Teach you how to choose a digital oscilloscope in testing

A digital oscilloscope is a general-purpose test instrument, which is essentially a graphic display device, equivalent to a voltmeter or multimeter with a graphic display. It can intuitively display the waveform of the signal changing over time on the screen, and measure and analyze the waveform's period, voltage, frequency and other parameters. It is widely used in various fields such as scientific research and production. It is the main test instrument for engineers to design, debug and repair products, and plays a vital role in testing work.

Many engineers will encounter such problems when using digital oscilloscopes: the signal amplitude and frequency data measured by a qualified digital oscilloscope are sometimes far from the designed value of the signal itself, so they look for circuit problems, but after spending time and effort, they find nothing and don’t know where the problem lies. In addition, for the same signal, the results measured by different digital oscilloscopes are different.

The causes of these problems are directly related to the choice of digital oscilloscope.

1 Main performance indicators of digital oscilloscopes

When choosing a digital oscilloscope, we mainly consider whether it can truly display the measured signal, that is, the consistency between the displayed signal and the measured signal.

The performance of a digital oscilloscope greatly affects its ability to achieve signal integrity. The following is a detailed analysis based on its main performance indicators.

(1) Bandwidth

As shown in Figure 1, the bandwidth of a digital oscilloscope refers to the frequency value (i.e. f-3dB) when the amplitude of the output waveform drops to 70.7% of the actual amplitude with the frequency change when the input equal-amplitude sine wave signals of different frequencies are applied. The bandwidth determines the basic measurement capability of the digital oscilloscope for the signal. As the signal frequency increases, the digital oscilloscope's ability to accurately display the signal decreases. In actual testing, we will find that when the frequency of the measured signal is close to the bandwidth of the digital oscilloscope, the digital oscilloscope will not be able to distinguish the high-frequency changes of the signal, and the displayed signal will be distorted. For example: a signal with a frequency of 100 MHz and a voltage amplitude of 1 V is tested with a digital oscilloscope with a bandwidth of 100 MHz, and the voltage displayed is only about 0.7 V. Figure 2 shows the results of the same step signal measured with digital oscilloscopes with bandwidths of 4 GHz, 1.5 GHz, and 300 MHz, respectively. As can be seen from the figure, the higher the bandwidth of the digital oscilloscope, the steeper the rising edge of the signal, the more high-frequency components are displayed, and the more accurate the reproduced signal. In actual applications, price factors are taken into consideration (the higher the bandwidth of a digital oscilloscope, the more expensive it is). Through the accumulation of practical experience, we have found that as long as the bandwidth of the digital oscilloscope is 3-5 times the highest frequency of the measured signal, an accuracy of ±3% to ±2% can be obtained, which meets general testing needs.

Figure 1 Frequency response curve

Figure 2 Measurement results of the same step signal based on digital oscilloscopes with different bandwidths

(2) Rise time.

Rise time is defined as the time it takes for the pulse amplitude to rise from 10% to 90% (as shown in Figure 3). It reflects the transient characteristics of the vertical system of the digital oscilloscope. The digital oscilloscope must have a fast enough rise time to accurately capture the details of fast-changing signals. The faster the rise time of the digital oscilloscope, the more accurately it can capture the fast-changing signals.

The rise time and bandwidth of a general digital oscilloscope satisfy the following formula:

Among them, tr is the rise time, f-3dB is the bandwidth, and k is a constant between 0.35-0.45 (different models of digital oscilloscopes have different values, you can refer to the corresponding instructions). Through calculation and referring to the bandwidth selection principle, it can be concluded that as long as the rise time of the digital oscilloscope is less than one-third to one-fifth of the measured signal, it can meet general test requirements.

(3) Frequency response.

Frequency response is the response performance when equal-amplitude sine wave signals of different frequencies are input. It includes the amplitude response in the entire frequency range from DC or AC low-frequency sine signals of a few hertz to the frequency where the amplitude cannot be displayed. In actual measurement, only considering the bandwidth performance is not enough to ensure that the digital oscilloscope can accurately reproduce the original signal. In the measurement work of digital oscilloscopes, we found that the frequency response curves of some digital oscilloscopes are not flat in the low-frequency band, and there will be large fluctuations. If the signal of the corresponding frequency is tested, distortion will occur. At this time, even if the bandwidth of the digital oscilloscope is much higher than the frequency of the measured signal, it cannot truly reproduce the signal. Therefore, when selecting a digital oscilloscope, its frequency response is also one of the key performance indicators to consider for different measured signals.

(4) Sampling rate.

The sampling rate of a digital oscilloscope is expressed as samples per second (S/s). The faster the sampling rate, the more realistically the original signal can be reproduced. According to the Nyquist theorem, the sampling rate must be greater than or equal to 2 times the frequency of the measured signal in order to restore the original signal without distortion. However, the premise of this theorem is based on infinite time and continuous signals. In actual testing, the technology of digital oscilloscopes cannot meet this condition. Based on the accumulation of practical experience, in order to accurately reproduce the original signal, the sampling rate of a digital oscilloscope is generally 2.5-10 times the highest frequency of the original signal.

2 Impact of probe on test

In actual work, a probe system (including probes and probe connection accessories) must also be used to introduce the signal of the circuit under test into the digital oscilloscope. Therefore, the performance of the probe system directly determines the degree of consistency between the signal introduced into the digital oscilloscope and the signal under test.

To obtain effective measurement results and truly reproduce the measured signal, the following three factors should be considered when selecting a probe system:

Physical connection, minimal impact on circuit operation, and consistency with the original signal. The first two factors require the correct selection of probe connection accessories, and the last factor requires the correct selection of the bandwidth of the probe system. In the entire test system, the bandwidth is determined by the lowest bandwidth part of the system, so the bandwidth of the probe system is also one of the factors that must be considered when testing the signal. At the same time, the probe system has also become a part of the circuit under test, and has a certain load effect. The load characteristics of the probe system are manifested in three aspects: the input resistance, capacitance and inductance of the probe system. The ideal probe system paired with a digital oscilloscope will minimize this load characteristic and give full play to the digital oscilloscope's ability, characteristics and tolerance to truly reproduce the measured signal. Therefore, when selecting a probe system, it is best to use the probe model recommended by the manufacturer and select the corresponding connection accessories according to the function.

3 Conclusion

In summary, the ability to achieve signal integrity is the core criterion for selecting a digital oscilloscope in testing. Several factors that affect the realization of digital oscilloscope signal integrity are: the bandwidth, rise time, frequency response, sampling rate of the digital oscilloscope, and the bandwidth and load effect of the probe system. When purchasing, in order to achieve the best cost-effectiveness, you can follow the following principles: the bandwidth of the digital oscilloscope is 3-5 times the highest frequency of the measured signal, the rise time is less than one-third to one-fifth of the measured signal, the frequency response curve is flat, and the sampling rate is 2.5-10 times the highest frequency of the measured signal. At the same time, choose the probe system recommended by the manufacturer to meet general testing needs.