How to understand the waveform capture rate and capture cycle of an oscilloscope

The waveform capture rate of an oscilloscope, as the name implies, refers to how many times the oscilloscope captures a waveform per unit time. Its unit is written as "wfm/s" in English (wfm is the abbreviation of waveform), and is generally written as "times/second" or "frames/second" in Chinese. For example, the waveform capture rate of the SDS3000 series of Dingyang Technology's intelligent oscilloscopes can reach up to 1 million times/second, which means that the oscilloscope can capture 1 million waveforms per second and display them on the screen of the oscilloscope. However, the waveform capture rate of many similar oscilloscopes based on the Windows operating system is only 2,500 times per second, and some are only a few hundred times.

Capturing a waveform 1 million times per second is equivalent to a single trigger of 100 times per second on the oscilloscope. Except for the DPX developed by T Company's oscilloscope in the early years, which is the waveform refresh rate, purely the refresh of "pixels", the waveform capture rate we are talking about now is actually the waveform trigger rate. The waveform on the oscilloscope is refreshed once the trigger is triggered. The waveform capture rate is ultimately reflected on the screen display of the oscilloscope, that is, the refresh of the waveform graphics, so the waveform capture rate can also be understood as the waveform refresh rate, but the waveform refresh rate cannot be understood as the waveform capture rate. This is a bit of a word game. The use of "refresh" once emphasizes that the DPX technology only displays "pixels" on the screen, not the display of data samples captured by the actual ADC sampling lag. This is the inherent disadvantage of DPX.

To put it more simply, we usually see the waveform on the oscilloscope changing with our naked eyes, but the rate at which the naked eye can observe the change of the signal is only about 60Hz, 60 times per second. The naked eye sees that the waveform on the oscilloscope is "constantly" changing, but in fact, a "huge amount" of the corresponding circuit signal between the waveform change and the next change has been "missed" and is not displayed on the oscilloscope screen. The time interval between the waveform captured this time and the waveform captured next time is the capture period. In other words, the waveform capture rate is the inverse of the capture period.

How to understand the "capture cycle" from the working principle of the oscilloscope? After the signal passes through the probe, it first enters the oscilloscope input channel, first enters the amplifier, ADC, acquisition memory, and then the oscilloscope will transfer the discrete data points in the acquisition memory to the CPU unit for display processing, measurement and calculation. The acquisition process and the display and processing process constitute a complete capture cycle. As shown in Figure 3, the acquisition process is actually very fast because it is all implemented through chip hardware. The time it takes for the ADC to convert the analog signal into a digital signal is negligible compared to the time it takes to send the data from the acquisition memory to the CPU for measurement and analysis and to send it to the screen for display.

Figure 3 Schematic diagram of the working principle of the oscilloscope

"A digital oscilloscope spends most of its capture cycle on post-processing of waveform samples. During the process of processing data samples, the oscilloscope is in a no-signal state and cannot continue to monitor the measured signal. Basically, dead time is the time required for a digital oscilloscope to post-process waveform samples. Figure 4 shows a schematic diagram of a waveform capture cycle. The capture cycle consists of effective capture time and dead time. During the effective capture time, the oscilloscope captures the number of waveform samples set by the user and writes them into the acquisition memory. The capture 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, which is directly related to the set number of capture samples (memory depth), horizontal scale, sampling rate, and selected post-processing functions (such as interpolation, math functions, measurements, and analysis). The dead time ratio is the ratio of the dead time to the capture cycle, and the reciprocal of the capture cycle is the waveform capture rate. Both are important parameters of the oscilloscope and are related to each other."

Figure 4: A capture cycle of a digital oscilloscope

Where is the starting point of each capture cycle? Trigger! Trigger once, capture once. In short, the oscilloscope discretizes the analog signal into digital "points", which are temporarily stored in the "acquisition memory". If there is no trigger, the acquisition memory follows the principle of "first in, first out", and the new data after discretization will continue to enter the acquisition memory, and the old data will be lost. Every time the oscilloscope is triggered, the oscilloscope sends the acquired signal to the screen for display once. Trigger once, capture once!

The explanation given in reference [1] is quite illustrative, so I repeat it here:

We can think of the memory of an oscilloscope as a circular memory. The oscilloscope continuously samples and gets new sampling points, which will be filled in, and the old sampling points will automatically overflow. This process will repeat until the oscilloscope is "stopped" by a "trigger signal" or forced to "stop" after a certain period of time. After "stopping" once, the oscilloscope "moves" the sampling points stored in the memory to the screen of the oscilloscope for display. The waiting time between these two "moves" is extremely long compared to the sampling time, which is called "dead time".

The above process is often compared like this: the memory is like a "water tank", and the capacity of the "water tank" is the "storage depth". If you use a "faucet" to fill the water tank at a constant speed, the water flow rate of the faucet is the "sampling rate". When the water tank is already full of water, the faucet is still filling the water tank, and some of the water in the water tank will overflow, but the overall capacity of the water tank remains unchanged. Under certain conditions, the water in the water tank will be poured out, and the cycle will repeat.

The "certain condition" here, relative to the oscilloscope, is the moment when the trigger signal arrives. So once the trigger is triggered, the oscilloscope will collect a waveform, but the waveform displayed on the screen "moves once" and the naked eye sees "a bunch of waveforms", because the oscilloscope captures too many waveforms in 1 second, and the naked eye can't react! At this time, you may think that it is better to be "slow", just like those traditional "slow" oscilloscopes, the naked eye can see better, and the naked eye can know what characteristics the waveform is "moving" according to.