What is the waveform refresh rate? What are the factors that affect the waveform refresh rate?

There was an article before, "Why Oscilloscope Manufacturers Never Mention Refresh Rate", which described the market status of various oscilloscope manufacturers on the market in terms of refresh rate parameters. Many oscilloscope users are all concerned about the refresh rate indicator of the oscilloscope. Recently, when our FAE communicated with customers, many customers were very interested in the high refresh rate of 330,000 frames per second of the ZDS2022 oscilloscope. How is such a high refresh rate achieved?

What is the waveform refresh rate?

The waveform refresh rate is also called the waveform capture rate, which refers to the number of times the waveform is refreshed per second, expressed as waveforms per second (wfms/s). In fact, the process from the oscilloscope acquiring the signal to displaying the signal waveform on the screen is composed of several capture cycles. A capture cycle includes sampling time and dead time. The analog signal is converted into a digital signal through ADC sampling and quantization and stored at the same time. The time of the entire sampling and storage process is called the sampling time. The oscilloscope must perform measurement, calculation, display and other processing on the stored data before starting the next sampling. This period of time is called the dead time. During the dead time, the oscilloscope does not perform waveform acquisition. After one capture cycle is completed, it will enter the next capture cycle. The reciprocal of the capture cycle is the waveform refresh rate, as shown in Figure 1.1, waveform refresh rate = 1/(Tacq+Tdeat).

What are the factors that affect the waveform refresh rate?

Sampling time and dead time

As shown in Figure 1.1, the waveform refresh rate is the reciprocal of Tacq (sampling time) and Tdeat (dead time), where the sampling time is determined by the sampling pane of the oscilloscope screen, which is calculated by multiplying the horizontal base gear by the number of horizontal grids. Once the horizontal time base is determined, the sampling time will be fixed.

The dead time is determined by the processing capability of the oscilloscope. When the oscilloscope's data processing capability is insufficient, the collected big data cannot be processed in time, the dead time will become longer, and the refresh rate will decrease. When the oscilloscope's data processing capability is strong, the dead time will become shorter, and the corresponding refresh rate will be very high. Therefore, the dead time is an important factor affecting the refresh rate.

Trigger holdoff time

Increasing the trigger holdoff time is equivalent to increasing the dead time in disguise, because during the holdoff period, the trigger circuit is closed and the trigger function is suspended. Even if there is a signal waveform that meets the trigger conditions, the oscilloscope will not trigger, so it will also affect the refresh rate. However, the trigger holdoff time does not refer to the dead time.

When triggering a large-cycle repetitive waveform, there are many waveform points in the waveform that meet the trigger conditions, which makes the triggered waveform unstable. In order to obtain a stable triggered waveform, we can set the trigger holdoff time so that the waveform is triggered at the same point each time and the triggered waveform is displayed stably. As shown in Figure 1.2, the holdoff time can be set to a value of >200ns but <600ns.

Figure 1.2 Trigger holdoff time