A deep dive into the internal principles and structure of an oscilloscope

Correctly selecting the trigger signal has a great impact on the stability and clarity of the waveform display. For example, in the measurement of digital circuits, for a simple periodic signal, it may be better to choose an internal trigger, while for a signal with a complex period and a signal with a periodic relationship with it, it may be better to choose an external trigger.

(2) Trigger coupling mode selection

There are many ways to couple the trigger signal to the trigger circuit in order to ensure the stability and reliability of the trigger signal. Here are some commonly used methods.

AC coupling is also called capacitive coupling. It only allows the AC component of the trigger signal to trigger, and the DC component of the trigger signal is blocked. This coupling method is usually used when the DC component is not considered to form a stable trigger. However, if the frequency of the trigger signal is less than 10Hz, it will cause triggering difficulties.

DC coupling (DC) does not block the DC component of the trigger signal. When the frequency of the trigger signal is low or the duty cycle of the trigger signal is large, it is better to use DC coupling.

When low frequency suppression (LFR) is triggered, the trigger signal is added to the trigger circuit through a high pass filter, and the low frequency component of the trigger signal is suppressed; when high frequency suppression (HFR) is triggered, the trigger signal is added to the trigger circuit through a low pass filter, and the high frequency component of the trigger signal is suppressed. In addition, there is a TV synchronization (TV) trigger for TV maintenance. These trigger coupling methods have their own scope of application, which needs to be experienced in use.

(3) Trigger level (Level) and trigger polarity (Slope)

Trigger level adjustment is also called synchronization adjustment, which synchronizes the scan with the measured signal. The level adjustment knob adjusts the trigger level of the trigger signal. Once the trigger signal exceeds the trigger level set by the knob, the scan is triggered. Turn the knob clockwise to increase the trigger level; turn the knob counterclockwise to decrease the trigger level. When the level knob is adjusted to the level lock position, the trigger level automatically remains within the amplitude of the trigger signal, and a stable trigger can be generated without level adjustment. When the signal waveform is complex and the level knob cannot be used to stably trigger, use the HoldOff knob to adjust the waveform's holdoff time (scan pause time) to synchronize the scan with the waveform.

The polarity switch is used to select the polarity of the trigger signal. When the switch is in the "+" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal increase. When the switch is in the "-" position, a trigger is generated when the trigger signal exceeds the trigger level in the direction of signal decrease. The trigger polarity and the trigger level jointly determine the trigger point of the trigger signal.

6. Sweep Mode

There are three scanning modes: Auto, Norm and Single.

Auto: When there is no trigger signal input, or the trigger signal frequency is lower than 50Hz, the scan is in self-excitation mode.

Normal state: When there is no trigger signal input, the scan is in the ready state and there is no scan line. When the trigger signal arrives, the scan is triggered.

Single: The single button is similar to a reset switch. In single scan mode, the scanning circuit is reset when the single button is pressed, and the Ready light is on. A scan is generated after the trigger signal arrives. After the single scan is completed, the Ready light goes out. Single scan is used to observe non-periodic signals or single transient signals, and it is often necessary to take a picture of the waveform.

The above briefly introduces the basic functions and operations of the oscilloscope. The oscilloscope has some more complex functions, such as delayed scanning, trigger delay, XY working mode, etc., which will not be introduced here. It is easy to get started with the oscilloscope, but you need to master it in application to be truly proficient. It is worth pointing out that although the oscilloscope has many functions, it is better to use other instruments and meters in many cases. For example, in a digital circuit experiment, it is much simpler to use a logic pen to determine whether a single pulse with a narrow pulse width occurs; when measuring the pulse width of a single pulse, it is better to use a logic analyzer.

04
Issues that must be paid attention to when using a digital oscilloscope

1 Introduction

Digital oscilloscopes are becoming more and more popular due to their unique advantages in waveform triggering, storage, display, measurement, waveform data analysis and processing. Since there is a large performance difference between digital oscilloscopes and analog oscilloscopes, if they are used improperly, large measurement errors will occur, thus affecting the test task.

2. Distinguish between analog bandwidth and digital real-time bandwidth

Bandwidth is one of the most important indicators of an oscilloscope. The bandwidth of an analog oscilloscope is a fixed value, while the bandwidth of a digital oscilloscope has two types: analog bandwidth and digital real-time bandwidth. The highest bandwidth that a digital oscilloscope can achieve using sequential sampling or random sampling technology for repetitive signals is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digitization frequency and the waveform reconstruction technology factor K (digital real-time bandwidth = highest digitization rate/K), and is generally not given directly as an indicator.

From the definitions of the two bandwidths, we can see that the analog bandwidth is only suitable for measuring repetitive periodic signals, while the digital real-time bandwidth is suitable for measuring both repetitive signals and single-shot signals. When manufacturers claim that the bandwidth of an oscilloscope can reach a certain number of megabytes, it actually refers to the analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, the bandwidth of TEK's TES520B is 500MHz, which actually refers to its analog bandwidth of 500MHz, while the maximum digital real-time bandwidth can only reach 400MHz, which is much lower than the analog bandwidth. Therefore, when measuring a single-shot signal, it is important to refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will cause unexpected errors in the measurement.

3. Sampling rate

Sampling rate is also called digitization rate, which refers to the number of samples of analog input signal per unit time, usually expressed in MS/s. Sampling rate is an important indicator of digital oscilloscope.

(1) If the sampling rate is not high enough, aliasing may occur.

If the input signal of the oscilloscope is a 100KHz sine signal, but the signal frequency displayed by the oscilloscope is 50KHz, what is going on? This is because the sampling rate of the oscilloscope is too slow, resulting in aliasing. Aliasing means that the frequency of the waveform displayed on the screen is lower than the actual frequency of the signal, or even if the trigger indicator on the oscilloscope is on, the displayed waveform is still unstable. The generation of aliasing is shown in Figure 1.

So, for a waveform of unknown frequency, how to determine whether the displayed waveform has aliasing? You can slowly change the sweep speed t/div to a faster time base to see if the frequency parameters of the waveform change dramatically. If so, it means that waveform aliasing has occurred; or the shaking waveform stabilizes at a faster time base, which also means that waveform aliasing has occurred. According to the Nyquist theorem, the sampling rate must be at least twice the high-frequency component of the signal to prevent aliasing. For example, a 500MHz signal requires a sampling rate of at least 1GS/s. There are several ways to simply prevent aliasing:

a. Adjust the scanning speed;

b. Use Autoset;

c. Try switching the collection mode to envelope mode or peak detection mode, because the envelope mode looks for extreme values ​​in multiple collection records, while the peak detection mode looks for maximum and minimum values ​​in a single collection record. Both methods can detect faster signal changes.

If the oscilloscope has InstaVu acquisition mode, you can choose it because this method acquires waveforms quickly and the waveforms displayed by this method are similar to those displayed by an analog oscilloscope.

(2) Relationship between sampling rate and t/div

The maximum sampling rate of each digital oscilloscope is a fixed value. However, at any scan time t/div, the sampling rate fs is given by the following formula:

fs=N/(t/div)N is the sampling point per grid

When the number of sampling points N is a certain value, fs is inversely proportional to t/div. The greater the sweep speed, the lower the sampling rate.

In summary, when using a digital oscilloscope, in order to avoid aliasing, it is best to set the sweep speed to a faster position. If you want to capture fleeting glitches, it is best to set the sweep speed to a slower position.