Brief Introduction of Dead Time for Oscilloscope

SHENZHEN, China - Feb. 18, 2014 - PRLog -- With the development of science and technology, digital oscilloscope becomes more technologically advanced and waveform capture rate now has been regarded as the forth technical qualification in addition to bandwidth, sampling rate and memory depth. It is essential for you to know the dead time when referring to the capture rate.

The dead time is an inevitable technical defect that can not be removed but to reduce for digital oscilloscope. Different from the display mode of analog oscilloscope by making electrons directly beat to fluorescent screen, digital oscilloscope is a typical system of “front-end data acquisition + rear-end digital signal processing”. For this system, the output data throughput of the front-end data acquisition system is much larger than that of the rear-end digital signal processing system, which means that the rear-end can not process data transmitted from the front-end timely, thus produce the dead time.

Siglent launched the new SDS2000 series digital oscilloscope recently. Its ADC sampling rate is 2GSa/s, which means the oscilloscope can output 2G data in one second. However, the data handling and display capacity of digital signal processor can only reach to Hundreds of megabytes point per second， which means the processor can only process very small part of the acquired data, the remaining data that known as the dead time will be discarded.

A waveform capture cycle is namely a signal cycle. The  waveform capture cycle consists of the effective capture time and dead time. During the effective capture time, the oscilloscope captures waves using specify sampling rate, and write them into RAM. The dead time includes fixed dead time and variable dead time. The fixed dead time depends on the own factors of oscilloscope, like the computation rate of FPGA/DSP chip and algorithms framework. The variable dead time depends on the time that ADC needs to acquire data, it is influenced by the set memory, time base, sampling rate and post-processing function like interpolation, mathematical function, measurement and analysis.

Waveform capture rate is defined as waveforms that can be displayed in unit time for oscilloscope, and it is of most importance to dead time comparing with other factors. The following is the formula calculating the dead time:

Dead Time%=[100×(1－waveform capture rate ×time base×horizontal grids)]%

So, the following is the effective time for waveform capturing:

Effective time%=100% - Dead time%

Using the formula above, we can calculate the effective time for SIGLENT new SDS2000 series super phosphor oscilloscope, which is featured with the 110,000 wfms/s capture rate. The SDS2000 oscilloscope has 14 horizontal grids, when the time base is set to 50us, which is known as the special time for the maximum capture rate, the dead time that calculated is 77%, namely the effective time is 23%.

As to traditional oscilloscope with the capture rate of only 2000wfm/s, like SIGLENT SDS1000L, which has 18 horizontal grids, the dead time calculated using the same way is: 98.2%. As a result, the effective time calculated is only 1.8%.In summary, the SDS2000 series is stronger greatly in capturing waveforms than SDS1000L series.

However, due to exist of variable dead time, and the capture rate and memory are variable in different time base, the exact dead time needs further analysis and calculation.

As shown in figure2, the abnormal signal capture using the high capture rate of SDS2000. In theory, the higher is capture rate, the quicker and more success in abnormal signal capture. It is wrong to understand that high capture rate definitely result in abnormal signal, while low capture rate unquestionably result in no abnormal signal.