What is dead time? To understand the source of dead time, you need to have a basic understanding of the structure of digital oscilloscopes. A typical block diagram of a digital oscilloscope is shown in Figures 1 and 2.
The measured signal enters the oscilloscope through the input channel and is adjusted by the attenuator and amplifier in the vertical system. Analog-to-digital converters (ADCs) sample the signal at regular intervals and convert each signal's amplitude into discrete digital values ​​called "sample points." The acquisition module then performs processing functions such as sample extraction. The default is generally the sampling mode. The output data is stored in the acquisition memory as samples. The number of stored samples can be set by the user by the record length.
According to the needs of users, these sample points can be further processed. Post-processing tasks include arithmetic functions (such as averaging), mathematical operations (such as FIR filtering), automatic measurements (such as rise time or fall time), and analysis functions (such as histogram or mask test). Other post-processing includes, for example, protocol decoding, jitter analysis, vector signal analysis, and the like.
For digital oscilloscopes, there are basically no restrictions on the processing steps performed on the waveform samples. These post-processing functions are either executed using the instrument's main processing program using software or implemented using dedicated ASIC or FPGA hardware depending on the oscilloscope's structure. The final result is then presented to the user through the oscilloscope's display.
From Fig. 1 and Fig. 2, we can see the difference between the R&S RTO series oscilloscope and the traditional digital oscilloscope in the signal processing process. It uses an independently developed ASIC chip RTC and FPGA to realize the post-processing of waveform samples, such as channel calibration, Sample extraction, digital filtering, math, histogram measurement, mask testing, and FFT, automatic measurement, protocol decoding, etc., greatly reduce the workload of the main processor. At the same time, the digital trigger replaces the analog trigger circuit in the RTO chip. The trigger jitter caused by the analog trigger circuit, the traditional high-end oscilloscope in order to reduce this part of jitter, requires a lot of DSP post-processing. The innovation in hardware structure greatly shortens the time spent on RTO oscilloscope waveform sample post processing.
The processing of the oscilloscope's capture from the signal sample to the waveform sample shows this period, called the capture period. After the previous capture period, the oscilloscope can capture the next new waveform. Therefore, digital oscilloscopes use most of the acquisition cycle for post-processing of waveform samples. In this process, the oscilloscope is in a no-signal state and cannot continue to monitor the measured signal. Fundamentally, dead time is the time required by a digital oscilloscope to postprocess waveform samples.
Dead-time and capture period versus waveform capture rate Figure 3 shows a schematic of the waveform capture period. The capture cycle consists of an effective capture time and a dead time cycle. During the effective capture time, the oscilloscope captures the number of waveform samples set by the user and writes it to the acquisition memory. Captured dead time consists of fixed time and variable time. The fixed time depends on the architecture of each instrument. The variable time depends on the time required for processing. It is related to the number of capture samples (record length), horizontal scale, sample rate, and selected post-processing functions (eg, interpolation, mathematical functions, measurement, and analysis). There is a direct relationship. The ratio of dead-time and capture period Dead-time is also an important characteristic of an oscilloscope. The reciprocal of the capture period is the waveform capture rate.
For example, if the effective acquisition time is 100 ns (the number of samples is 1 k, the sampling rate is 10 G), and the dead time is 10 ms, the time taken for the entire capture cycle is 10.0001 ms. The resulting dead-time ratio is 99.999%, and the waveform capture rate is less than 100 waveforms per second. Most of the current oscilloscopes on the market have a waveform capture rate of a few hundred orders of magnitude below the normal measurement mode. R&S's latest RTO series oscilloscopes can achieve a waveform acquisition rate of up to 1,000,000 times under the same conditions, and the dead time ratio can be Reduced to 90%, far higher than other oscilloscopes. Some oscilloscopes with bandwidth ≤ 1G can achieve a waveform capture rate of 50,000 times per second at their highest sampling rate, and their dead-time ratio is as high as 99.5%.
Effects of dead-time and waveform capture rate on measurement results Many engineers may have encountered this situation during the hardware debugging process: In the later stage of debugging, the welding of the main components of the circuit board is basically completed, during the function verification process, It will not be long before the system is found to operate. However, the oscilloscope can be used to view the critical clock and enable signals. The problem is “softâ€, and the cause of the fault is finally determined as the cause of the software. Then the code is checked line by line and the software is optimized. Now that you have a clear understanding of the oscilloscope's dead-time, there is a possibility that the oscilloscope missed an accidental signal that caused the system to fail. Figure 4 can be a very graphic description of this problem:
Due to the existence of the oscilloscope's dead time, the oscilloscope may miss the key abnormal signal and display a deceptive result to the user, ultimately misleading the user's judgment, which will greatly prolong the debugging time and reduce the debugging efficiency.
