The choice between analog oscilloscope and digital oscilloscope

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The choice between analog oscilloscope and digital oscilloscope [Copy link]

For electronics enthusiasts, engineers, and technicians, the wide variety of oscilloscopes on the market may make people dazzled. Oscilloscopes have a variety of parameters and functions, coupled with a variety of prices, it is indeed easy to make people feel overwhelmed.

For beginners, let's forget about the parameters and price of the oscilloscope and its various functions. Ask yourself a few questions first:

1. Do I need to take an oscilloscope with me? Do I need a portable oscilloscope? Do I need an oscilloscope that can be used outdoors with batteries?

2. Based on my actual work situation, how many signals do I need to measure simultaneously?

3. What is the range of the signal voltage I am measuring, and what are the maximum and minimum values?

4. What is the maximum frequency of the signal I am measuring?

5. Am I mainly measuring repetitive signals or capturing abnormal signals?

6. Do you need to perform frequency domain and time domain analysis on the signal?

If we have clear answers to the above questions, it should be easy for us to select a suitable oscilloscope. But if we are confused about the above questions, we should learn some of the following knowledge.

Comparison and differences between analog oscilloscopes and digital oscilloscopes

Whether it is an analog oscilloscope or a digital oscilloscope, its function is to measure signals. Both mainly observe signals based on the time domain. Since the signal changes at different time points, the vertical and horizontal axes in the oscilloscope represent the voltage value and time of the signal respectively. We can also observe signal anomalies and noise from the oscilloscope screen.

The fundamental difference between a digital oscilloscope and an analog oscilloscope is that a digital oscilloscope converts analog signals into digital signals through an analog-to-digital converter for processing and display. After a digital oscilloscope digitizes a signal, it can not only display the signal, but also perform various calculations on the signal, so the digital oscilloscope has more functions. Old analog oscilloscopes often use cathode ray tubes (CRTs) to display signals, and old digital oscilloscopes also use CRTs, but modern digital oscilloscopes generally use LCD screens. Since these two do not change the principle of the oscilloscope, there is no such thing as a CRT oscilloscope or an LCD oscilloscope in the industry.

As time goes by, the only advantage of analog oscilloscopes is price. Analog oscilloscopes do not have the ability to store data and analyze waveforms, their trigger functions are limited, and their ability to capture single-shot and sporadic signals is also poor. Moreover, since they use a large number of analog components inside, these components will change over time and temperature, so their performance is unstable.

At present, digital oscilloscopes have begun to surpass analog oscilloscopes in almost all parameters. In the future, the elimination of analog oscilloscopes is almost inevitable. Considering the very long service life of oscilloscopes and the future development of the electronics field, I strongly recommend that if you buy an oscilloscope now, just buy a digital oscilloscope. Let's take a look at some parameters of digital oscilloscopes.

bandwidth

The front-end attenuator and amplifier of the oscilloscope determine the bandwidth of the oscilloscope, and the bandwidth of the oscilloscope determines the maximum frequency range of the signal that the oscilloscope can measure. According to the definition, when an oscilloscope with a bandwidth of 100M measures a 100MHz frequency, 1V sine wave, the signal size will be attenuated to 0.707V. If you use an oscilloscope with a bandwidth of 50M to measure a sine wave with a frequency of 500M, the signal will definitely be distorted. According to the theory of oscilloscope bandwidth, assuming that the maximum frequency of the signal we measure is 20MHz, then you can buy an oscilloscope with a bandwidth of more than 100MHz.

Sampling rate and memory depth

ADC analog-to-digital converter and RAM high-speed memory affect the other two major indicators of the oscilloscope: sampling rate and memory depth. According to Nyquist's theorem, if the bandwidth of the measured signal is limited, then when sampling and quantizing the signal, if the sampling rate is more than twice the bandwidth of the measured signal, the information carried in the signal can be completely reconstructed or restored without aliasing. In fact, the sampling rate of oscilloscopes on the market is generally more than 5 times the bandwidth.

What needs to be noted here is the difference between the equivalent sampling rate and the real-time sampling rate. Generally, if an oscilloscope with a very low price has a very high sampling rate, you should pay special attention to whether the rate marked by the manufacturer is the equivalent sampling rate.

Oscilloscope sampling rate = storage depth ÷ waveform recording time

So you see, the sampling rate of the oscilloscope changes with the length of waveform recording, and the speed of change is affected by the storage depth. The greater the storage depth of the oscilloscope, the less likely the sampling rate will decrease when recording signals for a long time.

Vertical resolution

Most oscilloscopes have an 8-bit vertical resolution, which means the voltage value can be divided into 256 parts. If the measured voltage is within the range of ±1V, each part can be as fine as 7.8125mV, which is sufficient for analyzing digital signals.

The DC gain accuracy of an 8-bit ADC is typically within ±2 to 3%.

trigger

The trigger function of an oscilloscope is its soul function. It is because of it that we can observe the waveform stably. Otherwise, the signal changes on the screen all the time, and we will not be able to analyze the signal and make judgments.

Most oscilloscopes have edge triggering, which is the most basic trigger function. However, some oscilloscopes provide more diverse triggering methods to meet users' diverse signal triggering needs.

Measuring range

Generally speaking, due to the existence of the probe attenuation ratio, the voltage that an oscilloscope can measure is only related to the oscilloscope probe. For example, to measure high voltages of thousands of volts, you need to buy a high-voltage probe. The standard probe of the oscilloscope can generally only measure peak values of a few hundred volts.

However, you should also pay attention to the channel attenuation ratio selection of the oscilloscope and the vertical range of the oscilloscope's channels. First, set the oscilloscope's channel attenuation ratio to 1X and check the vertical range, such as 1mV/div-10V/div. Then, based on the channel attenuation ratio that the oscilloscope can set, you can know the oscilloscope's voltage measurement range.

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