splicing together the 6 GHz low-bandwidth amplifiers in different frequency bands to achieve more than 6 GHz bandwidth on a certain channel, for example, 3 The 6GHZ band of the channel "splices" to reach a bandwidth of 18 GHz. The advantages and corresponding defects can be clearly seen from the method implemented by DBI technology. The most obvious advantage is the combination of low-bandwidth multi-channel and high bandwidth of single channel over 10 GHz. The most expensive amplifier and ADC in oscilloscope design are The low speed design is very beneficial to control costs. Since the DBI technology essentially first distributes the signal frequency to different channels, samples it through a relatively low-speed ADC, and finally "splices" these frequency numbers containing different components by DSP technology, it leads to the following limitations.
1. Channel number limitation: When using different channels, the bandwidth is different. When using 3 channels or 4 channels, only 6 GHz bandwidth is provided, and the ADC sampling rate is also limited.
2. Spectrum “splicing†error: As can be seen from the amplitude-frequency characteristic diagram, each frequency “splicing†point has obvious nonlinearity. When the spectral component of the measured signal is in this area, the waveform displayed in the oscilloscope time domain will Waveform distortion occurs.
3. Low waveform capture rate: Because DBI technology requires software processing and “splicing†waveforms in the digital frequency domain, waveform processing and display speed are very low when the amount of data is large.
4. Function limitation: When the DBI is turned on, although the single channel bandwidth and ADC are improved, the bandwidth of the trigger system cannot be improved by the DBI technology, and the maximum is only 800MHZ. In addition, the external reference input of the oscilloscope and the fine adjustment of the vertical sensitivity will be DBI is open and limited.
Digital storage oscilloscopes have also made great strides in triggering systems. As can be seen from the block diagram, the trigger system of the digital oscilloscope is a completely independent circuit dominated by analog circuits. A high-performance trigger system is like a camera's shutter, which helps testers pinpoint signal behavior. For a variety of special signal characteristics, digital storage oscilloscopes can be equipped with a variety of advanced trigger modes such as glitch trigger, runt trigger, transition time, communication trigger, serial trigger, window trigger, status trigger, pattern start and bus trigger. Tektronix' PinpointTM triggering system is the industry's most advanced triggering system, using full SiGe technology for edge triggering and advanced triggering, so trigger sensitivity can be reached at very high levels, such as the TDS6124C, edge triggered and The sensitivity of the advanced trigger can reach 3div@9GHz at the same time. This dual-trigger system is complemented by a trigger delay setting and a trigger reset, and the trigger mode can be set almost unrestricted.
With these features, digital storage oscilloscopes can have much higher bandwidth performance than analog real-time oscilloscopes; with the combination of triggering and sampling, digital storage oscilloscopes have greatly improved the capture capability of single-shot signals (low-repetition probability signals); The testing and analysis capabilities are not the same as before...but, after enhancing the ability to capture and analyze single signals,
With these features, digital storage oscilloscopes can have much higher bandwidth performance than analog real-time oscilloscopes; with the combination of triggering and sampling, digital storage oscilloscopes have greatly improved the capture capability of single-shot signals (low-repetition probability signals); The ability to test and analyze is not the same as before... However, after enhancing the ability to capture and analyze single signals, it also introduces inevitable weaknesses, which are mainly reflected in the waveform capture rate and monotonic display capability. Let us explain these weaknesses:
The structure of the digital storage oscilloscope has been decided, it must work in a serial mode - the signal is conditioned and enters the ADC sampling; the sampled data of the ADC is sent to the acquisition memory under the control of the trigger system; after the acquisition memory is full, The waveform data is sent to the computer system; the microprocessor processes, calculates, and analyzes the data according to user requirements; finally, the waveform and analysis results are displayed on the display (the workflow is slightly different in the scroll mode, and will not be described in detail here). ). In this process: from signal conditioning, trigger monitoring to ADC sampling, almost real-time, will not affect the efficiency; and data from the acquisition memory to the computer system, microprocessor processing, calculation process, the final display, will Because of the different architecture of the oscilloscope, its real-time performance is affected. The most critical part of this is the processing of the microprocessor. We all know that the popular oscilloscope sampling rate will be tens of GHz (GS / s) per second, no universal microprocessor can handle such data stream in real time, so the oscilloscope microprocessor can only be processed " Grab a piece, slowly process it, control the display, and repeat. In this way, during its "slow processing" time, the oscilloscope will not be able to monitor the waveform. This is what we call "dead time". Events that occur during the dead time are not displayed on the screen. To measure the ratio of the dead time of a digital storage oscilloscope to the total observed working time, we introduce the concept of “waveform capture rateâ€, which is the number of waveforms captured and displayed per second by the oscilloscope. “Waveform†here refers to all the information that is triggered at one time. The test proves that the high-performance (bandwidth 1GHz or higher) digital storage oscilloscope with the highest waveform capture rate in the industry has a waveform capture rate of about 8000 times, and the total time of capturing the waveform accounts for about 1-2% of the total observation time, that is, Say: More than 98% of all signals are missing due to the oscilloscope's dead time.