According to Equation 1, if the waveform capture time (ie, the number of samples × resolution, or 10 × horizontal scale), the waveform capture rate, and the rate of signal events (eg, the repetition rate of pulsed interference) have all been determined, increasing the measurement time will result in Increase the probability of capturing and displaying signal events:
P: Probability to capture incidents of repetitive signals [in %]
GlitchRate: Signal failure frequency (eg, repetitive pulse interference) [unit is 1/s]
T: effective capture time or waveform display time (record length/sampling rate, or record length × resolution, or 10× time quantum/div) [unit is s]
AcqRate: Waveform capture rate [in wfms/s]
Tmeasure: measurement time [in s]
If you know the probability, transform Equation 1, you can calculate the time required to capture the contingent signal:
Formula 2:
Suppose a signal has an exception that repeats 10 times per second. The signal itself is displayed on the oscilloscope in data format with a horizontal scale of 10 ns/div. If the display used has 10 horizontal divisions, an effective capture time of 100 ns can be calculated. In order to ensure high confidence in the capture of the desired signal events, a 99.9% probability is used. Now, the required test time depends on the oscilloscope's waveform capture rate. The following table shows the required test time for several different waveform capture rates.
Table 1: Time required to capture anomalous signals at a probability of 99.9% (T=100ns, GlitchRate=10/s).
Although R&S's RTO series oscilloscopes still have a dead-time ratio of approximately 90% under this condition, compared to other oscilloscopes whose dead-time is above 99.5%, they found that the number of occasional anomalous signals increases by an order of magnitude. Can help engineers greatly improve debugging efficiency. Question: How many engineers can see more than 7 seconds on the oscilloscope when checking each signal?
As mentioned earlier, the waveform capture rate is related to the horizontal scale, record length, and sample rate setting. In the actual measurement, how to find a balance point in these parameter settings based on the actual measured signal, with the highest capture probability Looking at waveforms to improve debugging efficiency is an issue that engineers need to consider when using digital oscilloscopes. This section will be discussed in future articles.
Take a deep breath, close your eyes, the world has the freshest oxygen ... Yu Quan's "Deep Breath" is well known to us, but as the environmental problems become more and more serious, the haze sky also increases, PM2.5 PM10 and so on Vocabulary has also been mentioned more and more, and air purifiers have quickly become popular on the market with this "air quality" trend. Along with this, the air-clean brand has sprung up, and various technologies are dazzling. Which ones are indeed effective and which are just tasteless propaganda? How does the air purifier bother us to get rid of pollution? And look at the editors for you.
Analysis of air purifier
The critical value of PM2.5 content is 75 μg / m3
The pollutants in the air mainly include soot, total suspended particulates, inhalable particulates (PM2.5) and fine particulates (PM10). The smaller the diameter of the particulates, the deeper the part that can enter the respiratory tract. 10 micron diameter particles It is deposited in the upper respiratory tract, and particles below 2 microns can penetrate 100% into the bronchiole and alveoli.
1k-5k air purifier prices vary greatly
The air purifier products on the market not only have various functions, but also have prices ranging from thousand yuan to tens of thousands. The pricing of imported products is generally higher than that of domestic brands. Except for the core technology of many brands, the promotion of purification effects is similar, and consumers need to choose according to demand.
The air quality standard of 74 key cities is only 4.1%
According to the data of the press conference held by the State Council Information Office on June 4, according to the new "Ambient Air Quality Standards", the six pollutants SO2, NO2, PM10, PM2.5, CO and O3 were evaluated, 74 new standards In the first phase of monitoring and implementation, the proportion of cities that met the ambient air quality standards was only 4.1%, and the other 256 cities implemented the old air quality standards, and the proportion of cities that met the standards was 69.5%.
Air purifier sales increased by 90.5% compared with last year
According to a report recently released by the China Electronics and Information Industry Development Institute, due to the haze weather, air purifiers have become the most concerned home appliance products, with sales of nearly 2.4 million units nationwide last year, a year-on-year increase of 90.5%; retail The amount reached 5.6 billion yuan, a year-on-year increase of 105.9%.
HEPA filter needs to be replaced in March
The filter of the air purifier is the same as the filter of the water purifier, and it is not effective for a long time. After long-term use, the accumulation of dust on the surface of the filter will also cause the filter to gradually fail. Therefore, when the haze is more serious, the non-washable HEPA filter needs to be replaced every three months.
Merchants claim that the purification rate is more than 99% for the concept of stealing
99% or even 99.99% seems to have become a slogan for many air purifiers on the market. Many brands of products promote their high removal rate of PM2.5 and formaldehyde. In fact, this kind of publicity is more of a kind. The slogan of stealing the concept, the removal rate of 99% is the effect of the material itself on PM2.5 and formaldehyde, not the effect of actually running in space.
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