Every engineer believes that the instrument provides the right information, but few engineers will consider that the oscilloscope they are using can only provide so little waveform detail—for example, if there is an average in the signal you are observing A failure occurs once in 1 second, so the probability of finding this failure within 1 second of the digital storage oscilloscope is less than 2%, and the probability of discovery within 15 seconds is only about 26%. In fact, due to the tight development time, the average engineer will not take more than 10 seconds to observe a signal - as a result, you have less than a quarter of the chance to capture the fault and debug it effectively.
Almost all oscilloscope vendors are aware of the low waveform capture rate of digital storage oscilloscopes and have developed many ways to increase the speed of oscilloscopes. However, no matter the architecture of using two pairs of 1.25Gbps Gigabit Ethernet links when data is transferred from the acquisition memory to the microprocessor, the acceleration technology that displays partial and snapshot displays on the display is not the most fundamental. The problem of solving the throughput problem - in the serial architecture, the microprocessor is the bottleneck of speed, only the complete change of the serial structure, liberation of the microprocessor is the key to solving the problem.
In this respect, Tektronix is ​​at the forefront of the industry, starting with the transformation of the serial architecture from the beginning. From the InstaVu? in the mid-1990s to the real-time DPO in early 2006, the third-generation oscilloscope based on parallel architecture: digital phosphor oscilloscope has gradually matured from appearance. The figure below shows the structure of the DPO digital phosphor oscilloscope:
As can be seen from the structure, the parallel processing core of the DPO digital phosphor oscilloscope is the DPX parallel imaging processing chip. The DPX completes the storage, rasterization, and statistical processing of the acquired data to generate a three-dimensional database. And the rasterized waveform image information can be directly imported into the video memory. In this architecture, the microprocessor only does display control and so on, and no longer acts as a bottleneck in the data processing process.
The parallel structure of DPO digital phosphor oscilloscope fundamentally solves the defects of low waveform capture rate and serious waveform loss of DSO digital storage oscilloscope. The waveform capture rate of DPO7000 and DPO70000 series real-time digital phosphor oscilloscopes can reach 250,000wfm/s, DPO71000 and DPO72000 series ultra-high performance digital phosphor oscilloscopes can exceed 300,000wfm/s, and the ratio of captured waveforms to the total signal can reach up to 60%. Provided); and the new generation of DPX acquisition does not have the limitation of the highest 1.25G real-time sampling rate of the previous generation of “quasi-real-time fluorescence oscilloscopeâ€, but can work at any sampling rate to further enhance the signal capture capability. The best tool for the industry to find problems. The figure below shows a clock with the same level of oscilloscope from three different manufacturers simultaneously with a sporadic fault (once every second), after 15 seconds. It can be seen that when the previous two oscilloscopes barely found any problems, Tektronix' digital phosphor oscilloscope (pictured right) captured the multiple faults that occurred during this time, and the difference was clear at a glance.
DPX generates a three-digit database that also has a huge advantage in display. This database, recorded by the hardware buffer, preserves the amplitude, time, and amplitude of the waveform (ie, the frequency of occurrence of each point signal), regardless of the cumulative speed or buffer depth (26 bits per point). A database that is much larger than the software generated by other vendors. The display waveform generated by the three-digit database can simultaneously inform the user of the amplitude, time and probability information of the signal in terms of color temperature, spectrum, brightness level, etc. The effect is very similar to that of an analog oscilloscope.
The digital phosphor oscilloscope has the same waveform capture rate and display mode as the analog oscilloscope. The ability to capture and observe repetitive signals and repetitive signals (such as digital signals and serial communication signals) greatly exceeds that of traditional digital storage oscilloscopes. Improve the efficiency of debugging and verification. At the same time, digital phosphor oscilloscopes also have the ability to fully analyze a single acquisition of a digital storage oscilloscope. Moreover, due to its architectural advantages, digital phosphor oscilloscopes are fully ahead of digital storage oscilloscopes in terms of test items, test speeds, and test accuracy.
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Yuheng Optics Co., Ltd.(Changchun) , https://www.yhenoptics.com